Critical Understanding #5, Knowing “+ROC/5” Rate of Climb on Five cylinders

Builders:

While some builders have taken proper steps to document both their expected and actual performance numbers for Take off roll and for Rate of climb in normal operations, the really critical number to understand about your own plane is the “+ROC/5”, the Rate of Climb on Five cylinders.

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This is critically important, because you should NEVER operate your Corvair powered plane under a set of circumstances where it wouldn’t have a positive rate of climb on five cylinders. Dan Weseman and I have done elaborate and accurate dyno, ground and flight testing to confirm the Corvairs we promote make 78% of their rated power after one cylinder becomes inoperative. This includes the RPM loss of losing the cylinder. This is achieved because the Corvair has an incredibly flat power band, and few other aircraft with six cylinders could match this, and none with four cylinders could without magic.

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78% output  will produce a positive rate of climb at standard conditions, at gross weight on the applications we promote, but it requires the PIC (pilot in command) to have accounted for other factors such as propeller selection. Additionally he must account for the local atmospheric conditions. It needs to be specifically known for the individual aircraft. To get in a plane and fly it, particularly with a trusting passenger, without knowing if the plane could still climb after fowling a plug , is not acting in a responsible manner required by the title PIC.

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How you test this: You note the static rpm of the motor (Critical Understanding #2, Absolute Minimum Static RPM.) Open the cowl, remove one plug wire and conduct the same test. Note the exact difference, which will be about 200 rpm. Later with the plug lead replaced, go flying on 6 cylinders over your airport. Reduce the power to the simulate the rpm loss of one cylinder, and then conduct rate of climb evaluations. Developing a number of data points for different weather conditions will provide a picture of the factors. If you are using a ground adjustable prop, try one degree less pitch in the blades, on a Corvair it can easily drive up the gross weight of “+ROC/5” by 200 pounds.

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Keep in mind that these tests need to be done at Vx and Vy speeds. Pilots who inexplicably always climb planes well above their Vx speed will find in the sudden loss of a cylinder that their plane will not climb at their artificially high chosen climb speed. A Zenith 601XL has a Vx speed near 60 mph. the rate of climb in the plane will be half as much above 90mph and may not be positive on 5 cylinders at that speed. Pilots that don’t ever fly their plane at the right airspeeds, and may never have written them down in the logs at the end of phase one, are a danger to themselves and their passengers.   This is the kind of thing I am speaking of in the general decay of flying standards in the country. Such people are not being weeded out in an era of very lax biennial flight reviews. You can’t fix that, just be determined to be better than that yourself.

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Notice how the ‘expert’ who says “that plane climbs like a dog”  Never says, ” last week I conducted a test at 1300 pounds and a density altitude of 6,300′ and found the ROC at the Vx to be..” This is how you identify bullshit artists. While it isn’t legal for you to harm them, for some reason it is OK for them to spread disinformation that hurts other people. Steer clear of such people, you can’t learn anything from them.

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Factors that affect “+ROC/5”:

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The gross weight of the plane:

The PIC must know the max gross weight at which his plane will still achieve “+ROC/5”. Surprisingly to many builders, this number will be far above the gross weight of the plane, even on a 100F day. We tested this many times on our Zenith 601XL, and the number was near 1500 pounds at 100F. Do not take that as a guarantee, the PIC must develop his own chart for the specific plane and conditions he will be flying in.

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The atmospheric conditions:

The PIC must know the effect of the local Density Altitude on his plane. People often ask about performance in mountainous areas, and make comments like “Sure corvairs work in Florida, but what about Denver?”  Go here: http://wahiduddin.net/calc/calc_da.htm  and run some numbers. We live on a 2,500′ airport, surrounded by 60′ trees, where the surface in the summer is 6″ of lush grass. Even though the field elevation is 75′ we frequently are operating with a density altitude of more than 4,000′ in the summer. Being near sea level doesn’t allow getting lax about DA awareness. Higher field elevations almost always have much longer runways without obstructions.

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The propeller selection and adjustment:

This is part of the reason why I am a stickler for having a prop that turns enough static rpm: (Critical Understanding #2, Absolute Minimum Static RPM.)

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First, a comparative example: I have seen a great number of people fly a passenger in a Pietenpol powered by a 65hp Continental on a hot day. I doubt many of these pilots have tested it their plane would climb if one magneto failed at that point. I have serious doubts that any of these planes would climb with a stuck valve on one cylinder, a serious possibility on an A-65. (Read note “A” below) It isn’t my mission to police such pilots who’s responsibility for passengers is reduced to wishful thinking. My mission is to make our own builders, people who have willfully chosen a more considered path, better educated.

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Comparatively, a Corvair powered Pietenpol has great reserves of power, and has the potential to easily have a positive climb rate on 5 cylinders, even on a 100 degree day at a gross weight of 1320 pounds. My own Pietenpol could do this. It is a significant safety advantage.

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No offense to any builder who wants to make his own Piet prop, but you have to look at what you are getting over a Warp Drive prop besides looks. At Corvair College #39 it is my plan to carefully document Weight/ROC/Weather data on Bob Lester’s 2,700 cc Warp drive 66″ two blade Pietenpol. I want to use this as a base line and look at his “+ROC/5”, which I believe will be a lot higher than any wood propped Corvair/Piet.  Kevin Purtee had both a wood prop and a WD, and used the WD when he knew that he would be climbing with passengers. The point is to make sure that pilots, particularly those with passengers, are not flying with a negative “+ROC/5” value because of an underperforming prop. The day to find this out is during solo flight testing, not when you fowl a plug over unfriendly ground with a passenger aboard.

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Note “A” ; There will be Continental fans who come back and claim that the planes will easily fly on one mag, but they are referring to Cub’s and T-Crafts with 36′ wingspans and substantially more wing area. They will make the same claim on stuck valves, but I have hundreds of hours in a C-85 hp T-craft, which includes sticking a valve twice, and I have serious doubts that 20 less horse power and seven less feet of wingspan would climb. I have a 71 x 38 B-90 metal prop on it now, which statics near 2,400 rpm. This is dramatically more potent than any A-65, and I believe it would take such an installation to have a positive rate of climb on 3 cylinders. If anyone in your EAA chapter wants to debate this, note that they will not offer to weigh the plane and conduct the test in front of you, it will all be conducted with “I had a buddy who had one once and it…” for evidence.

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Note Book Section:

Make line 5.1 in your Hand Book a entry that reads the full static RPM. It should also note the prop and pitch, and the atmospheric conditions at the time. It must also include the fuel and the timing settings.  

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Make line 5.2 in your Hand Book an entry under the same conditions as 5.1, but with one plug wire removed.  Through subtraction note the rpm loss and write this down.

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Imbed in your mind that any time the plane loses this amount of rpm, it has lost a cylinder. I have had second owners of planes that turned 2700 static, later in the flight day think nothing of doing a take off and flying to a new airport with a static of 2500. This was justified by a 5,000 hr pilot as attributable to the OAT being 25F higher. Don’t be that kind of an idiot, and don’t trust people just because they have a lot more flight hours than you. Read. think, understand and be in charge of your own life.

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When your plane is still in phase one testing, pick a typical solo flying weight, and climb your plane to 2,000 AGL. Set the plane up in a Vx climb and time it from 2500 AGL to 3500 AGL with a stopwatch. Back on the ground, making note of the DA,  Make line 5.3 in your Hand Book this value. Repeat the test with 100 pounds more fuel in the plane, and make a similar note. Work  your way to gross weight and graph the results in your Hand Book. If you will fly at higher DA’s Perform these evaluations at those altitudes. Don’t be the kind of dolt that flies around for 40 hours in phase one boring holes in the sky. Learn something.

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Don’t use live humans for ballast, EVER. People who conduct gross weight vs performance tests by adding live humans to their plane are placing a very low value on peoples lives. three 60 pound bags of play sand cost about $10. I don’t love people, but I operate on the general assumption they are worth more than $10 a piece to someone. If anyone flies any of their test period with a passenger, they are making a serious judgement error, even if the FAA says it’s ok now.  If anyone flies a new high gross weight in a plane using a passenger for ballast, they are making a serious judgement error. If anyone tells you this is wrong, nod politely, but focus on how you would be responsible, not him, in the event of an accident.

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Repeat the tests of 5.3, but when you set up the climb,  do it at a power setting that is at the lower rpm that reflects the measured  differential between 5.1 and 5.2. Make 5.4 in your Hand Book a series of notes on your “+ROC/5” climb rates. Graph it out, and you will be able to predict your service ceiling when +ROC/5 = 100. Make notes on this at different DA’s. Study the trends, they will give a very good idea of when you are operating within the intelligent margin of safety provided by having a positive number for “+ROC/5”.

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If the numbers are  not satisfactory to you, going for a lower prop pitch is the immediate solution. If you have a ground adjustable prop, this takes very little effort, but you should go back and document with a climb test series to have hard numbers for what you gained. 

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Lest anyone tell you that the service celling of a typical Corvair powered plane is low, Consider that on a standard atmospheric day Lynn Dingfelder’s 601XL with an Elison EFS-3A fed 2700cc Corvair and a 66″Warp Drive prop still had 200 foot per minute ROC at 17,000′, while flying at 1150 pounds. Obviously it’s +ROC/5 = 100 is a lot lower, but it isn’t sea level as some people would have you believe. Andy Elliott’s 601XL took off loaded with a DA near 11,000′. The point of this exercise is that you will know exactly what your own plane can and can’t do with a good margin of safety.

-ww.

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