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Whats wrong with a pusher?

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Simon Chaddock20/12/2012 17:01:18
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For some time I have been puzzling over the relative aerodynamic/structural efficiency and handling characterisitics of the pusher layout against tractor, specifically with electric power.

The pusher layout (with either the motor or the wing mounted on a pylon) is typically used on foamy beginners planes where prop protection and robustness are obvious benefits but I suspect most wish to move on to something that looks more like 'proper' plane so it quickly gets left behind.

However with a higher power/weight ratio does it have other characterisitcs that would not normally be seen by a beginner but that might actually be benficial?

I believe it does.

First is the fuselage is not bathed in the high speed prop wash which must create drag. On the down side there has to be the extra surface area of a pylon to raise the motor and/or the wing above the fuselage although it is still not in the high speed prop wash.

Second the high thrust line places it above the CofG and also above the centre of drag suggesting the motor axis can be left exactly in the direction of flight.

Third the high thrust line also creates a stable fast steep climb where the nose down moment from the low CofG counters the normal pitch up resulting from increased speed.

I think there are more but that will do for the moment!

bouncebounce crunch20/12/2012 17:09:26
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I still have a Hi fly Glider that flew with a .049 Cox engine Pylon mount in Pusher config and she was fine.(was given to me built) servos, engine and all, no radio but what a gift.

Now you have got me inspired to get in roof (spiders and their webs) tommorrow and get her down, will post pics

MikeS20/12/2012 18:13:01
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I had a Hi -Fly when I was around ten. I also had a cox .049 on the plyon. I remember it well. It never got of the ground. I would love to build another one.

Mike

Simon Chaddock20/12/2012 21:57:58
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I was thinking beyond a pylon glider, although that too works well, but more of a true 'power' plane

My own view is that a pod and boom fuselage with a pylon mounted wing coupled with a pusher motor at the pylon/wing intersection had potential benefits in aerodynamic and structural terms.

The fuselage pod can be reduced in size to just encompass the largest single component, the battery.

The boom itself can have the minimum possible surface area possible and it also allows the biggest diameter prop for a given wing pylon height.

Research and my own experiments has shown that the flow approaching a prop flows in an inward radial pattern. Even when stationary the airflow is little disturbed much beyond one prop diameter ahead where as the high speed airflow extends many diameters past it.

This indicates that if the pusher is mounted exactly at the wing and pylon intersection it will have little effect on the prop inflow however any turbulence generated from the wing & pylon itself will effect its efficiency so a fine trailing edge is beneficial. The prop itself creates an airflow disturbance ahead of it so if it is mounted close to a wing trailing edge it creates a considerable noise (propeller slap) however if the gap is increased to 1/2 a prop diameter this effect is largely eliminated. Most out runners tend to place the prop some distance from their mounting so this becomes an advantage in a wing mounted pusher.

Hollow spinner

The pylon also provides additional fuselage side area around the centre of pressure. This can improve the effectiveness of the fin/rudder (so it can be smaller) as well as providing a measure of pendulum stability reducing the need for aerodynamic dihedral.

A 'hidden' advantage of a pusher is the complete protection offered to the prop means the motor mount itself can be built much lighter and anything that saves weight improves performance! wink 2

Of course I may be completely wrong but I suspect that a pusher installation can be as efficient (possibly even more so) than a nose mounted tractor prop.

The layout does seem to suit my preferred light weight construction resulting in a plane of unexpected performance yet still has a measure of robustness.

 

Edited By Simon Chaddock on 20/12/2012 21:59:41

John Cole21/12/2012 10:09:58
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Wing-mounted pusher configurations were tried just after the war, as they were reckoned to be 5 or 10% more efficient. Convair B-36, for example.

I have found that the high-mounted pusher works well, but that it tends to result in a steep full-power-on climb and a rapid power-off glide. I have fixed this by mixing power with down-elevator.

I was however puzzled by this power-trim interference. I eventually concluded after static tests that it was caused by the (conical) prop-wash hitting the tailplane. Power-off the elevator would have a low negative alpha, but power-on the air from the prop would be flowing "down" onto it putting it at much-increased negative alpha, causing the steep climb.

Turbycat21/12/2012 10:25:13
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There are plenty of microlight aircraft that use a pusher configuration. One that springs easily to mind is the Shadow. There are plenty of others and all the flexwing trikes are pushers.

Andy

Radge21/12/2012 12:07:43
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Very good thread Simon, I suspect that's why we are modellers, we have an intelligent curiosity which we may relatively inexpensively 'tinker' with. Having mostly completed my electric 'tractor' glider I have reconsidered my power options and am just about to complete a top pylon pusher arrangement. Now then, is this to be my first 'twin' build? Purely as an evaluation of research, the pusher motor is to be mounted with a slight up and left thrust, will this be a correct assumption?

Erfolg21/12/2012 13:06:32
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The advantages of pushers are well recognised in full scale aircraft. As has been mentioned with respect to B36, then there was the Lear Fan and a number of others such as the XB 35. All seeking the benefit of lower drag.

With high mounted engine a number of these advantages are lost. With respect to the microlight, the motor is mounted very near the CG, so the disadvantage of the increased drag, is compensated for by control of the CG and supporting structures. Additionally, speed is not the a major feature of the repertoire of these types. Although not true of the Republic sea bee, but this is about water operations and the advantage of a high mounted propeller.

With respect to many multi engine aircraft, it is the prop wash over control surfaces that can be useful, particularly during the take of cycle. Then there is the installation and maintenance issues. Finally most engines were designed to take thrusts in the tractor configuration.

Some of the large pusher aircraft were conceived with the engines buried in the wing, to reduce drag. Which on aircraft like the Brabazon design exercises resulted on a thick wing. Eventually maintenance on many was seen as more important to the airlines than the improved efficiencies projected.

Simon Chaddock21/12/2012 13:14:54
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John

An interesting observation although not an effect that I have noticed.

This may be due to the fact that generally I have used 'mid fin' mounted tail planes both for ground clearance and servo positioning so perhaps this has put the tail plane closer to the central axis of the prop wash 'cone' so the power on/off effect is less noticeable.

 

Radge

The amount of angle to the thrust line is rather going to depend on its distance from both the CofG and the centre of drag and the planes thrust to weight ratio.

If you are after a near vertical climb performance a motor on a pylon could be problem as the thrust line will a long way from everything so no amount of up/down can compensate for a substantial 'power on' pitch change. By the same token adding left or right so far above the CofG will induce a roll component that is likely to work in the opposite direction to that intended!

If only 'power assisted' glider performance is required then the pitch change forces will almost certainly be small enough not to be of a concern and the motor thrust line can be left alone.

In my opinion a 'power pylon' is not an ideal aerodynamic solution as apart from the problems of a high thrust line the pylon itself creates additional drag. Of course in some cases the simplicity and robustness of its installation can out weigh any aerodynamic deficiencies.

To me only when both the wing and motor are on the pylon do the advantages & disadvantages really start to balance out.

A 3D aerobatic pusher?

Edited By Simon Chaddock on 21/12/2012 13:15:27

John Cole21/12/2012 13:51:32
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If the motor is high-mounted, then to keep it from being too high designers will compromise by fitting a small-diameter prop - losing "power-efficiency".

I've just (nearly) built a pusher with twin booms and an inverted V-tail. The booms droop relative to the fuselage so the V-tail is in the centre of the prop-wash.

I just have to build a trike u/c. I have fitted a 11" prop on a 1-kg 50" E193 wing-section model. I am hoping for 30 minute gentle flights on a 3S 2200 maH LiPo.

Edited By John Cole on 21/12/2012 13:52:37

Radge21/12/2012 14:42:37
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322 forum posts
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Ooh that is interesting Simon! Are you saying that for pitch control a T tail is preferrable?

Simon Chaddock21/12/2012 16:34:59
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I would agree that a motor pylon probably does limit the maximum prop diameter that can be used successfully but by mounting both the wing & motor on the pylon the prop can be as big as you make the pylon tall.

As the wing creates a large part of the total drag this configuration also puts the thrust line much closer to the centre of drag so reducing power on pitch changes.

On my most successful aerobatic pusher the motor is mounted just below the wing which has probably put its thrust line very close to the centre of drag although it still has a slight pitch up under power, which probably a good thing with the speed of my reactions!.

Where I have a relatively tall pylon and a heavy battery you do get an pitch down tendency on acceleration that delays the effect of the aerodynamic pitch up but it is really only noticeable at slow speed and when applying maximum thrust although to be honest it only lasts a fraction of a second before the aerodynamic forces take charge.

Radge

I am divided on the benefit of raising the tail plane into the middle of the prop wash.

By placing the tail in the fastest part of the prop slip stream it creates extra drag and removing the fuselage/tail from the direct prop wash is one of the benefits of a pusher.

To get the tail full in the prop wash with a motor pylon design will require a full height 'T' tail which can have implications for the planes handling power off.

A V tail like this is probably more in keeping with the pusher philosophy of minimising drag from the direct prop wash.

60inch complete

Although I am not sure how much difference it really makes.

Edited By Simon Chaddock on 21/12/2012 16:36:57

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