Hello all,

I am building the Guillows Thomas Morse kit for electric RC. I have asked this question directly of Gerald O'Docharty who posts over on the scale forum and who has built this same model, but I have not received an answer yet (maybe he is indisposed), so I will ask it here as I am keen to finish the model.

Can anyone suggest what wing incidence and motor thrust angles to use? The original rubber powered plans show the top wing lower surface parallel with the horizontal stabiliser, and the bottom wing with about 2 to 3 degrees positive. But I'm not sure whether these are right for the electric conversion.

Some relevant facts:
Wing span approx 24" (top wing), slightly less on lower wing.
Lower wing chord is approx 60% of top wing chord.
Both airfoils are flat bottomed, and very thin, maybe 8 or 9%.
Weight should end up around 230g.
Wing loading approx. 5.5 oz/sq.foot.

Sorry about the vague specs. but I'm in the office at work and don't have the plans with me.

What are the theoretical considerations when designing biplane incidence angles? Any advice would be welcome.

Thanks guys.
Graham Allen.

The angular relationships between the wings and tail do not change because of the type of power or the wing loading. If the angles shown on the plans were right and the CG is in the same place then the conversion should have the same incidence angles.

Ollie,

Thanks for your reply. I've been busy with work so am only just responding now.

I accept your point that altering wing loading or power source on their own would not require any change in wing incidence, but I am still concerned about the decalage because the model will be flying at much greater speeds than it was originally set up for.

As I understand free flight low powered rubber models they are set up with large wing incidence angles and a forward C of G to give very strong longtitudinal stability. So if they are upset say, into a dive, they speed up then the high decalage angle causes the nose to rise thereby slowing the model down and correcting the dive. On the original Guillows model this would produce a nice stable, but slow flight path.

But now, I am increasing the weight so will need to fly faster to maintain level flight, and furthermore I do not want the strong longtitudinal stability as the model is now R/C. So do I still need such a high decalage angle?

I probably sound totally confused, which I am, and any clarification would be welcome.

Graham.

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CG location affects both stability and pitch trim. Decalage affects pitch trim but not stability. CG location should be used to establish the desired stability and decalage should be used to establish the desired flight speed (pitch trim). The problem is, if you decrease the decalage too much, the desired pitch trim may result in an unstable CG location. Changes in CG and decalage should be made in small increments between test flights. The structure should be modified so that the incidence of the stab, and hence the decalage, can be adjusted with temporary wedges of balsa during the flight testing phase. Just make the slot in the fuselage oversize and put wedges above and below the stab to take up the extra space. Do not glue the wedges permanently in place until CG and decalage are finalized by flight testing.

Ollie,

That's much clearer now. I think you have confirmed what I felt intuitively, that the high decalage angle of the original kit would produce a trimmed flight speed too low for the added weight of my conversion and I would have had to correct it out with down elevator trim.

I understand what you say about moving the C of G too far back, and I will test carefully as I go. The only complication being that I need to consider the relative incidence of the two wings, as well as the incidence relative to the stab., and this is not so easy to alter. The original kit has the lower wing about 3 degrees positive relative to the top wing. I seem to remember reading something about the pinching effect the top wing has on the airflow over the lower wing, delaying the stall, but my gut feeling is that they should be less than 3 degrees relative incidence to each other. I also remember something about biplanes having one wing that deliberately stalls first, which kind of smears the stall over a wider angle of attack (and speed) range.

I will box on with some assumptions and keep the airframe modifiable. Thanks again for your help.

Graham.

I would not modify the wing mount or incidence but I would arrange the stab mount and motor mount so that the decalage and thrust angles could be changed relative to the wings. The fuselage is just there for appearance, the other angular relationships and to hold the equipment. The fuselage angle to the direction of flight varies greatly with changes in air speed and the effect is mainly on drag not so much the relationship between the other angles and forces.

I would start flight testing with about three degrees of down thrust and about zero degrees of stab incidence relative to the lower wing. This will result in the original, safe and forward CG location for pitch trim. You can adjust from there to suit your flying style.

You are right to think about this.

Incidence. A high main wing incidence will cause the model to fly nose down in level flight as speed increases

Decalage. A large positive decalage will give excessive longitudinal stability. On a well trimmed model with downthrust, the effect will be to give a strong climbing angle under power, and a stable and safe glide.

C of G. Too far foward and its very soggy on elevator response. To far back and its almost impossible to keep in level flight.

To convert a slow rubber model, I'd be tempted - to get the 'sit' right, and to get rid of some of the 'stability' - to drop the main wing incidence to a degree or three, and put the decalage at a similar amount, and move the C of G back to compensate. My best flying models are those which are relatively neutral in setup. They fly level under power, and need up elevator to climb, and glide properly. That's personal taste tho. I don't like a well forward CofG. I like em a bit sensitive.

Froim experience a high wing model for rudder/elevator control needs at least 5 degrees dihedral, maybe up to ten, c of g about 40% chord. main wing incidence about 3-5 degrees and zero on the tailplane.

I'd start about there and see how it flies. If the tail is dragging around the sky, up the main wing incidence till the fuselage sits level, and use the elevator trim to get the trim right.