I am collecting stories on testing we have done here. Virtually every month for the last 24 years has brought some type of testing or data collection on the Corvair flight engine. Some tests are fairly simple, such as building a new manifold and testing the output of a simple 1 barrel automotive carb, others like building and dyno testing EFI systems were more complex. This goes on continuously. Many of the tests go undocumented, or show themselves to be fruitless or economically unusable. Many test only provide a puzzle piece that is useful on another project years later.
Most alternative engine outfits are only interested in selling stuff, and more often than not, they did almost no testing before going to market. Many companies start selling engine before the first example has flown 100 hours, and I can think of a number of now defunct companies that sold engines long before even a single example had left the ground. The common element with all of them was viewing testing as just some useless overhead that cuts into quick profit margins. We are just the opposite. Remember that teaching builders about their engine is our primary goal. A a learning focused company, reasearch, testing and evaluation has provided the very material our program is made of.
Below are links to several stories that give a glimpse of the practical testing that has always been integrated into my work with the Corvair. Just stop and think about how many time you have read in a magazine article or sales brochure that the horse power output was from “Dynamometer tests”, yet, how many times have you ever seen a picture of any of these engines actually on a dyno? Personally I have seen at least 200 claims of HP output alleged to be measured on a dyno, yet I have only seen pictures of 4 different engines on an actual dyno run. In an era were virtual everyone has a cell phone, and every one of them is a camera, why do you think that 196 companies didn’t end up with a photo of their engine? Just maybe, the only “dyno” run they did was an imaginary one for the brochure.
I have said it before, If your goal is to Buy something, any salesman will do. If you goal is to learn, build and fly, to be the master of what you are doing, they you need someone you can learn from. I am willing to share what we have learned in many years of reasearch and testing with anyone who came to experimental aviation to learn and build.
Above, the EFI 2,700cc Corvair in 2007, at power on my dyno. The urethane wheel directly reads foot pounds of torque off the digital scale.
Click on any title below to read the full story of that test.
Below is a story from the spring of 2008, taken from our ‘Hangar Update” on our main page Flycorvair.com It is a good indication of how testing is integrated everything we do. I am sure than many other companies headed to Sun-n-Fun that year focused their attention on getting spiffy polo shirts ready and glossy brochures. Not us, we were out on the ramp in front of the hangar testing engines.
In the second part of the story, we are running a test on a perfect “standard day” These are set of circumstances that rarely occur in real life, but which all dynos are supposed to be corrected for. By running the test on that day we developed very accurate correction factors for our testing. It is taking the time to do things like that, and not ow you dress that makes testing valid.
The story was shot in front of our house and hangar in northern Florida. Mark Petz and Kevin Fahy are prominently featured. Mark still builds outstanding Corvair flight heads at Falconmachine.net, but Kevin a member of our original “Hangar Gang” is long since retired. Shortly after this story he He married a very attractive woman with a PhD in Aerospace Engineering who long worked for NASA at Huntsville AL. Kevin made a tee-shirt that said “Trophy Husband” and set out to lead a life of quiet leisure.
A few days before Sun ‘N Fun, Kevin came up to give us a hand readying the display engine for the show. Above, he’s prepping our Fifth Bearing engine for its run on our Dynamometer We have run more than 50 engines on this dyno. The the run stand we had before the dyno broke in and test ran about 75 more. Research, testing and years worth of study and learning make our recommendations valid.
A week prior to the show, Mark Petniunas of Falcon Automotive drove down from Wisconsin to our North Florida hangar to give us a hand assembling and test running our Fifth Bearing test engine. I told him on the phone I thought it was a day or two away from running. Late into the sixth 18-hour day of his visit, Mark said: “I’m going to have to fire my travel agent. I have yet to see one girl in one bikini on one sunny sandy beach. This Florida vacation is nothing like the brochure.” Above, Mark on the right confers with Kevin right after the first start up of our Fifth Bearing Motor.
Above is our Fifth Bearing Engine at power on the Dyno. The natural aluminum CNC billet Bearing Plate is between the case, Ring Gear and the Gold Prop Hub. It is intended to address both thrust and bending issues.
I came up with this design myself, but the CAD modeling was done by our aeronautical engineer Spencer Gould. Sharp eyes will notice that this utilizes All Our Regular Production Components. The added 1″ round spacer in front of the CNC Starter Bracketshows the length of the engine. The engine has a Gold Billet CNC Pan on it.
The day after Sun ‘N Fun we were back at our North Florida Hangar conducting more tests and unloading and unpacking the trailer after the show. Here, Kevin, myself and Mark on the other side use all hands on a run of The Fifth Bearing Engine.
Above is the balancer on The Fifth Bearing Motor. The timing scale on the back of the Corvair engine shows 0 to 16 degrees. The length of this scale can be transferred to the balancer to show 16 and 32 degrees BTDC (before top dead center).
As stated in my conversion manual, the proper way to set the timing on your Corvair engine is to know what the full advance is at full static rpm. I have long told people to tie down the tail of their airplane and check the timing advance at its full static rpm. Installing the distributor and not setting the timing this way is foolish. All aircraft engines, including those with magnetos, have their timing checked at maximum advance.
The difference is that aircraft with magnetos have their timing set statically at full advance, and then their impulse couplings retard their timing. The Corvair engine can have its timing set statically at idle for an idle setting, but it must be run to its full static rpm to have the timing checked because distributor ignition has mechanical advance, not retard.
If you’re a builder and you didn’t know this, that’s perfectly okay. That’s why we issue instructions. If you hold an A&P license and you don’t know this, you can stick the powerplant section of your license in an envelope and mail it back to Oklahoma City. This is a good example of how I’ve intentionally patterned the Corvair engine to philosophically duplicate the proven aspects of Lycomings and Continentals.
Dyno calibration after Sun ‘N Fun.
Above, you’ll notice Kevin and I are wearing jackets. We’re waiting just before sunset for a rare weather phenomena to occur: a perfect standard day of 59F 50% relative humidity and a pressure of 29.92. Any time you read a dyno report and it says “corrected horsepower,” they’re making a calculation, sometimes accurate and sometimes not, to adjust for their test conditions not being at standard atmosphere. Because we live in Florida near sea level, there have actually been three occasions in the past four years when these conditions were met during daylight hours on testing days.
Our dyno relies on the super accurate optical Prop Tach for the rpm measurement and it will only reliably pick this up in daylight. A few minutes after the photo above was taken, we made a dyno run which required no correction. By testing the same engine later in the week, we reconfirmed our correction factors for this particular dynomometer and we retained accurate measurements all year round.
As the post Sun ‘N Fun work wound down, we stopped for a photo op with Grace’s Taylorcraft. From left above: Dan Weseman, Mark Petniunas of Falcon, Kevin, myself, Grace and Scoob E were on hand for the last hour of tests. Although it marked the end of another Sun ‘N Fun as it became a collection of good memories, friends and fun, the talk already centered on what we were going to do this summer, plans for Oshkosh and good times ahead.
The pace of the Corvair Movement affords little time for reflection. And certainly the best of times are ahead of us. If you are new to the land of Corvairs, there’s time to express creativity, make your mark, enjoy new friends and join the adventure.
Here is a story about thrust testing two engines from our time at the Edgewater hangar. This work was the start of our efforts with turbos on Corvairs. The testing below lead directly to flying a turbocharged 2700cc engine in our Stitts Skycoupe test aircraft the following year. The StolGlass aircraft had a very nice 912S rotax in it, with an inflight adjustable prop on it.
If legends, hangar flying stories and old wives tales were to be mistaken for testing and data, the Rotax would have delivered great numbers, which it didn’t. Few people understand that if the gearing on the Rotax was for maximum performance, it would be in the range of 1.8:1, but the gearing the factory selected is far higher, 2.54:1, and this is driven by the need for the engine to meet very stringent European noise restrictions.
Turbo Corvair and 912S Thrust Testing
At the hangar, we do testing all the time. It’s not a special process, but rather integrated at every opportunity. In these photos, you’ll see two tests that we ran during the summer. The photo above shows a direct drive 164cid Corvair engine we used as a test mule for our simple turbo setup. Our previous tests have more photos of this same engine, but here we’re testing a 72″ two-blade Warp Drive propeller. In this photo you’ll clearly see that this is not a rebuilt engine. We used the engine as is to confirm the initial sizing of the turbo. At this point, we did not have it heavily instrumented. Without an accurate EGT gauge, it’s quite easy to harm a test motor when initially developing a turbo installation. However, I had no worries here. This particular engine, nicknamed “Old Greasy,” was purchased running for $200, putting a very low cap on my potential loss. Notably, the engine ran through all the tests with flying colors, and never broke anything.
Above, Dave is holding the digital optical tach and the pressure gauge. If you look closely, you’ll see the engine is turning a wood prop, the thrust output here is about 360 pounds. This is an appropriate prop for a 180mph aircraft. When this propeller was replaced with the 72″ Warp Drive, a prop appropriate for a plane with an 85-100mph cruise speed, the thrust shot up to 470 pounds. This is roughly equivalent to the static thrust available from an O-320 powered Cessna 172. The main difference between the two props is primarily the pitch, not the diameter. Lower pitched props appropriate for aircraft with lower cruise speeds produce significantly more static thrust than props with higher pitch. The 72″ prop and the turbo is a combination we’re looking into for STOL airplanes. My line of thought: The 20 pound turbo setup is lighter than any gearbox or belt reduction, comparitively immune to torsional vibration, and a whole lot less expensive. More testing to follow, but the few runs we made here already exceeded my expectations.
Above is a line of airplanes outside our hangar. The Cessna 120 belongs to Gus, The Taylorcraft is Grace’s BC12D (C-85 powered). The Corvair powered KR2 belongs to Steve Makish. Of interest here is the StolGlas in the foreground. This is a factory built aircraft from South America. It is imported by CR Aviation in Miami. It is a popular aircraft in South America, and is now being brought to the U.S. as a kit/LSA. Steve Critelli of CR Aviation brought the aircraft to our hangar to explore the possibility of re-engining the aircraft with a Corvair. When we tested it for a baseline, it had its factory installed Rotax 912S 100hp powerplant and a 3-blade, 72″ diameter, in-flight adjustable Ivoprop. The engine and propeller were in first class condition with 140 hours on them.
The results of the test were surprising to say the least. Let me start by acknowledging that the Rotax is a good engine, it’s known to make its rated power, and it is something of an industry standard for experimental engines in its class. Although it’s a small motor, barely more than 1,300cc, it’s heavily geared, 2.58 to 1. Common consensus holds that a combination like this should be capable of producing a lot of thrust. We carefully rigged the airplane for thrust testing to make allowances for the thrust line of the aircraft, and also to protect the airframe.
After several full power runs, carefully checking the propeller’s low pitch setting, and confirming WOT, the engine pulled 340-345 pounds of thrust. The propeller rpm was about 2,200. With the gearbox, the engine rpm was near 5,700. This amount of thrust was far less than expected if old wives’ tales of low rpm propeller efficiency are to be believed. Compare this with direct drive Corvair powerplants we have built turning 68″ props at 2,800rpm. The Corvair powerplant easily produces 10-15% more thrust. This is contrary to what most people have been led to expect. I’ve been teaching people for many years that higher rpm props are better right up to the point where the tip goes supersonic, and that low rpm props with low tip speeds are actually a disadvantage. The time to climb capabilities of aircraft like Pushy Galore are graphic presentations of my point. So why did Rotax gear this engine down this far? The most plausible explanation is for noise abatement. Although not yet a design consideration in the United States, European engines are required by very strict laws to meet extremely stringent noise restrictions. It is illegal to operate engines which don’t meet these standards throughout much of Europe. The Rotax engine is a European product designed to meet these standards. While the Corvair engine is not particularly loud by American standards, it would be hardpressed to match the Rotax. Having worked for the German firm MT-Propeller, I can attest to the great efforts European manufacturers go to in order to meet their noise standards. There’s nothing wrong with the Rotax, but there’s certainly no magic in its gear reduction when it comes to thrust output. Of course a 1,300cc powerplant needs some type of a reduction to be a viable 100hp aircraft engine. But this testing has shown just as obviously that a 2,700cc engine does not need a reduction to more than match the smaller engine’s thrust output. While theories have their place, testing in the real world has far greater value for people who want to build and fly airplanes, not just talk about them.
Here is another set of testing from the days at our Edgewater hangar. Thrust testing is a very common number to quote, but it is also the most commonly faked or deceptive piece of data people quote. It is very easy to set a ground adjustable prop so low on pitch that it produces fantastically high thrust numbers, but the plane would be required to have a 40mph cruise speed to use them. (airboats operate in this range)
For data to be useful for more than inpressive sounding number in a brochure or website, it must have two elements. First, the prop pitch must be realistic to the type of flying you will be doing. Second, you have to use the same equipment on the same day to test known engine prop combinations like the O-200 C-150 for comparison.
All of this can take time and be a real bother comparied to just making up a number that sounds good. After years of this type of testing, a am going to guess that 75% of the numbers people quote on this topic are simply made up. Just stop and think about how many times the numbers you have seen came without any kind of photo of the test being conducted. I have found that people wo like to talk about planes they will build one day most often cite numbers known to be fake. On the other hand, buiolders who are working on the plane they will finish and fly follow data like this story.
Over the years we’ve done a lot of thrust testing in order to compare the output of engines, the thrust of different propellers, and the effects of systems installations. The method used to measure thrust is a hydraulic cylinder attached to a remote gauge. It is easy to calibrate because you can hang a known weight from it. In our case, the thrust is 1.54 x the number shown on the gauge. This is because the piston in the hydraulic cylinder has more than 1 square inch of area.
A few days ago, we tested a lot of different combinations at the hangar for comparative purposes. All tests that we’ve done recently are conducted on 100ll fuel. All of the Corvairs were tested with 32 degrees total ignition advance. The only exception to the ignition was the turbo engine, which was set at 22 degrees total. A $300 digital, optical tachometer was used to measure rpm. Weather conditions are measured on the spot with digital instruments. Here you’ll see tests of certified engine and propeller combinations also. Over the years I’ve been working with alternative engines, I’ve noted that many people who are fans of alternative engines know very little about certified engines. Being an A&P mechanic, I have the greatest respect for certified powerplants. I like everything about them except for the expense of obtaining and operating them. All my work with the Corvair motor is patterned after the success of certified engines. I use their performance as a baseline, and their level of reliability as a goal. Anyone who tells you that alternative engines have superior reliability, or fantastically better performance than certified powerplants is either not telling the truth or has no practical experience with them. In our case, we own, maintain and fly certified powerplants in addition to our work with Corvairs. This gives me a greater range of experience and a more balanced view of the capabilities of alternative powerplants, specifically the Corvair. The next time you hear somebody comparing alternative to certified powerplants, either pro or con, ask them if they’ve owned and operated both and you’ll find that very, very few people have personal experience in both fields.
The Zenvair 601
Above is our 601’s engine measured as installed in the aircraft. The only thing different about this engine is that it has roller rockers and our modified cylinder head intake pipes. I doubt either one of these mods would have a substantial effect on the output of the engine. The prop pitch setting of 11.5 degrees at the tips would be an appropriate setting for a direct drive Corvair motor to move the 601 at 140-150mph. If the prop was set flatter for a slower speed airplane, or used a slightly larger diameter prop, the thrust numbers would be even higher.
Propeller: Warp Drive 2-blade HP hub and blades, stock tips, 66″ diameter, 11.5 degrees pitch measured at tips
Density Altitude: -174
Wind: 4-9mph headwind
Thrust: 347 pounds static
1946 Cessna 120
In the photo above is Gus Warren’s 120 that he rebuilt from a basket case to 1998 Oshkosh Champion. It lives in our hangar. The engine has about 100 hours on a first class overhaul. It has flow matched Superior cylinders.
Propeller: McCauley 71×46 Met-L, aluminum (This is a climb prop for a 120)
Density Altitude: -174
Wind: 4-9mph headwind
Thrust: 340 pounds
Larry and Cody Hudson’s Corvair Engine
This father/son team from Indiana built their own engine, in the photo above, from our Conversion Manual and components last year. They dropped it off at our shop before Sun ‘N Fun for a break in on our test stand. The prop installed is appropriate for a 180mph airframe. This is why it has low static thrust numbers. It is good for comparative purposes, and is the same prop used on some of the 2002 tests. This engine is not fully broken in, as it has less than two hours of test stand time on it.
Displacement: 164cid, .030 over
Exhaust: Cast iron manifolds, automotive muffler
Cowling: None, cooling baffle only
Propeller: Sterba 62×58
Density Altitude: -122
Wind: 5-7mph headwind
Thrust: 225 pounds
Pictured above is our neighbor Arnold’s 1959 Cessna 150. The engine in this aircraft is one that is the subject of the AD that requires the timing to be reduced to 24 degrees. The engine is a mid-time engine that just came out of a 100-hour inspection. It can be considered to be in good working order. Contrary to what most people think, O-200s in 150s are only certified to use propellers up to 69″ diameter. No 150 left the factory with a propeller diameter of 72″.
Engine: Continental O-200, 100hp, 2750rpm redline
Exhaust: Stock 150
Cowling: Stock 150
Propeller: McCauley Clip Tip 68″ diameter, aluminum, standard pitch
Density Altitude: -122
Wind: 5-7mph headwind
Thrust: 335 pounds
Shop Test Engine
We built up a test engine, below, from parts in our shop. We built it up to use in potentially destructive ground testing. Since it’s made of used parts, it is not only dirty, but also fully broken in and has very low internal drag. I believe this is why it will turn slightly higher numbers than the Hudson engine. We utilized the same distributor, intake, carb and exhaust on this engine and the Hudson engine. The only difference would be the status of the internal assemblies.
Displacement: 164cid, standard bore
Exhaust: Cast iron manifolds, automotive muffler
Cowling: None, cooling baffle only
Propeller: Sterba 62×58
Density Altitude: -133
Wind: 2-3mph headwind
Turbo Test Engine
The engine above is the same as the test engine, with the addition of a new Garrett turbocharger, which we had specifically sized and set up for a drawthrough condition. I wanted to test this on a junk motor with a mild steel exhaust to evaluate the sizing of the turbo, and to ensure that it produced boost in the rpm range we wanted. Turns out that the sizing and the trim of the turbo are nearly dead on. We’re going to run a lot more ground tests, and then develop our flight installation package. Based on early tests, we should have absolutely no problem getting 100hp at 10,000 feet on a 164cid engine. While the installation looks very Mad Max, it gave us the data we needed. Keep in mind that everything on this installation was less than optimal, and it has already met my expectations. Despite being told by armchair experts of the antiquated nature of drawthrough installations, and the requirement for an intercooler, this simple installation of a modern, efficient turbocharger worked exceptionally well. At full output, you could reach up and put your hand on the steel intake manifold, and it was not too hot to touch. While it would be hotter at altitude, I think the installation’s off to a great running start. A little practical testing has once again shown that you can learn a lot more by testing rather than talking.
Displacement: 164cid, standard bore
Exhaust: Cast iron manifolds to Garrett T04B turbo, 2.5″ outlet pipe 18″ long
Cowling: None, cooling baffle only
Propeller: Sterba 62×58
Density Altitude: -1
Wind: 3-4mph tailwind
Thrust: 331 pounds
RPM: 2950 (there was more power available, but I did not want to boost the motor past 45″ without working EGT in place)
We have more testing lined up on the turbo engine, and we’re going to maintain a separate Turbo Testing Page on http://www.FlyCorvair.com for it. We have a 72″ Warp Drive propeller we’ll be installing for a maximum thrust test, which will give fans of 80-120mph aircraft a better idea of the potential of the powerplant in their speed range. Please keep in mind when you read these statistics and look at the pictures that all the data is factual. I frequently read stories where people claim to have VWs which produce 500 pounds of thrust and Subarus which produce even more. We professionals in experimental aviation get a good chuckle out of inflated numbers from advertising brochures and press releases. But, people new to sport aviation should know that you can come down to my hangar any time and I’ll gladly duplicate these tests.
Here is a look at a classic testing story from our days at the Edgewater Florida hangar . The years we spent there (2004-2008), maked the sucessful completion of many planes and hosting a number of outstanding colleges. The pace of testing was going into high gear then. The story notes that we had previously run 50 engines on the run stand before it was converted to a dyno. In the years that followed, we ran more than 100 engines on the dyno, most of the in public or at college where we invited people to study the system and verify the conclusions.
Today, the run stand we use at the college is made from many of the same parts we used on the dyno. It has been simplified to allow much faster engine changes required to run many engines at a college. If you look at the engines in all the tests, you can see that they are all 2700s that pre-date any of our Gold system components. (Gold oil systems eleminate the need to verify the stock oil bypass as shown in the story) The engines we build today, the 2,850s and 3,000cc power plants, are even more powerful than the engines in the tests, with no sacrifice in reliability. As we have extracted more power, we have done it largely by increasing the displacement of the engine, so the power output per cubic inch has stayed about the same, keeping the stress on the engine to very conservative levels.
Torque, Horsepower and Thrust Testing
September 21, 2004
Here’s the second engine being run on our dynamometer. We ran this a few hours ago. In the photo above, Gus is checking the timing with a light. We spent some time this week upgrading the dynamometer in its details. Visible is its new paint job, but it got a lot of detail work to facilitate rapid engine changes, and multiple tests. I contacted Darryl at Warp Drive Propellers today, and he’s sending us down a matched set of smaller diameter blades which will allow us to graph the horsepower at high rpm settings. These blades will become part of the permanent setup. The engine shown here in the test is a virtual clone of our 601 engine. The only significant difference is that this engine has GM steel rockers, while our 601 engine has roller rockers. I built this engine to represent our standard engine configuration, and use it as a test engine for the dynamometer. It’s fully airworthy and has many nice details like ARP close tolerance through case studs. After the break in and dyno runs, this engine will be for sale.
Here’s a side view of the same engine running, above. This engine has the inward leaning, welded on aluminum intake tubes that fit in our 601 Nosebowl. The intake manifold has four rubber slip joints in it that allow it to mate to many different intake pipe configurations. In photos below, you’ll see the PC Cruiser engine, which has a different intake pipe configuration. The adjustable intake manifold for the dynamometer quickly mated to both motors with a minimum of fuss. These types of details will allow many engines to be evaluated on the dyno. There are 5/8″ and 3/8″ tubes welded into the left valve cover. These mate up to the readily available and highly effective air/oil separator sold by Wick’s and Spruce. The gauge measures mechanical oil pressure. Temperature and rpm are measured remotely. We’ve painted the heavy duty baffle box green. The exhaust system is currently stock iron manifolds with 1 1/2″ down tubes. Shortly, we’ll have mufflers in place.
Above is the front and right side of the engine. The right valve cover has the oil filler neck welded in at the back. The starter motor is the Ultra Low Profile configuration, which fits inside the 601 Nosebowl. The alternator is not yet mounted, but the corner of the front alternator bracket is visible at the edge of the Ring Gear. The Pulley to drive the front alternator is just ahead of the Ring Gear. Pushrod tubes on all of our production engines have always been painted white. While many people believe that oil returning to the crankcase through the pushrod tubes is cooled by airflow through the motor, our testing has shown that just the reverse is true; the pushrod tubes run significantly hotter than the oil in the sump. We paint the tubes white to help them reflect heat.
The photo above shows a very important piece of test equipment that we use on all the motors we build. We use this to evaluate the condition of the oil cooler bypass in the engine. The cooler bypass performs a crucial function in the Corvair. It has a tradition of trouble free operation, even in engines 30 years old. The Corvair’s oil system has a very good track record, and the design of the cooler bypass contributes to this. However, in the interest of truly knowing its condition, we built this tool. The bypass is a pressure sensitive check valve set to relieve at 7 to 8 psi differential. When operating correctly, it allows the motor to warm up the oil quickly. But if it leaks or has a weak spring, the engine will have hot oil temperatures no matter how big a cooler you put on it. This tool, made from a modified oil cooler mount, bolts on to the stock cooler mount and allows me to measure the exact pressure at which the bypass relieves. This is done when the oil pump is being primed with a drill motor, long before the engine is ever run.
Above is the cooler bypass tool in action. Although the photo is a bit blurry, you can see an 8psi differential on the two gauges. The engine is sitting on the dyno being dressed out. What’s driving the oil pressure is a half inch electric drill using a dummy distributor housing and distributor shaft without a drive gear on it. You’ll see this priming tool in many photos. It’s painted orange like many of our shop tools. I primed this motor for 20 minutes with the drill turning the pump at a speed that would be comparable to a high idle on the engine. During this time, I turned the propeller over slowly by hand a bunch of times, allowing the oil to flow through all the passages in the engine. This technique is very effective. This motor has Sealed Power hydraulic lifters, and these, combined with priming, did not let out a single tick when we started the engine an hour later. These hydraulic lifters will maintain their adjustment for the life of the motor. It’s attention to detail like this that pays rewards no matter what type of engine you’re building.
Stay tuned for the test data and horsepower calculations. We’re going to run this motor for a few more hours before we give it full power runs on the dyno.
September 9, 2004
Here is the first run on our newly built engine dynamometer. There are many types of engine dynamometers. One of the most simple and easily made measures the engine’s torque reaction. Our own stand has a motor mount which is free to pivot along the crankshaft axis. This is restrained from rotating by a hydraulic cylinder. It is a simple calculation verified by a simple test, and you can ascertain the amount of torque the engine is producing at any given instant by reading the hydraulic pressure. It is accurate, and if you have the capability of measuring the rpm of the engine very accurately at the same moment, a simple calculation will give you the exact horsepower that the engine is producing. Shown above is the very first run we did on the dynamometer. Its details are still being finished, but it works very well.
Above is the view of the dynamometer with the engine removed. Its operation is very simple. Everything seen in blue rotates on the crankshaft’s axis. If you look closely, you can see that the bearing is the front spindle, hub and wheel removed from a late model Corvair. The bed type mount is slung low so that the crankshaft centerline lines up exactly with the spindle. The reinforcements below the engine contact a bearing at the bottom of the stand for additional support. This is a Corvair blower bearing rolling sideways on a steel plate. It effectively has no drag. Below the spindle is the mounting point for the hydraulic cylinder. The green oxygen bottle has been converted to a gravity feed fuel tank on the test stand. It hold 2.5 gallons of fuel, and has a very accurate sight gauge on it which allows precision measurement of fuel burn.
Another view of the first run is above. Just ahead and above the battery is the hydraulic cylinder. A stainless braided line running out of the picture goes to the remote gauge. Our rpm measurement is by digital optical tachometer. This is one of the few types of tachs accurate enough to give good test information. Many people will recognize the chassis of the dynamometer as our previous engine run stand. The old stand served us well, and broke in many famous Corvair engines, such as Mark Langford’s 3100. Although we never kept count, I’m pretty sure 50+ engines were run on it. The new dynamometer is capable of everything that the old run stand could do, plus its obvious new function of measuring horsepower. In the coming year, we’d like to run as many engines as possible and anyone converting a Corvair for flight reading this is certainly welcome to bring their engine for a run. We took the time to manufacture a very special intake manifold for the dynamometer which is compatible with any style of cylinder head used in Corvair flight motors. Details like this will make installing and running engines a quick and simple process.
Gus monitors the engine run, above. Note the newly constructed heavy duty baffle box to provide cooling air to test engines. I’ve said it many times, but it’s worth repeating that you should not run your motor even briefly without a cooling system in place. The carburetor in this run is an MA3-SPA from an O-200. This will be the dedicated carburetor on the dynamometer, although we will be able to evaluate others. The propeller is a 72″ 2-blade Warp Drive. In the background in this photo is the Corvair Trimotor fuselage.
Above is another view of the running engine. The baffle box is made of 50/1000″ aluminum, although 25/1000″ would be plenty for a box you’re only going to run for a few hours. We plan on getting a lot of work out of this, so we built it heavy duty. You’ll notice that the 12-plate oil cooler is outside the baffle box. Engines run on the test stand traditionally have very cool oil temperatures. I kept the cooler outside the baffle to give the oil a chance to warm up. When set up for flight inside a cowl, the engine will have normal oil temperature and it will be appropriate to have full air flow over the cooler.
Here is the same engine pictured running on this page. We built this engine specifically for the Corvair Personal Cruiser, a single seat aircraft designed for Corvair power. This engine will be installed on the prototype, now under construction. In the photo above, the engine is sitting on the mount for the Cruiser. Shown in silver is the intake manifold for the aircraft. We built this from mild steel tubing. The horizontal inlet is built specifically to mate with an Ellison EFS-3A. Also of interest, note the layout of the sparkplug wires. When oriented like this, the cap can be removed for inspection without removing the wires. Additionally, the distributor can be rotated to set the timing without the wires becoming slack or taut. Doing dozens of engine installations over the years has allowed me to perfect small details that allow the operation and maintenance on the motor to be done far more easily.
Very shortly, we’ll provide the next batch of photos sharing the test data and the calculations. Additionally, we’ll show you the calibration procedure, which allows everyone to understand how accurate this simple machine can be.
Here is an older story of testing from 2004. It is a good example of how our testing has been an integral part of the work we have done. Although the machinery is simple, the comparitive testing is sound and the meathod is valid. The information gathered in these tests has served builders for a nearly a decade. In the story I mention that the three of us totaled 55 years of work as A&P’s. Today that number is now 84 years of working experience.
O-200 Torque and Horsepower Testing
Here’s the O-200 on our dynamomemter, and your test crew from left to right, above: Gus Warren, Detroit Institute of Aeronautics, A&P 1990; Steve Upson, Northrop University, A&P 1976; yours truly, William Wynne, Embry-Riddle Aeronautical University, A&P 1991. While the way we dress may be slow to catch on in high fashion circles, we certainly know our stuff about all types of aircraft powerplants. This is 55 years of A&P experience working on engines in the field nearly every day. This experience, along with a good technical background, puts us in a good position to do real world testing.
On the left above is the Continental O-200 as removed from a 1959 Cessna 150. This engine is considered the standard against which all other 100hp class engines are measured. It is a direct drive 4-stroke, 4-cylinder engine of 200cid. It carries a horsepower rating of 100 at 2,750rpm. I have read that Continental produced about 50,000 O-200s. On the right is a 170cid Corvair engine. For size comparison, the O-200 is 32″ wide without the baffling. The Corvair is 28″ wide.
To adapt the O-200 to our dynamometer required making a new mount. Everything seen in gray, above, is part of that mount. The red cap at the center is a dust cap covering the bearing on which the mount pivots. This red cap is in exact alignment with the O-200’s crankshaft. This way, the rotation against the torque is in line with the crankshaft. The mount was made from a Corvair wheel, a pirated VariEze motor mount, some spare tubing, and a Corvair blower bearing. This bearing is at the bottom, and rides on a steel U channel. This provides additional support to the mount, and restrains it from turning full circle. The vertical element is a 1.5″ diameter steel tube. There is a pin on the back of this tube that engages the hydraulic cylinder. By comparing this mount to the blue Corvair bed mount seen in previous dyno tests, It is apparent how we can change the configuration in a few minutes. This is the charm of using the 5×4.75 bolt pattern wheel as the basic element.
The first photo on this page shows the engine with its stock McCauley prop. Above we see the engine fitted with our primary test prop, a 2-blade 60″ Warp Drive. Since we normally use this prop on Corvairs, the blades here are turned around to work as standard rotation pusher blades. This will effectively load the engine for torque testing. The prop is ground adjustable, so we can fully load the engine at any rpm we desire. The carburetor is an overhauled MA3-SPA. The engine has new Slick mags. It is less than 500 SMOH. We first tested the timing at 24 degrees as per the AD, and then tested it at the pre-AD setting of 28. Differential compression showed all over 70. The engine turned its full rated static rpm with the certified McCauley prop, indicating it was a very healthy engine.
Above is an overview of the test rig. We used two different methods to measure the torque output. First, we used a hydraulic cylinder. This cylinder is located just above and in front of the battery. Second, we measured it with a digital scale. The scale is located just out of view, but it is driven by the 8-foot metal beam clamped onto the mount. We had 4 feet of it extending on each side, so that its weight would not affect the scale reading.
Above is a closeup of the hydraulic cylinder. The braided line runs to a remote gauge. You’ll notice it’s on the opposite side of the stand now. The O-200 has a different rotation than the Corvair, requiring the hydraulic unit to be moved to the other side of the stand. The gauge reading was calibrated by hanging weights on a 4-foot lever arm in 5 pound increments. The needle valve on the output of the cylinder is required because pulses on the line when cranking the starter motor are so fierce, they will damage the gauge. This is true with both the Corvair and the O-200.
In the photo above, you can see the electronic scale sitting off to the side. Pressure is put on the scale by the vertical stick clamped to the steel arm. We’re going to refine this and make it a lot cleaner looking shortly, but for these tests, it worked flawlessly and provided repeatable accuracy. If you’re wondering how all this stayed together in the prop blast, you’re forgetting that the prop is functioning as a pusher. I was only concerned that some of the equipment would be inhaled. Although we got both methods of torque reading to agree, I feel in the future we’ll probably use the electronic scale more often because it’s subject to fewer variables. The dynamometer is also rigged for simultaneous thrust measurement, so we’re going to put the hydraulic unit to that function for simultaneous readings.
After a full day of testing, which included several dozen test runs, we came up with some surprising data. The engine performed substantially below its 100hp rating. I initially suspected that the engine was not performing at peak power, or that the test equipment was flawed. During the testing, we conducted all of the standard mechanics’ tests to evaluate the condition of an engine, including differential compression, timing, and fuel flow. All of these showed the engine to be in good condition. The most telling test was that the engine turned its full static rpm with the certified propeller. It would not do this if it were down on power. Keep in mind that we use a digital optical tach to ensure that there is no error in rpm measurement.
We retested and calibrated the hydraulic cylinder system. It showed itself to be accurate. To doublecheck it, we came up with the digital scale system to corroborate the data. They both told the same story. As an A&P mechanic and a big fan of certified engines, I was very reluctant to conclude that the O-200’s 100hp rating is probably a “gross” rating, as opposed to a “net” rating. If you’re a fan of car engines, you probably know that in the 1960s, many car engines had gross hp ratings. These optimistic numbers had things like the fricitional drag of the engine and the accessories factored out. In the 1970s, net hp ratings became more popular. This is the power output you’d actually see at the prop flange. All of the numbers that we test are net. This is the only type of external measurement we can do. It is also the real world power output of the engine that you are going to use to go flying.
The torque peak of the O-200 occured at 2450rpm. The engine produced 160 foot pounds of torque. If you use the formula Torque x RPM / 5252 = HP, you’ll see the engine was producing 74.6hp. We established the torque peak by running the prop at many different pitch settings until we homed in on the peak of 160.
The hp peak of the engine was very close to its rated peak of 2750rpm. We tested numbers slightly higher than this. However, I was reluctant to run the motor in the 3000s because it’s above the engine’s redline, it’s a borrowed engine, and it’s a certified piece of equipment which will very likely go back into another certified plane. So it behooves us to operate it accordingly. At this rpm, we measured the torque at 144 foot pounds. Using the formula, you’ll see that the engine produced 75.9hp. Again, these are net horsepower numbers.
The temperature outside was 85F, and the RH was 60%. The pressure was almost standard, and we’re only a few feet above sea level. A rudimentary calculation to account for the temperature difference above a standard 59F shows that the corrected hp output of the engine is in the neighborhood of 80-81hp. Again, keep in mind we worked all day in an attempt to raise this output. If you’re reading this and thinking there’s something we’ve missed, I can understand that. It’s difficult to convey the work of three mechanics over a 12-hour period in a few paragraphs and photos, but I can assure you we left no stone unturned in our search. At the end of the day, I largely came to the conclusion that the 100 horsepower rating was a gross rating.
Keep in mind that I’ve been doing installations on planes for 10 years. In this time, we had numerous comparative studies which showed that the Corvair was a very powerful engine, and in many circumstances, could easily match the O-200. One which stands out in my mind was the break-in run of Mark Langford’s Corvair engine at Corvair College #3. He had a pusher prop from an O-200 mounted on his Corvair. The manufacturer of the prop told him on the phone that the Corvair could never turn up the prop to any substantial rpm. When it did, the propmaker was something between impressed and stunned. Even though Mark’s engine is a 3100, it was exceeding what the O-200 could do by a good margin. Over the years, a lot of circumstantial data like this makes more sense in light of finding that an O-200 has a far lower net output than previously suspected.
Does this mean that an O-200 is a bad engine? Does this mean that the VSI in every Cessna 150 isn’t telling the truth? Of course not. The engine is and remains the standard measuring stick of 100hp engines. They have worked for nearly half a century, and will continue to do so. This said, I can assure you from our dynamometer testing that standard displacement Corvair engines will exceed the O-200’s power output handily. This being true takes nothing away from the O-200’s status. It just puts numbers on the success we’ve seen with the Corvair motor over the years.
As a coincidence, a few days after the testing we had a visit from Al Jonic. I worked with Al on the installation of the V-8 in Jim Rahm’s Lancair IVP. Al won the EAA’s highest honor for engineering, the August Raspet Award, for this work. He’s a veteran of thousands of dynamometer runs. Although he’s used much more sophisticated equipment, he was duly impressed with our setup and approach. He offered to send us sophisticated programs to use to correct the conditions for standard day performance. He also offered 40 years of insight on the value of dynamometer runs, correction factors, and gross vs. net ratings.
This dynamometer testing is an ongoing business. I didn’t build it to run it a few times, and prove a few points. I regard it much more as an everyday tool. It takes all the talk out of engine building, and replaces it with hard testing. It is the perfect complement to our ability to rapidly flight test any modification. We’re currently running an entire series of Corvair engine tests. Most of these will be done by Corvair College #8. The Corvair is already exceeding the power output of the O-200. We’re just working to define by how much. When we have this data complete, we’ll put it all on the Web page here.
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