On the Pietenpol discussion group, a well meaning guy reposted a story from the 1996 Pietenpol newsletter. The subject was on prop efficiency. It included the comment:
“The Corvair engine is another compromise. They have a loss of efficiency due to the small diameter propeller and accelerate poorly (due to the tall gear effect) but produces good power. ”
The guy who posted this probably didn’t know it, but I know that the comment is without merit, and no one who actually conducted a test, or understood propellers would make such a comment. Yet, here we are, 18 years later, in the information age, following the same myths that have been floating around for decades.
The above comment has no educational value. Take it line by line: All engines, not just a Corvair are a compromise, period. Testing shows that a 100HP engine climbing at 60 mph with a 66″ prop may be close to 95% as efficient as one with a 72″ prop; Ask any tester you like, there is no such thing as an engine that produces good power but accelerates poorly. This only happens when it has a bad prop on it.
OK, the whole point of having a dialog, reading thoughts or communicating about the building of planes is to learn something and use this knowledge to improve the plane you are building. The comment above serves none of these functions. It is only valuable to people who which to reinforce false realities they believe in. You can divide almost every story you read that allegedly shares information on airplane building into to camps: Valid testing that supports learning, and opinion or out of context stories that support myths. Only one of these will make your plane better.
This division cleaves all discussion, any story can be put in one pile or the other. What always gets me is this simple fact: Less than 20% of homebuilts are completed. I have been around Pietenpols a long time, and I would guess that their completion rate is far below 10%. What no industry magazine or salesman is going to tell you is that the completion rate, as a whole for our branch of aviation is actually dropping. Yes more planes get completed, but the sell many more these days. Given this fact, you would think that builders would all recognize that to personally beat the poor average, they have to make some smart choices, and a critical one is learn from valid tests, and don’t waste time listening to the same myths and old wives tales that lead 80% of previous builders to failure. But, for some reason, the myth mill still works every day, and people participate in it, directly sabotaging their own chances of learning and success
It is somewhat frustrating to conduct a mountain of public tests, but they are cited less often than dubious sources from decades-old news letters. For people who wish to see real side by side thrust tests and dyno runs, get a look at this link to our testing page: Testing and Data Collection reference page. For Pietenpol builders who want to see that a 72″ prop doesn’t hold a candle to a 66″ one bolted to a powerful engine, look at: Pietenpol Power: 100 hp Corvair vs 65 hp Lycoming. If you would like to read about how most of the things said about “prop efficiency” are myths, get a look at this:The case of the Murphy Rebel, “eyeball vs. testing” I would hope that the next time the myth machine goes to work, someone will share a link to these pages, or even this story.
Here is a simple example of testing and common sense from the last story link:
“Props with diameters of 74″ are only efficient on engines like the Continental 65 with a low red line of 2300 rpm. Low rpm isn’t efficient in itself. A 65 Continental becomes a 75 continental with respect to power output by just a jet change and an RPM increase to 2600. If turning the prop 300 rpm faster and using one with less diameter actually made less low speed thrust, than no one would have ever converted a 65 to a 75.”
Some pretty basic logic. I only ask people to believe what I can show them with tests and common sense like the point above. To counter this, the myth makers only have old newsletters and stories like “I heard a guy tried that once but it didn’t work.” Occasionally there will be input from a guy who touts a long dusty engineering degree as some credential as credibility for his favorite myth. If that is more valid than a specific test done in public, then I have a Unicorn to sell you.
If you would like a simple example to destroy the myth that low speed aircraft have to have large slow turning props to have performance, let us take a look at some work from a man who only valued one thing in planes: Performance. This man was the greatest air racer who ever lived, and almost all of his work was done on planes that could be well powered by a Corvair. The Mans name was Steve Wittman.
Let’s look at the plane below and the prop on it. It is Wittman’s “Big X”. It is a 4 seater powered by a 150HP Franklin. It was noted for having a very wide speed envelope. Did Wittman use a big slow turning prop said to be efficient? No, he used a high rpm, smaller scimitar prop. This plane climbed at 70 mph, it had to have good low speed thrust, and it did. How long have people known that the only thing slow props have going for them is sound suppression? Well the photo here is from 1947.
Below, the Buttercup. The photo is from the EAA museum where the plane was retired to after flying for 60 years. It is reported to have 3,000 hours on it. The prop is not the correct one, it is just for display. This plane will fly and climb at speeds well below the stall speed of a Pietenpol. it will also do 145 mph on the top end. How does it cover such a wide envelope with a C-85? Simple, it has a smaller diameter prop that it spins faster. For people who claim that high rpm props don’t make thrust, please explain what was making the thrust that drove the plane forward at 145 mph when the engine was turning 3,400 rpm. Again the ideas are not new, the buttercup was built in 1937.
How about a slightly faster example? Below is a youtube link to Steve flying his Tailwind N-37SW, powered by a direct drive inverted Olds 215 V-8, bored and stroked to 262 cid. It has a 62″ diameter prop on it. It was cut down from a Cessna 150 aluminum prop. It climbed very strong and topped out at 3,600 rpm and 195 mph with out wheel pants. How do I know this? Because I flew in the airplane with him for a very vigorous flight in 1993. Notice that people who present myths always have a mysterious “guy who tried it”. I am essentially doing the same thing here, with the exception that I was there, my “guy” actually existed and was one of the greatest builders and pilots of all time, and, conversely, it worked for him. Other than those details, my story is just the same as any other internet myth.
Something I wrote about real aviators at the core of flight:
“If you look at their lives close enough, all of the greats offer something to guide us in pursuit of the timeless truth of flying. Pietenpol teaches that we are more likely to find it in the simplest of planes; Lindbergh knew that you started your search inside yourself; Gann said that we will not see the truth directly, but you can watch it at work in the actions of airmen; and Wittman showed that if you flew fast enough, for long enough, you just might catch it. These men, and many others, spent the better part of their lives looking for this very illusive ghost. Some of them paid a high price, but you get the impression they all thought it was worth it.
While it is possible that someone who rents a 172 or even a person who reads Fate is the Hunter has some access, I honestly think that the homebuilder who dreams, plans, builds and eventually flys his own plane is infinitely more likely to experience the timeless truth of man’s quest for flight. All of the aviators who had some insight to guide you found it while they were in action, in the arena. If you inherently feel that you want to build a plane, you feel just like Pietenpol did. When you’re building it, you will find out how determined you are and what kind of perseverance you have. Lindbergh evaluated these qualities in himself every day. As you finish and prepare to fly, you will find others of enormous qualities and flaws, and you will learn to sort them and their counsel, as Gann always did. And when you fly your plane, and come to trust it because it is your creation, and you cut no corners, you will never want to stop, the way Wittman never did.” -ww-(2008)
4 Replies to “Myths about propeller efficency”
Good read, William.
The old wives tales, myths and ignorance with regards to propellers is as bad as it is with airfoils…and this inclu.des prop manufacturers and repair shops! Anyone who wishes to learn about props should get a copy of Jack Norris’ book: Propellers The First, and Final Explanation. Norris’ writing style is different, but he gets his point across and covers the work by Theodorsen, Goldstein, & Betz. The NACA archives have lots of info too, but it takes a lot of time and effort and one has to filter through the documents. Well worth the time and effort, though.
There is a mindset at work here that I fight all the time at work. I design and specify coating dies that are complicated mechanical devices that weigh about the same as my ’76 Nova. The design should be strongly influenced by what kind of tape we are making and what kind of adhesive we are using. Yet, inevitably, when a division wants to put in a new die for a new product, they want it identical to the last one that worked for someone they know in a factory, not to have me run experiments to determine what kind of coating geometry will work for them. The “authority” of someone who has a die that works great is more influential than the data and experimentation of some head in the clouds researcher in headquarters. It’s an aspect of human nature that I find frustrating but have to learn to accept. I have 3 ways to fight this: 1. Let them build it wrong, and fix it when it’s done. This is the easiest, but costs the most money, and I reserve this for people I just can’t find a way to deal with. 2. Use my influence with the CAD designers to try and work in the features I know will work, or make the fix easy. 3. Pretend not to be a PhD – talk and act as if I was an operator or technician or process engineer, and start my story – “I remember back in plant X we tried this, and it seemed to work then.” Hope you are well. I hope to get my plane flying this summer Ready to put in the fuel system when I get back out there. Wiring, plumbing, and the canopy are all that’s left. Take care, my good friend, Becky
Every airplane that flies is filled with a series of compromises set to maximize the achievement of performance goals set by the designer and builder.
Each element of a design has a whole series of variables that affect performance. Propellers, engines, wings, fuselages, and airplane configurations not only have those variables, but putting them together also adds more variables because they interact.
Considering each part that makes up an airplane in isolation without considering how they interact others is unrealistic, and even foolhardy. What may be theoretically ideal in isolation will often not be ideal when put together with the other parts of an airplane.
That’s why absolute statements about props, engines, airfoils, configurations, wings, and fuselages or any other element of a design are not wise.
There was some other erroneous information in the article, namely: “You also need to think about cooling. At 60 mph there is not much ram air. The Corvair requires a blower to get proper cooling. If you go this route you have two projects instead of one!” Somebody should tell all those people who are flying behind Corvairs that their engines need a blower to get proper cooling.