Carb applications, choices people make
December 5, 2012 Leave a comment
Yesterday at our airpark, 8 RV’s took off in succession, on 6 second intervals. At most airports this kind of flying is forbidden/frowned upon/ illegal. At our airpark it is just called going flying. We only have 40 houses here and 64 planes, but we have a lot of activity. The skill level here is very high, 7 out 8 of the RV’s were flown by former USN attack pilots. The 8th plane was flown by a Vietnam vet, USAF F-4 Phantom pilot. Flying an A-4, A-6, A-7 or an F-4 doesn’t automatically qualify you to pilot a light plane. But it is worth noting that often people who flew in serious settings have a different mental picture of what they want out of planes, how important reliability is, and the value of known proven information.
What does this have to do with Carbs for Corvairs? A lot actually. All 8 of the planes that left are very reliable performers, they have more than 500 hours on them on average, and you could start any one of them and fly it to the other side of the continent without mechanical consideration. Yes, they are all Lycoming powered, but more specifically, Every single one of them has a Marvel Schebler Carb on it.
The MA3-SPA we suggest using on a Corvair is the kid brother to the MA4s and MA4-5s on 320/360 Lycomings. (235/290 Lycomings used MA3s). At a glance, most people can’t tell the difference between a 3 and a 4. They are of the exact same design, and for that reason, they have the same excellent reliability record.
I spoke with my friend Jeff Lange on the phone the other day. Jeff is a well-known VW pilot and an air racer. He is a mechanically clever guy, positive, friendly and out going. He is working to improve a friends KR/Corvair engine installation. The plane has flown about 100 hours on an Aerocarb, the very simple, red, floatless carb sold by the Sonex people. The KR builder did a great job on the airframe, but the engine install had some weak points. The fuel flow was restricted by having 9 (nine) 90 degree 5/16″ brass elbows in it. Additionally, the intake manifold that he fabricated had several vacuum leaks in it. Jeff was in the process of correcting these things.
The biggest thing that Jeff was having an issue with was getting the carb to be jetted correctly. Fuel flow at different throttle positions on an Aerocarb is determined by the taper of a 2″ long needle. Going all the way back to Posa carbs, VW guys in search of a really insepensive carb have often tried to make these carbs flow correctly by grinding a custom taper in the needle. This is not easy, and Jeff was now working on his fourth needle trying to get this to work. Let me first say that the Monnetts get these carbs to work on VW’s first by making the slide parallel to the crankshaft. For some reason, people who put them on Corvairs tend to miss this, which gives the engine poor left/right fuel distribution. Jeff is a skilled guy, and he certainly made the engine run better, but how good compared to an MA3? At the end of the conversation Jeff mentioned that he was looking into a carb from a jet ski as an inexpensive option. He may very well have something that could work eventually. But I have to ask myself if Jeff’s entry point into aviation had been seated in a F-4 headed to attack the Paul Doumer bridge instead of messing with VW powered planes would he have the same point of view? Probably not. These polar entry points are extremes. which end of the spectrum you gravitate toward says a lot about what you want out of your homebuilt.
The RV guys just want something proven that works. This is because they understood the value of reliability in machines that fly. In all their time, I can’t think of any kind of carb issue that any one of them has had, none of them has even taken their MA4 apart once, far less ground down any internal part to make it run. They fly very strong aerobatics every week, they fly to the mountains of Montana and Idaho every summer, and the fly without any internal adjustment to their carb even if it is 20F or 100F outside. Builders who have selected MA3’s for their Corvairs have had pretty much the same experience.
Is using a certified carb the only answer? No. People are free to do what they want, its their plane, their time, their goals. The only point that is very true, and is a hard and fast rule that I, in 25 years of being around experimental aircraft, I have never seen broken; You will not get the reliably of a proven certified aircraft component from something that is extremely low-cost and adapted to flight, especially if it is modified with a file and sand paper. You can’t have it both ways. Plenty of people have heard me say this before. Some people have an instant cop-out sarcastic emotional reaction; “I guess that means that we all have to fly certified Lycomings.” I don’t say it for that reaction. People who are really interested in learning don’t have emotional reactions to challenging thoughts. We are speaking of risk management of a finite budget, finite amount of time, and a skill set that improves, but isn’t going to make a guy into a fluid dynamics engineer in time to build a carb and go flying.
For each person there is a path and a reasonable answer, and I try to share my perspective and experience so that builders can make more educated choices for themselves. I want them to understand that there is marketing pressure to buy an EFIS, but an MA3, Stromberg or an Ellison will serve you better. GPS is nice, but get a 5th bearing first. Rubber hoses, plastic barbed fittings and hose clamps are a great deal compared to AN fittings and braided lines, that is until you factor in the cost of skin graphs at $6,000 per square inch. (there is a bulk discount on them when you get 3 square feet, price drops to much more reasonable $1,500/square in.)
If every single homebuilt that was started got finished, They all had perfect reliability no matter what they had for carbs or other components, and if no one ever got hurt in planes, there would be absolutely no point to writing anything here. Home building isn’t a tee-ball game without score keeping. 80% of planes don’t get finished, it makes a big difference on what your installation is, and this isn’t a ‘sport’ like bowling, people can, and do get hurt here. Your personal politics may be against the death penalty, but understand that Physics and Chemistry are the two unswayable referees in the game of flight, and they both are known to bench players without consideration for good intentions or nice guys.
Here is the good news: success, reliabilty and risk are not random here. They are completely controlled by you and the decisions you make. It’s actually one of the things I like best about flying. No legislature is going to change the laws of physics. Play by the rules, and physics and chemistry become the most reliable allies anyone ever had. If your new to home building, your only task is to dispense with the consumer/marketing infomercial trash that fills most of our industry magazines and airshows, and get down to learning the real rules of the game, the ones that never change. It all begins with deciding who you’re going to learn from. Over time you will come to know that any system or product that promises an end-run on the unchanging rules is likely made of Unicorn dung, and that the very definition of reliability is a system that respects physics and chemistry and has harnessed them as allies.-ww
To show that my positions on these issues has never changed, I share a 2005 excerpt form our Flycorvair.com website below. If you thought my previous post about companies that promote plastic barbed fittings for fuel systems was unfair, get a look at the photo below. Don’t think about it being on me, or even what it would feel like on you. Take a minute to think about how you would feel about a child who was your passenger getting this kind of lifetime souvenir, and the free nightmares that go with it. Thats why it is completely ok to criticize the ownership of any company that promotes, sells, and profits from plastic fuel fittings in the cockpit.