Scale twin-engined biplane

[Bill Harris]

I'm building a 1/14 scale Curtiss Condor twin bipe http://www.rcgroups.com/forums/showthread.php?t=737927 and I'm wondering what effect the motor nacelles would have on the stall characterisitcs of the bottom wing. The model, and as far as I can tell, the full-size aircraft also, have an incidence of 1.5 deg on the upper wing and 0.6deg on the lower wing.

--Bill

[pmackenzie]

Nice subject for a model.
Two wings and two round engines.

On a bipe with the top wing forward it is normally set at a bit more AoA.
The idea is that it will stall before the lower wing and the plane will tend to pitch nose down.
If the lower wing were to stall first then the tail could drop and the stall would not be recoverable.

That is my theory at least :)


Pat MacKenzie

edit - just looked at the 3 views and there is not much stagger :o

[pmackenzie]

FWIW, I checked out the FM article, and the jig Laddie describes would result in the same AoA for both wings.

Pat MacKenzie

[Bill Harris]

The wing incidences are one item I'm trying to resolve. Careful measurements on the plans suggest a lower wing incidence of 0.6 deg and an upper incidence of 1.5 deg or so, assuming that the tailplane is at zero. This is my first bipe and and I'm learning as much biplane technology as I can-- I likely won't start serious work on it til first of next year. I'm not saying that the designer is wrong, I just like to understand _why_.

Another Condor model with a similar fuse and wings, but with a twin biplane tail will be the "sister" ship to this one... the Golden Age was indeed golden.

--Bill

[pmackenzie]

If you read the text of the article the describes a simple cardboard jig to set the incidence. Talks about drawing parallel lines to lay the wings on.

Pat MacKenzie

[JetPlaneFlyer]

On a bipe with the top wing forward it is normally set at a bit more AoA.


Actually the opposite is usually true...
When air is passing the wings of a biplane it is deflected downward. The downwash of the upper wing would have the effect of decreasing the angle of attack of the lower. In order properly to distribute the lift between the two wings it is then necessary to set the lower wing at a greater angle.


Steve

[vintage1]

then we can add the propwash on the lower wings, which will increase their lift under any sort of power..

This is more or less where I give up and just build the thing. Biplanes were a design compromise..at low speeds two wings plus rugging wires allowed thin sections and loads of wing area at low weight. That meant minimal power to weight could be utilised effectively.

Apart from extreme medium speed manoeuvrabiliy they have nothing else to recommend them aerodynamically :D

Mostly they belong to an era when design was largely done by the test pilot. If he survived.

Right up to WWII and beyond you see radical changes to tail areas and incidences happening between prototypes and production, and even at the hangar by riggers.


The fact that wind tunnels still abound shows that design by theory is not yet as comprehensive as anyone cares to claim.

I have to say I tend to copy the full size slavishly with scale models. Possibly a more forward CG, to make them less twitchy, and rigged up ailerons to prevent vicious tip stalls.. heck its a scale plane, not a competition aerobat..;)

[Bill Harris]

So true, Vintage1. Early aeroplanes, up to and including the "Golden Age" aircraft, were largely designed on the "TLAR" engineering principles. Things got better after the first successful aircraft, that Wright Bros Canard Biplane.

I'll likely fly it as Laddie designed it, but I am going to use this plane as a learning experience on Bipes. I'll incorporate a degree or two of washout into the wings, as is my usual practice, and fiddle with the (root) incidence so that the tip incidences won't go negative.

But ain't it fun obsessing about these little details...

Attached is a cut-and-paste of the twin biplane tail I mentioned earlier. That will have a presense in flight.

--Bill

[Dereck]

Hi Bill
Neat project, but the snap roll rate might be a bit slow :)

Seriously - your twin tail, don't get fretful. Sometime in the last century (around 1982, IIRC) I designed my first RC model. It was also my first RC scale model design too...

The Hannover CLIIIA - a WW1 German biplane with a biplane tailplane. I basically blew up the outline from a 3 view I'd found, added the technical and structural bits based on Gordon Whitehead's great book on RC scale models and went out to fly the thing.

Despite my best efforts, it set off, took off and flew around fine.

An electric powered version off my published plan showed up in Australia earlier this year, so there was at least one built off my plan!

I had both tailplanes at the same incidence - zero to the model's datum line - and directly hooked together with external connector rods. It was too small to figure out an internal linkage, but it all worked fine anyway.

Go ahead and do it - you can then be certain you won't have to remember which is your model when it comes time to pack up and go home.

If you're worrying about tipstalling, try a lot of aileron differential - around 90% up, 10% downgoing. I've used that on very pointy wing scale models - Chilton DW1 - to good effect and if it was good enough for the DH Tiger Moth, it should be good enough for us too.

Have fun, make sure you have something simple to keep your flying hand in with before this one sucks you in too deeply!

Regards

Dereck

[Bill Harris]

Thanks, Dereck. I think that this plane will make a fun project.

--Bill