Large Model Association Forum

Im at that point with the 60% Yak where those moments arrive making you think 'have I covered off all I need for a successful flight test programme' . Then someone throws a curve ball.
My issue/concern is: do I need to mass balance the Aielerons or not. So like most I trawl the web and come up with more confusion than when I started.
I understand the principals of mass balance and have used it in the past on smaller stuff, by putting weight into the tips of the aielerons where they have the extensions that fit into the wing tip. Simple enough, but on the Yak although I have cut outs at the end of the Aielerons to facilitate the idea I dont want to add weight there. I figure my roll rate would increase but stopping it rolling after inertia takes over might be an issue
So if it is necessary I want to go the spade route as per scale to full size. These would be fitted about 1/4 of the way up the Aieleron so the weight is inboard not at the tips.
I accept flutter can be induced by several factors ie, rigidity of surface, airflow,air speed etc. But has anyone got any advice on this one. It is also stated somewhere on the WWW that the spade also exerts an aerodynamic load when it is digging the air, another consideration( or not).
Each aieleron is 1.8m long and 220mm wide,4 x20Kg servos in pairs driving 2 mount points equal in distance to each other and the hinges etc are scale. I guess with 210 Lb of static thrust on a 115Lb model and taking into account drag etc. it will have a max speed of 50/60mph.
any thoughts or am i getting a bit anal about it.

Robbie,
I can't help with the science of mass balance but looking at the dimensions you have given for the ailerons and your statement that the model will fly at 50-60mph, then 2x20kg.cm servos per aileron may be marginal - it depends on how much aileron deflection you want at your maximum speed.
On the assumption that you will set up each servo so that you use the normal 45 degreees of movement at the servo and you want a max aileron deflection of 20degrees, then you will be ok at 50mph as the torque required will be 32kg.cm but at 60mph the torque for 20degrees will be 45kg.cm. If you want 30degree of deflection(though you may not use high aileron deflection at high speed) the figure then jumps to 68kg.cm at 60mph.
If you intend doing nothing but prop-hanging all will probably be ok (no airspeed over the surfaces) but for general aerobatics, if you don't want to stall servos from time to time you may want to choose ones with a bit more torque.
Bear in mind that a very large model may actually be travelling a bit faster than estimated!

John,
I think Robbie is using 4 servos per aileron, ie 2 pairs of 2, which would give sufficient torque using your figures,

Barrie

Barrie,
Well done, I was making the silly mistake of reading his data as 4 servos in total ie 2 per aileron. He did begin the sentence by saying "each aileron" - I should have read the statement more thoroughly!

Robbie,
I used mass balancing in all the surfaces of my Magister but after a very short time, after killing a load of servos due to flutter around neutral, I stripped it all out and have never regretted it.

John Townsend

Thanks for the comments. The interesting point is about flutter around the neutral point. Ive read (again on the WWW)in a white paper from circa 1955 from the RAF about such issues.
Me thinks other than being a well read guy on mass balancing now, im going to go the route of the scientists and 'suck it and see', If I balance wrongly I can get flutter, if i dont balance I could get flutter. Initial test flights ill build up speed slowly and see where we go, I cant think of too much more I can do. Im off to much Marcle for the weekend so will probably bend some ears there also

John Townsend 389 wrote:Robbie,
I used mass balancing in all the surfaces of my Magister but after a very short time, after killing a load of servos due to flutter around neutral, I stripped it all out and have never regretted it.

John,

Just out of interest, how did you mass balance your surfaces?


Mark

Mark,
As per full size on ailerons, lead weight on a rod extending from the centre of the aileron leading edge which was contained within the wing structure and protruded below through a slot with up aileron. Elevator was similar but fixed to an extended aluminium horn and the rudder had weight added to the leading edge of the aerodynamic balance.

John

John,

Sorry, I wasn't very clear.

More how did the surface balance after you added the weight, i.e. did it balance level or nose down


Mark

Level

That may have been the problem that caused the flutter. The times I've mass balanced control surfaces, (full size), they have all been balanced against a load, i.e using the hinge line as a pivot you add weight in the nose with a spring balance holding the trailing edge down. The only times I've seen where a surface is not mass balanced is where there's no mechanical connection between stick and surface, (fly by wire), or pre mid-20's aircraft although they usually had aerodynamic balance.


Mark