As I had alot of spare time the other day my mind started wondering into the realm that I'm sure we all escape to in our free time, that of RC airplane. And I started wondering about the pusher type aircraft. Is there a special type of setup that you need to have the engine facing rearwards? And what type of effect would a pusher have on the airplane, if any, if the engine was located between the twin booms of the tail plane? if the air stream? (for lack of a better word) running over elevators make the plane more responsive in that regard?

just wondering


Palms

Good question. You have to put the fuel tank in a logical place and make sure the engine/prop when running is going to make pushing forces. I believe the reason that most airplanes use tractor engines is that the airflow is cleaner and it allows the power transfer into thrust to be better than a pusher. Think of the air going around a fuselage with bumps, etc and the effect of wing lift getting in the mess and the flow into a pusher prop is pretty messed up. It will work but not as good as a tractor.

The thrust over the elevators would make that surface more effective but that alone isn't worth the propulsion loss. You can just make the elevator bigger and regain the same effectiveness. Certainly in the long run the location of just the thrust component from front to rear doesn't effect the airplane adversly. A modern jet has the thrust at the rear and works fine.

This would be a fun thing to investigate with a small foamy type of airplane. Build a tractor/pusher configuration and compare the results flying on each one and then both.

Back in 74--I built some pusher -twin boom pattern models - they were dreadful flops at that task.
The thrust seemed puny on both - (little 40 powered and larger 60 powered designs)
The design was a swept constant chord --with the prop on the CG- and twin booms and a tail group which was of relatively small area.


I tried some tests -as I did not have a wind tunnel .
These were tests to see what the air going to the prop and from the prop was actually doing.
I used an old setup - a simple piece of string -held in various positions to see what the airflow looked like.
The strange thing was that the string -held aft the prop --showed the air stream rapidly became a very small cone !
It was shaped almost like a water whirlpool , going down a drain.
So - are there any bits of knowledge that explain this?

.
In Andy Lennon's and Martin Simons books.

Well--- it was worth a shot --

Tweak, tweak.
From a more formal source, leaving out the differential equations.. It seems your ad-hoc test confirms theory..

On the other hand it is surprising how well full scale pushers perform. As you know Dick Rutan has designed several composite high performance airplanes that all are pushers. There are also many ultra light planes that are pushers. I have never heard that they are less efficient than tractors. Often the opposite is stated.
In the mid forties there was a Swedish fighter that also was a pusher. This fighter had a higher top speed that was calculated. The explanation was that a pusher prop was more efficient than a tractor. Of course this proves nothing. There could have been many other reasons.
All this just as a curiosity. Everything does not work well when scaled down. But it would be interesting to understand why.
/PO

The problem in looking at Rutan's designs is finding an airplane to compare them to that is the same except for only the tractor-pusher change. They are beautifully designed airplanes but since so much changes from something like a Piper Cub (insert an appropriate airplane name) you can't just isolate the engine location effects.

Having a Swedish fighter that went faster than what was calculated just means that the original calculations were wrong, not that the pusher engine was better, etc. Remember those were times when the state of the art was pretty backward compared to today's knowledge base.

If a model like Dick's twin boom airplanes were made with two fuselages, one tractor and the other pusher, and some tests made with respect to speed or fuel flow or endurance or ??? then a reasonable conclusion could be made. I will accept Dick's analysis on this since he has designed and analyzed the configurations and has enough experience to evaluate the results.

Do keep in mind that things like the prop working in messed up airflow behind the fuselage and wing can have impact on whether or not the design is acceptable. You can get destructive vibrations built up as a result. Again the prop working in clean air at the nose is the best place for all around work, maneuvering, high load factor, and the like. Having a surface of any kind, whether prop or wing, in clean air is nicer.

OK there is some formal info - I will go one step further - in the test -I also found that the string could actually be held lower than the tip of the blade (about 1/4") and was undisturbed.
This said to me (remember, I am a country boy ) " goll--y thet prop is not producing at the tips .
So to borrow from the blue sky guys - it looked like the faster air actually sucked the flow inward -from the tips - into the vortex.
back in 84 at the Reno Nats, we got a chance to look over and peek into a prototype pusher ,composite high speed personal transport.
the blade configuration said that something different was going on ,
they had a wild twist to them. The plane was a bit of a flop I understand.
I know that the Rutan planes work very well but I assumed the pusher position was a concession to the canard layout
I would not want a P39 type drive shaft passing between my legs --

I'll have some empirical evidence for you in a month or so, when I finish this baby. ( http://www.nextcraft.com/j47_sportjet01.html )

All parts of the design were intended to both reduce drag, and keep the flow over the fuselage as organized as possible, even at high alpha. (Hence the continuous curvature, and extended dorsals on the twin vertical fins.) Cooling is by via two fairly large NACA inlets at the rear of the fuselage. Of course, it's all theory until we fly the thing, (hopefully before the end of December) so cross your fingers for me.

I built a 1/3rd scale Berkut (basically a Long EZ with retracts, in modeling terms) and it flew very well (and fast) at 15 pounds, using an OS 1.60 2-cycle. Burt certainly knows what he's doing. (!)

Aside from the need for a clean, slippery fuselage ahead of a pusher prop to keep the airflow as smooth as possible into the prop, a pusher layout results in more net torque reaction on the airframe. In a tractor layout, the wing and stab act as flow straighteners of the spiral propwash, and in the process, cancel out a lot of torque reaction. For example, engine torque can be completely cancelled while hovering simply by deflecting large chord, full-span ailerons against the torque; Hovering, hanging on the prop with no roll would normally be impossible with a pusher.

I almost built a pusher pattern model, but had second thoughts at the last moment, when I realized that in a crash, the heavy stuff tends to find its way forward, smashing whatever is in front of it, and a rear mounted engine could make mincemeat of a fuselage in a relatively gentle crash that might result in little damage to a tractor layout. Always bear in mind that if you're going to fly, you are eventually going to crash.

Interesting...

Can't speak to the torque issue, since I wasn't hovering the Berkut, and I won't be hovering the jet.

But, I'd argue about a "gentle crash" doing more damage with a pusher. In testing of the Berkut, when we had some Reynolds-related "issues" with the canard, we plowed the prototype into the rough at the end of the runway on more than one occasion.... at full throttle, moving at about 40 to 50 mph. The most severe "crash" was enough to break the fiberglass at the nose, and break the nose gear former, but the power system, including the tank, which was secured with velcro, and even the prop, were undamaged. With less mass in the nose, the nose is more likely to deflect away from a surface, rather than just plow into the dirt, like a typical tractor system.

I've only got limited experience with pushers, and don't claim to be an expert on the subject, but that's my experience so far.

Lear Fan?

Ah yes, the low hanging fruit. I would be concerned with gearbox maintaince.

All the fast prop airplanes have been tractor. Wing down wash and non laminar air does not give great propeller efficiency. The Seawind configuration looks interesting though.

An interesting plane to study is the Cessna 337B. This plane has two identical engines and props. When flown with one engine only it has better performance with the rear engine than the front one.
See link
[link]http://www.skymaster.org.uk/perform.asp[/link]

Also remember that Rutan decided to use the rear engine as the main engine in his Voyager. The front engine was only used for start and landing (and back-up). So the pusher configuration can´t be that bad if designed correctly.

I think I read that the Cessna Skymaster has better engine-out performance with the rear engine running because the airflow pattern happens to be a little bit better. The fuselage does about as good a job as possible to give the rear prop a clean airflow, and there are no struts, fuselage, etc. to get in the way of the airflow aft.

Even though it's not a 'hot' performer, I've always liked the Skymater.

.
What could be "cleaner" than nothing in front, like the front engine?
The difference in performance front only to rear only is more due to the stuff aft of the front prop, than ahead of the rear prop. The increased drag of air going aft from the front prop isn't there with only the rear motor going.
My push-pull Kadet, at 13 pounds, will fly equally well front engine only or rear engine only. Two 40 FPs, which aren't the most powerful..
The rear motor MUST have a cooling air scoop. Even uncowled it would overheat.

Paul, Good way to describe it!

Something not touched on yet is that a pusher design has an inherent pitch stability with the prop behind the CG. When the aircraft angle of attack is increased, the reaction at the prop attempts to decrease it.

The largest problems to overcome with the pusher are engine cooling and keeping the CG forward with the heavy engine at the rear of the plane.

CJ

I guess if the fuselage is nicely streamlined (not that Paul's airplane isn't :-) the air presentation to the prop isn't that critical. That does leave you with the difference in drag (due to difference in velocity) over the length of the fuselage as the big factor. It doesn't show up in Paul's flying but should in a cruise power comparison where a few mph or fuel flow can be measured.

It certainly indicates that there isn't enough difference in the model world to drive a design.

Mike, I was so looking forward to a short movie of your designs hovering!! -- maybe as a Christmas present for the forum???

CJ - I am not too sure why the pusher design should have inherent pitch stability. Certainly if the horizontal tail is behind the rear prop, and gets the higher dynamic pressure due to that, it would maintain control effectiveness and the tail contribution to stability over a large range of angles of attack from 0 to 90degrees.

But I don't understand where the reaction at the prop comes from that you write about??

Tall Paul,

Interesting to hear that you have solved the heating problem associated with the rear engine. I am building a scale model of the Swedish fighter I mentioned in my earlier post. Still unsolved is how to cool the rear engine. I thought of having an electrical fan and putting the battery in the nose where weight anyway is needed. But I have no idea how strong fan I need.
I have tried to estimate how much heat the engine generates. The engine an YS120 has roughly the power of 2 hp. That is about 1500 watts. If I assume the engine to have an efficiency of 25% it will also produce 4500 watts of heat. That is an enormous amount of heat to cool away. If these figures are correct it is easy to understand that cooling can be a crucial issue.

Maybe ducts would be a better solution even if they are not so simple to implement on this plane.

Paul, it would be interesting to hear how big ducts you have on your plane and if there is any complicated baffling. A picture of the installation would be of great help.

/PO

Where can we find a photo of the airplane??

Here it is:

[link]http://hem.passagen.se/hankri/21_1.htm[/link]

/PO

Here's the rear motor installation.
The upper scoop made all the difference in engine cooling. The small one on the side was supposed to be enough, but it didn't do the job.
The gas tank is put in conventionally, with the fuel and pressure lines wrapped back to the motor.
Both motors use the same throttle servo.
There's a 10x5 prop on the front, and a 10x6 pusher on the rear.
When both motors are running, it flies like an .80 powered Kadet is supposed to!

The liquid cooled version doesn't have any provisions for aircooling the engine.
The only scoop is on the starboard side for the carburetor.
The jet version scoops would be a good option, but not scale unfortunately.

Neat looking airplane, would the NASA (or what ever they are called) inlets work at this Rn? They would (maybe) be easier to make less noticable.