Pusher Vs Tractor Efficiency

NoWingsAttached

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Reading some hits on a Mozilla Yahoo search (I hate Google everything and only resort to Chrome or their search engine when not happy with Yahoo results) I came across a lot of "informed" writings pronouncing tractor prop aircraft were more efficient - until I came across this, which is a direct rip from aviation.stackexchange.com.


"The pusher design is more efficient, because the suction forward of the prop reduces flow separation, and the accelerated flow behind it is not streaming around the fuselage (or wing), where it would create additional friction drag. In case of the Do-335 (see picture below), the single-engine top speed was 30 km/h higher with the rear engine running than with the front engine (both were DB-603s with identical power rating).
Do-335

On the other hand, the puller prop will help to maneuver the plane on the ground (this is a big benefit for taildraggers - note how many two-engined, taildragger airplanes have an H-tail (two rudders as endplates of the stabilizer). They were placed in the prop wake and this gave much better directional control at low speed on the ground. Also, the prop wash helps to increase the lift from flaps.
The main disadvantage for a single-engined aircraft, the reduced tail clearance, has already been mentioned. If you cannot really rotate, takeoffs and landings are high-speed affairs. But there is another disadvantage to a rear-mounted prop: It stabilizes the aircraft, much as an additional tail, but without control surfaces. Especially for a fighter aircraft, this is the opposite of what you want. THAT is why almost all high-powered, singe-engined aircraft have their propeller in the front: Maneuverability!
The stabilizing effect increases in proportion to the propeller surface area and the thrust, of course. Since a regular airplane needs to have basic stability with the engine running at idle, any additional stability change due to propeller placement comes on top. At full power and with the long lever arm of a single pusher prop on a central fuselage (think LearAvia Learfan), the aircraft becomes stiff as a brick. A two-boom layout (think Saab J 21, pictured below) is better, but creates additional friction and interference drag, so the advantage of the pusher arrangement is reduced. Note, however, that studies of a tractor engine variant (Saab J 23) showed inferior performance to the pusher design.
Saab-21

If you want hard data on that: There is an old NACA report (NACA TN 2586) on this by John L. Crigler and Jean Gilman, called Propellers in Pitch and Yaw."

Things that make ya go, "Hmmm."

One day all I did was add some plastic side panels around the Hirth 65 HP engine on my Honey Bee, and got another 5 MPH out of the little blue gyro! Chuck Beatty mentioned that cleaning up the air behind the seat was probably more important than in front of it.

The reason for this thread is in wondering about the effectiveness of straightening out the circular prop wash of a pusher gyrocopter.

Now I've only recently just flown a tall tail Air Command with the HS under the VS, an inverted "T". I had done extensive wind tunnel testing of several tail designs, but not the invert "T". So I had nothing to go on, really, but always had my suspicions that they weren't as efficient as the Ernie Boyette RFD Cruciform flying tail. I had lots of experience with RFD tails, and knew they were the best way to go for as long as I have been flying. No question about it.

Turned out that I had to induce as much corrective rudder angle with the Air Command tall tail and HS under it on the keel as I do when I build or tune a shorty tail with HS directly under the VS. This was shocking. The top of the tail does not adequately compensate for the bottom half's prop wash yaw in the least! ItT may as well not even be there at all for all the good it does (doesn't) do.

There were many valuable lessons learned in the wind tunnel testing I did c2010: The HS traps air against the VS. It turned out that the very worst thing you could do was build a "T" with the HS on top of the VS, up near the center of thrust. It was far worse than turning that upside down, but what was really amazing was that with the inverted "T" when I moved the HS forward of the VS, the short VS was amazingly remarkable - almost as good in preventing adverse yaw and perhaps even more efficient overall (especially) in terms of drag as the full-height RFD tail!

Ron Awad's Gyronimo was built this way, with the HS in front of the short VS. When he brought it to Bensen Days I told him about the wind tunnel testing that proved it was perhaps one of the best designs for a pusher empenage of all. He and Jon Carleton, who bought the gyro from Ron, both reported it flew remarkably well with very little adverse yaw.

When you see the old style Bensen and Air Command tails with no HS, you can be certain that those gyros fly with FAR less adverse yaw from the prop wash than any other tail in popular use with the exception of the RFD.

Of course it still was incomparable to the RFD tail. But there was one design that topped the charts: "V"

Which was what I suspected going in to the test, and was the entire reason I built a wind tunnel and did the testing. But was what was truly eye opening was that placement of the bottom of the V was critical. Too low and it was junk. Too high and it was junk. But get the crotch in the sweet spot and you really have something to write home about. Somewhere about 1/3 the way up from the bottom of the prop OD.

But that's not what I was thinking about this week. What got me here today was wondering if straightening out the swirling prop wash by using a RFD tail mounted very close to the prop could actually improve thrust.

Jet (& turbo) engines mount stationary flow straighteners behind each fan blade to increase efficiency. Can an advantage be gained by doing something similar on a pusher? And if this is so, shouldn't the flow straightener be as close to the prop as possible?

I guess I could determine if this works by doing some thrust tests with the RFD tail installed, and then removed. But perhaps someone has already done this and tested for it.

It would be interesting and much appreciated if anyone knows about this already. I would be quite surprised if such a thing has not been tested on a Dominator in the past.
 
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C. Beaty

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Thrust does increase as a result of flow straightening vanes centered in the propeller slip stream for the same reason that a sailboat can sail into the wind..

A simple vector diagram explains how.
 
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Jazzenjohn

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I've designed, built, and flown 4 different ultralight gyros. Amassing parts for a 2 place now.
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I tried testing the thrust on my gyro with and without the tail and I don't remember having seen too much of a difference, but, my tail is much farther away from the prop than a Dominator IIRC, and my tail is half height and with a HS about 1/3 below the center of the prop diameter. I think my tail is about 2 feet away or so? I should do the test again since my tail is removable now.
 

Brent Smith

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Mar 14, 2014
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Could adding a duct around the pusher prop help, perhaps with veins incorporated just aft of the propeller to straiten the flow? (not an aerodynamicist and don't play one on TV)
 
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