Fantasizing about twins

[Tom Harper]

In my 'things I gotta do someday' notebook I have sketched a number of twin configurations. I have always rejected them because the single engine makes better sense. But a twin version of this model has advantages.

Two Norvel .25s with headers will deliver 1500 watts to the prop(s). Thats a lot in a small light package. I can get the same thing with a ST.60 or .90 but then I have to deal with a very big prop. With two 9x4s I get double the power without raising the thrust line. Since the airplane can be 1.2 times larger I have in effect lowered the thrust line.

The engines are almost on the CG and would have 4 deg out thrust.

[Guille]

Hi Tom; I think that you'd better keep the single configuration since you will clearly avoid problems with asymmetrical thrust so near of the CG and with a small tail moment. I shall recommend you to keep it single and use 3 blade props for less daim.
There is company that manufactures an twin powered electric UAV named "evolution" which instead using rudder, manages the thrust in each engine.
Bye!

[Tom Harper]

Good point. I do have some three bladers.

The idea here is that the engines are close together to minimize the difference effects. Is being on the CG the best or worst position?

[Sparky Paul]

With glow motors, asymmetric power will be a problem.
You''l have to be ready to cut to idle when one quits, and then figure out which one did.
Push-pull would be a better configuration for glow.
Gonna try this one today.. (an idle mind....... ) :)

[Guille]

The real problem is your short tail moment (i.e.: distance from the CG to the aerodynamic center of the tail; it's named tail momentum). Note that there are a lot of twin engined planes that also have twin rudder, each one aligned with prop; note also the large rudder area of the single rudder ones. I'd suggest: 1- One engine and 3 blade prop or 2- Twin engine as near as possible of CG and enlarged rear fusselage (or enlarged rudder).
another detail: how did you decided the incidence angle of the engine? Beware; you migth have trouble on pitch (nose up or down natural tendence).
Perhaps if you send me a 3 view sketch of the plane I could make a better analysis...
Q: Are you planning to use a camera in the nose?

[Tom Harper]

Sparkster,

I like it!! Let me know what happens.

Paul Matt drew an Alcor C.6.1 that used two engines like I show above only they were hung out on the front end of the fuselage. It used 3 1/2 deg of outboard cant on both engines. Claimed excellent single engine performance.

[Tom Harper]

Guille,

I am using a 9 ounce camera in the nose. This is an extension of the design shown in the LD-4 design thread. The short moment is my solution to making it transportable and very rugged. This shows the field conditions:

http://www.rcgroups.com/forums/showthread.php?t=400130

The version I am considering as a twin is:

[Guille]

Look at this plane: http://www.designation-systems.net/dusrm/app4/dragoneye.html
you shall not have any problem with it... send me an e-mail so I can send you the coord. for a pretty good airfoil. You can also make single-pusher engine...
Regards

[Sparky Paul]

Sparkster,

I like it!! Let me know what happens.

Paul Matt drew an Alcor C.6.1 that used two engines like I show above only they were hung out on the front end of the fuselage. It used 3 1/2 deg of outboard cant on both engines. Claimed excellent single engine performance.
.
Don Incoll in Oz showed a twin with the motors mounted top and bottom of a deep fuselage..I disrecall if the props were overlapping.. one set back from the other.
The GL push-pull twin doesn't handle any differently than as a twin pusher.
See the Playing with twins thread.
Dick Skoglund's Twin Otter Kadet with 2 OS 40FPs handles well on one motor.

[Tom Harper]

Guille,

Thanks for the link. There are some problems with using the Dragoneye design. They have access to molding equipment that allows a very light structure. The wing has a laminar section which accounts for a lot of their performance. Also they are not using Lipos from the hobby shop. I understand these things cost around $65,000 each.

I doubt that they come close to the published spec. Note that the thing was not useful under operational conditions. Still R&D.

I am working at the other end of the spectrum (did you read the design link?). Thanks for the offer of coordinates but I am not using a curved section. Rather it is a straight line approximation of the camber line. The present models are pusher but I am changing to tractor for mundane, practical reasons. My models can be built in a week for less than $20. They operate in very hazardous environments and when they break you tape them back together and put them in the air. The only purpose here is to get the camera over the target. And to do it as cheap as possible.

[Guille]

OK; now I understant some of your designs reasons... Good luck!!
If you still have interest on any Flying wing configuration; just let me know... I've a nice bunch of data and own designs flying nicely...
Regards