Twin Tractor v. Twin Pusher

[wgt40w]

I have been experimenting for the last two years (I'm not in a hurry!) to evolve what I consider to be a really great design of FPV Fixed Wing plane.

To achieve my decided overall C of G design, I am considering using a twin pusher motor setup; twin motors and props on the rear, trailing edges of the fixed wing.

Ignoring any 'single motor v. twin motor' discussion, are there any particular aero design reasons why a twin pusher / single tail design would be unworkable ??

[Brucey]

A twin tractor will be quieter, but a twin pusher should still work fine

[BlueFlyer]

with a tractor setup, the props are encountering clean undisturbed air and will therefore operate very quietly

However, with any pusher prop configuration, the props will encounter air that has already been disrupted by either the fuselage or the wing and will therefore operate a fair bit louder

How this affects efficiency is a subject that can go on and on (my view is that unwanted noise is energy wastage)

My opinion is that whatever the difference, it's negligible enough for you to simply decide on pusher vs tractor as a case of aesthetics/equipment layout rather than worry about the intricacies of fluid dynamics

[wgt40w]

with a tractor setup, the props are encountering clean undisturbed air and will therefore operate very quietly

However, with any pusher prop configuration, the props will encounter air that has already been disrupted by either the fuselage or the wing and will therefore operate a fair bit louder

How this affects efficiency is a subject that can go on and on (my view is that unwanted noise is energy wastage)

My opinion is that whatever the difference, it's negligible enough for you to simply decide on pusher vs tractor as a case of aesthetics/equipment layout rather than worry about the intricacies of fluid dynamics

Well written comments - many thanks.

[electrotor]

With a tractor configuration the air passing over the part of the wing immediately behind the prop is accelerated and will therefore produce more lift. This also extends to the air passing over any control surface, eg an aileron if the configuration is conventional main plane and tail plane. This is not the case with a pusher where the accelerated air blows into free space behind the model. From my own experience with a Mugi then a Mugi Evo (delta winged single engine pusher) a great deal of the noise is produced by the prop tips being close to the trailing edge. The Mugi Evo has a notched trailing edge to help reduce the noise.

[skyscraper]

One point In favour of a twin pusher. I reckon that the aerodynamics over the wing will be cleaner so the wing will work better with the motors behind the wing. You could use inrunners with streamlined fairings and folding props. That might work for a thermal soarer type plane.

regards
Andy

[Billy_boy_2010]

Trappy made a big wing- 70-80" from memory- and used a twin pusher setup on 6s. Said it was so loud it received complaints almost every time he flew it. Guzzled through battery pretty quick too.

Conversely, FliteTest made a big wing with a twin tractor setup and it seemed pretty good. Had landing gear too.

I think there's a factory version of the funjet running twin pusher- it doesn't seem to sell that well though.

[Gundummy]

negligible being the key word i think. This has been asked before on this forum with the same result.. "chicken and egg" ..

the flip side to this with esc and motor inefficiency is that i fail to see how a twin motor/esc rig will ever out perform a single motor/esc set up?

:+

[Ratty]

I can answer the "chicken and egg" question if you really want to know.  ;D

Ipeel has a twin pusher wing. Its pretty loud, but sounds awesome. Might be worth asking him his thoughts on it.

[wgt40w]

Thanks for all the answers / comments. Think I might explain why I asked the question.

Over the last few years I have acquired a number of FPV foamies and realised they all have, in my eyes, design faults. I am therefore getting close to considering designing and building one I have designed.

Basic requirements
1) A steady flier with good lift - no aerobatics - suggest 1900 to 2000 mm wingspan
2) Ability to carry motorised gimbal on the nose - same as Coyote's TwinDream.
3) Belly lander - no undercarriage so can land 'anywhere'.
4) Adjustable battery location to adjust c of g for large / smaller lipos.
5) Design to allow easy access to lipos and electronics.
6) Twin motors - less stress and facilitates redundancy.

To achieve all of this I am considering a design of:-
a) Twin pusher motors - less stress, gives redundancy and locates motor / esc weight to the Rear of c of g, Also, using a single tail configuration, will be easier to incorporate large efficient propellers - no fouling on tail as happens with Bixlers, Skywalkers etc.
b) Single tail will have a braced high elevator - as Skywalker - easier to use on a catapult launcher ramp. Ele and Rudder servos mounted on tail assembly.

With all this weight, in a) & b) being to the Rear of the c of g, I hope it will easily (?) balance against Front of c of g gimbal, fpv camera and lipos on a sliding tray. Receiver / flight controller to be located under wing on c of g.

PS regarding single v twin motors. A few years ago I was involved in a commercial start up where initially we used a single motor Bormatec Maja. To hand launch you really had to throw it in the air and it never seemed to have much motor power to spare. After a few months we switched to using several Bormatec Explorer's - twin motor (same motor as on Maja). To hand launch, one guy just held the plane, the pilot opened the throttle and the plane pulled itself out of the launcher's hands ! It would cruise at less than half throttle giving the impression the motors would last a very long time. Also handled stronger winds much better.

[iPeel]

I've got a twin pusher:



The sound the thing makes is phenomenal. It's not fast but can climb vertical at 55mph. It also flies really well with no torque roll, I did try to add "rudder" through a differential mix on the two ESCs but it has the opposite effect of causing the bird to roll instead.

I've also got a twin tractor for the VTOL project:



That hasn't flown yet though, due to concerns with the VTOL code.

[wgt40w]

iPeel - thanks for your impressive pictures - makes me feel less of a 'freak' wanting twin motors !!

[electrotor]

Only a twin? This one has a mere 18.  :o
Check out the thread : http://www.rcgroups.com/forums/showpost.php?p=31542811&postcount=86

[iPeel]

iPeel - thanks for your impressive pictures - makes me feel less of a 'freak' wanting twin motors !!

Haha, there's plenty would argue me having one makes it the other way round!

FYI the twin pusher still needs 100g nose-weight to balance up even with 7000mah 4S up front. I cambered the motors by a couple of degrees to combat wobble which works really well. I'd also totally recommend a bungee launcher, even though this isn't the wing that ended up biting me I now wouldn't think of hand launching it after my new found respect for propellers.

Bruno also caught some air to air, maybe a bit too closely...  So close in fact you can hear the twin props at work:

https://www.youtube.com/watch?v=F2j1nXPIrA0

[electrotor]

wgt40w have you had a look at these?

http://www.crashtesthobby.com/hercules-xl-7278.html
http://www.crashtesthobby.com/hercules-66.html

Slow access to my memory banks means that I just recently remembered these from a few years back. Might give you some ideas. The building instructions and videos are linked.