Push-pull design questions

[D-Rock]

I am working on a twin engine aircraft that is a push-pull. I plan on using 2 GWS 4-blade propellers, more for looks than efficiency. For the pusher motor I will be reversing the motor rotation and mounting the prop on “backwards”. The tractor propeller sits about 6” behind the pusher. Will this pose any problems such as prop wash? I realize torque may be a factor but are there other issues with this set up I should be concerned with?

D-Rock

[skyflyer]

The rear prop will not be able to take as big of a "bite" out of the air, both because its oncoming air is faster and has some vorticity. Shouldn't be a huge problem, though, as you're not trying to maximize efficiency or performance.

Are there no 4-blade pusher props of the right size? That would be ideal, second choice would be the same prop with higher pitch, which, if it turns out that you need it, you can do with a pitch gauge and a heat gun.

I can't wait to see some pics.

[D-Rock]

Thanks Skyflyer. So far it’s in its preliminary stage, testing to see if this will work. Was thinking of powering it by two BP 9Y BL motors and building it mostly with Depron. I have attached a copy of my basic outline and where I got my inspiration from.

[skyflyer]

That'll definitely turn some heads. Will it be a floater, too? The wingspan is relatively short compared to the prop diameter. You might have to mix in some aileron with throttle because of the torque.

[JetPlaneFlyer]

With props so close together I'd definitely look to making them contra-rotate. If they both spin in the same direction then the rear prop would largely be along for the ride.

A bigger problem with this design would likely be directional (yaw) stability. I'd estimate the CG will be close to the LE of the wing. With the deep twin booms, the forward fuselage nacelle and the two big props all in front of the CG then the side profile 'moment area' ahead of the CG will be greater than that behind the CG... This equals an aircraft that will try to fly backward :rolleyes:

Steve

[Rickenbacker]

Wow, that plane would most definitely win the "least efficient way to mount two engines" category. but it looks fun, which is the main thing :).

[D-Rock]

Yeah, I understand this is probably not the most efficient form of a twin. The idea was from a "fantasy scale" website and I am looking into the feasibility of some of the designs. Not sure if it will get beyond the design stage but we will see.

D-Rock

[BMatthews]

One option would be to just make it a single motor model. But otherwise you could easily make your own 4 bladers by using 4 two bladers with the one side cut off and mounted between a combiner plate sort of deal that is hidden inside or part of the relatively huge spinners. That way your pitch options are fairly open and you could go for a lower pitch on the forward prop and higher pitch on the rear prop.

I'll also second the concern about yaw stability. Those belly fins are no where near big enough or rearward enough to compensate for all the forward areas of the booms, forward pod and floats. Without making them absolutely huge or extending the booms to the rear of the wing and mounting the fins actually at or slightly behind the rear wing's trailing edge this just isn't going to work.

[Brandano]

I can see plenty of cooling problems for those motors, so it might be sensible to open some air scoops for them. Also, couldn't the two facing spinners be made just cylindrical, to streamline the design a bit?

[D-Rock]

Thanks for all the help guys. I was thinking if I do go ahead with this as a model I would just make the front-pusher a dummy and just use a mor powerful motor for the tractor set-up. Will do some testing with larger verticals to help with yaw issues. It will be interesting to see if it works. On the three view the estimated CG point is the black dot behind the rear propeller.

D-Rock

[BMatthews]

If you put the CG there it'll put a fair load on the front wing. You may want to start there and plan on moving it back towards the leading edge of the rear wing.

I'd strongly recomend that you extend the booms a little and put most of the fin area behind the rear wing's trailing edge.

A small all balsa test glider will soon tell you what's what. Make the profiles of the pods and booms about 20% less deep to allow for the rounded fuselage effect. Then play with tail areas and that CG location until you achieve stable glides where it can return to level from a slight banked launch and other odd ball throws that indicates that it's decently stable in yaw. I'm sure that you'll soon find that those underwing fins need to be HUMOUNGOUS if they are to work in that location. But rear mounted fins would provide you with more leverage for the fins to work with.

Also the rear canopy should be set up to look more like a gun turret with tail end disuaders installed. Right now it looks more like a second pilot's home.

This look like it came right out of Crimson Skies... :D

[D-Rock]

Bruce, I was thinking the same thing regarding the profile glider. The CG locations was figured using a formula on a website, but it is something I can move on the glider as needed. Speaking of Crimson Skies, I have Devistator about 85% done, retracts and all. Hope to get it finished soon.

D-Rock