Why so few pushers?

[thermal]

With respect to models of say 20 to 80 inch wingspan, what are the relative pros and cons of using a pusher. With the motor and prop somewhat protected in the pusher design it seems like a good setup. However I don't seem to see many pusher designs out there as compared to tractors...why is this? Is there some inherent problem with the pusher design?

Thanks for any insight into this area that you can provide.

Bob

[Ollie]

To balance the engine or motor problem is hard with pusher behind the wing and easy with a puller in the nose.

[slipstick]

Plus getting all the surfaces neatly arranged on the thrust line is difficult if you need a big hole in the middle for the prop. And if you put the motor right at the back the balance problem is even worse.

Steve

[Arthur P.]

I haven't found balancing of e.g. flying wings or delta's any harder with a purhser vs a tractor configuraton, particularly not if you do use a little fuselage to put the battery into. And thrustline is not anywhere near as big an issue with a pusher. If the motor is in line with the wing, it tends to be very easy: 0 degrees up or down and left or right.

Biggest advantage of pushers is that with crashes you tend to break less props and nose repairs tend to be easy as the nose is usually a non-critical part of the pusher model. Most expensive damage could be the battery which tends to sit somwhere in the nose area (but some foam padding can help here and so far my experience is that you have a lot of room for strong nose construction without too much balancing problems). From my point of view, a delta or flying wing actually is a very good elevon/aerelon trainer.

Downside of pushers is that you have less control at very low speeds as their is no prop flow over any control surfaces, only airspeed induced flow.

Biggest advantage of the tractor is that even at low speeds full throttle with the flow ever the rudder and elevator may already get you some response and thus might at least be able to avoid the worst possible harm (next to drilling the motor into the ground that is).

Biggest disadvantage of the tractor is, of course, the potentiall damage to the motor when you try drilling for oil.... And sometimes the damage does extend further as the battery tends to sit rather close behind the motor....

With a pusher you always need a bit of time to gain speed again before you regain sufficient control to get out of harms way. It takes a bit of getting used to, but actually isn't a bad learning experience. I now also tend to keep my speed up in the safe range with my 'tractors' instead of relying on full throttle to immediately be able to save me.

[thermal]

Thanks for the information guys. So aside from the balancing problem and the fact that there is less prop wash over the control surfaces there are no other detrimental factors that would keep one from using a pusher as opposed to a puller?

I assume you would get the same amount of thrust for a given brushless setup?

Would "cleaner" less disturbed air going over the wing give any additional lift?

Thanks,
Bob

[Arthur P.]

Actually there is one additional downside of a pusher: noise. If the prop sits very close behind the trailing edge of the wing it causes noise. Move the prop back a couple of inches from the trailing edge and the noise goes down.

Actually besides flying wings there are quite a lot of pushers. Have a look in the special forum for pusher jets in the electrics section. And in the foamies section there are also several pusher plans for less jet-like planes.

[thermal]

Thanks Arthur, I'll have a look.

Bob

[Tom Harper]

Thermal,

Lift is a function of projected area so the prop position does not impact it but the clean air of a pusher might give you a little less drag.

Some advantages of a pusher are:

1. You don't break as many props.

2. The forward fuselage is available for payload such as cameras.

3. The prop behind the CG is a more stable configuration than a tractor.

4. There are specific control problems that are avoided with a pusher (and you get some new ones, as pointed out above).

Try it, you'll like it!

[SkyPyro]

With a pusher, you don't get any propwash over the surfaces.

Dan

[vintage1]

I believe the big reaqson pushers were never exploited until the Jet era, was the basic problem of crashing.

In a crash, having the heaviest component behind the pilot just become intolerable.

With jets, the engine always 'pushes' but its placement is a bit more flexible.

But its always a compromise. Pod mounted engines are that way because it makes them easier to service. Aerodynamically they are not the greatest.

[Tom Harper]

Thermal,

Why aren't they more popular. Vintage gives some good reasons and then there is the inertia of convention. You may have to design your own. I prefer them.

Glo Pusher entering final leg:

[sawicki]

Thermal,

3. The prop behind the CG is a more stable configuration than a tractor.

4. There are specific control problems that are avoided with a pusher (and you get some new ones, as pointed out above).



Can you elaborate on these two ?

Thanks,

Felix

[Tom Harper]

Felix,

I'll do it tomorrow. It's too late tonight.

Tom

[Solcat]

Multiplex puts out a bunch of pushers, the Easystar, Micro Jet, Twin Jet, Sonic Liner.

[Tom Harper]

Sawicki,

I am not a mechanical engineer so I will appeal to authority on the stability question. Howard L. Chevalier PE, "Model Airplane Design and Performance", published by Challenge Engineering Inc.. Chevalier states that The distance of the propeller ahead of the center of gravity is destabilizing. In a pusher stability increases with the distance of the prop from the CG.

The prop creates circular flow around the fuselage when it is stalled or when the model is accelerating. This is most common on take off and landing. If there is a rotational vector to the air that reaches the wing, the roots fly at different angles of attack. This can cause a trim problem during take off or a sudden stall and spin on landing.

A pusher with twin rudders does not encounter these problems. No big deal but there is a difference.

[sawicki]

Thanks Tom, that makes sense

Felix

[gouch]

This whole pushers versus tractors things reminds me of a question I once read.

"does a horse pull the cart or push it ?"


With a pusher, you don't get any propwash over the surfaces.

HUH? How does my frog have any control if this is the case? or my wing for that matter.
I can see that maybe you are talking about the wing not having "prop wash" over it, but the wing as a whole should always have airspeed otherwise it is stalled and therefore ceases to be an aircraft.....

Cheers
Paul

[mtnmnstr]

What about.... You get clean air to the prop up front, Disturbed air in the back. more effective up front.

My .02
Gene

[Aio_1]

I assume you would get the same amount of thrust for a given brushless setup?

Would "cleaner" less disturbed air going over the wing give any additional lift?

The thrust and drag are both effected by whether you use a pusher or tractor prop. A tractor prop operates in smooth air and is more efficient but creates a high velocity turbulent wake over the fuselage which can increase drag. The pusher prop is less efficient but can reduce drag by thinning the boundary layer on the fuselage due to the suction created at the prop face.
Which is most efficient in any given case would depend on the exact design and won't be practical to determine.

Aidan

[Tom Harper]

Aidan,

Perhaps, but I've never seen any term in the propeller thrust equation to represent whether it is pushing or pulling.

[Aio_1]

Aidan,

Perhaps, but I've never seen any term in the propeller thrust equation to represent whether it is pushing or pulling.

It would be impossible to account for accurately in a basic equation. You would need to run wind tunnel tests and/or perform complex CFD modelling to establish an accurate indication of these effects. General propellor thrust equations are based on a prop in the freestream if I'm not mistaken. If you look at basic rotorcraft theory you'll see that a downforce is typically subtracted from the rotor thrust figure to account for the presence of the fuselage in the rotor wake (typically about 5% if I remember correctly). The same principles applies to propellors.

Aidan

[Tom Harper]

Aidan,

Exactly, in the case of the pusher there is no fuselage present in the rotor wake.

[AndyOne]

Pushers tend to have ground clearance problems on take off particularly so they tend not to have undercarriages which menas they have to be hand launched. The biggest problem with engine powered pushers that have to be hand launched is that it has to be thrown with the engine going. I have heard of people losing fingers doing this.

Andy.

[Tom Harper]

Andy,

Yeah, that smarts! Best to use high thrust line and low slung stab. It gives you something safe to hang on to.

[Ensignnolo]

Alternatively, you could use a bungee and rail concept ala MLB Company's Bat:
http://www.spyplanes.com/bat3.html

I don't work for them, just saw the system fly at an AUVSI demo at Patuxent River a few years ago. The launcher looked simple and reliable.

I'm looking for proven plans for a pusher platform like Dave Jone's series of planes. No commercial interest, just want to make my own. Any ideas?

Ensignnolo

[Ensignnolo]

You can find a video of the launcher here:
http://www.spyplanes.com/movies.html

[peterangus]

Eleven pushers in my gallery. Some more on my website.

[HELModels]

Here is a not so great picture of the pusher I'm learning on. I cant help but think the tail is a bit too big, as it only requires stick pressure to control and big movement on the sticks only leads to sloppy flying. It recovers on its own when upset.

[Aio_1]

Here is a not so great picture of the pusher I'm learning on. I cant help but think the tail is a bit too big, as it only requires stick pressure to control and big movement on the sticks only leads to sloppy flying. It recovers on its own when upset.
I doubt that's due to a problem with the size of the tail. The CG may be a little far back for your taste or the control surface deflections may be too big.

[Sparky Paul]

Pushers tend to have ground clearance problems on take off particularly so they tend not to have undercarriages which menas they have to be hand launched. The biggest problem with engine powered pushers that have to be hand launched is that it has to be thrown with the engine going. I have heard of people losing fingers doing this.

Andy.
.
I've handlaunched a lot of large-propped pushers, and haven't had anything remotely close to a skin injury much less loss of anything major.
I'd expect a prop cut on the arm, but fingers??????
You don't hold the spinner.. :)

[peterangus]

There are many possible pusher model configurations. Some posters seem not to recognise this.

I wonder what configuration[s] thermal had in mind in his post #1.

[HELModels]

I doubt that's due to a problem with the size of the tail. The CG may be a little far back for your taste or the control surface deflections may be too big.

Yup, the tail is probably sized o.k. As for CG, I actually think it is a little forward. If I stall and do nothing, it will nose down, pick up speed and recover by approaching another stall - that's when I push the nose over. That reminds me of the dive test and that a plane will do that if the CG is too forward.

I cant decrease the throws physically and dont have a computer radio, so I just have to learn to fly it using my own thumb mixes. I think part of the control sensitivity issue emerges when I fly it very slow.

I took it out tonight before a big rain, and the wind was easily 15 knots, and I needed most of the deflection. I had also moved the CG back a 1/4". It handles the wind like a dream. If I only had those west coast slopes to try it on, any west coast.

To focus on topic, The great thing about high wing mounted pushers is that after all the crashes I had while learning, I have not broken a prop. It's been almost a year in development, and I have the same prop. I broke one prop on a failed tractor design and that was when I decided to develop this one. I highly recommend a high wing pusher for learning to fly.

[LostintheTrees]

Nice drawings!!