CofG discussion Slopeside in the October issue.

[Roderic Craig]

I dont follow Andy Ellison's explanation.Surely if the Cof G is forward the plane will continue to dive and if its aft of the correct position it wil pull up! Or  do the Laws of Physics not apply to gliders in a 45 degree dive? If you add weight to the nose it must go down and towards the rear it should pull up! Is Andy Ellison wrong or am I missing some other factor. Certainly the BMFA  article to which he refers seems  the correct one to me!! I await an explanation from some aerodynamacist   Roderic.

[Bert]

Roderic

 

You don't need to be an aerodynamist to understand what Andy Ellison is telling you.

 

If the CG is forward, then to maintain level flight the wing will need to be at a positive incidence (bit of up elevator) - it will tend to pull out of the dive.

 

The reverse is true if the CG is too far back.

 

Andy Ellison is one of the most knowledgable magazine contributors there is - believe me he's right in what he says 

 

Bert

[Roderic Craig]

Dear Bert,

Thanks for your comments.We seem to be talking at cross purposes here! You agree that Cof G forward , into dive, release elevator and dive gets worse so you have to up elevator to maintain level flight path,which is just what the guy in BMFA was saying.

But look at page52 of RCME, middle column "with forward CofG, will begin to pull out of dive....as if up elevator had been applied" but surely that contradicts what you have just said. Any rate what is the point of Andy's article? My Easy GliderPRO has the Cof G where mulptiplex say it should be and flight is level.Is the idea that when slope soaring in strong rising air current that by adjusting the Cof G for those conditions that you can keep better control? Id love to hear from you further, Roderic.

[Slopetrashuk]

Hi Roderic - perrhaps I can clarify the situation for you as you sound like a relatively inexperienced model flier and this issue can be quite confusing.

The immediate reaction of many people is, "Why does the nose coming up mean it's too heavy?  That's back to front!"

Remember that the model was originally trimmed to cruise level at moderate speed.  At the higher speed associated with the dive the elevator has more authority or effect on the way the model is trimmed. If it was originally a bit "up" to hold a heavy nose (or a forward CG) it will lift it more at the higher speed. Of course the opposite is true for a rearward CG. Try this with one of your models to be sure. Trim for straight and level flight then try the dive test. If the model zooms, remove some weight from the nose and trim again for straight and level before dive testing once more.
As you approach a neutral CG you will end up with a model that can cover all speed ranges with no re-trimming required. hence the benefit of achieving the correct CG.

 

It is not at all related to slope lift and similar tests are advocated for all model types. Typically powered aircraft will be proven in a vertical line at low throttle. Other types in inverted flight etc.

 

The point of the article? Well I suppose it is to educate guys like yourself into getting the best from your models and clarifying the confusing surrounding a complicated topic.

 

All the best.

 

Andy

[Erfolg]

I have trouble with Andy's explanation, which does not mean it does not work, at least for himself and probably many others.

 

I accept that the model ay all positive angles of attack, has a nose down couple, which is counteracted by the tailplane maintaining that attitude, my developing a downwards force, to counter the couple. It does follow that the further forward the CG, the greater this force must be. For equilibrium conditions.

 

It is what happens when this equilibrium is disturbed, by control input or gusts, that is another issue. I suspect that this could well be influenced by section camber, tailplane section.

 

My own observations are if the CG is to far back, the model is very difficult to fly, in that constant corrections are required. Easily disturbed by gusts. Generally difficult to keep up in pitch with the oscillations of the model. On the other hand, if the CG is to far forward, the model, is heavy on the elevator, that is sluggish to respond. That is it wants to keep going in its set condition. Far to far forward and my observations are that it starts behaving something similar to far back, in that control becomes difficult at the extremes of the flight envelope. It keeps going till it stalls. If going downhill, it keeps going, until it snaps into going uphill, till it etc. Remembering that in the mid speed range it is just sluggish on elevator. I tend to judge if the CG is just right, by seeing if the model will bunt. If it does and the rest of the envelope is OK, in that the stall, is acceptable, the model responds well to looping, flaring out etc. The problem is not all my models want to bunt easily. 

 

Now I have tried the speed test and have not found the results that Andy talks of. That could be that I do not (did not, in that I chickened out) put the model into a 45 degree dive. My dives I guess have been modest, possibly as little as 15 degrees. To go steeper, results in a model often traveling far to fast for my liking. At modest angles I find a very different set of reactions to Andy.

 

Yet I guess it works, but not for me.

 

Erfolg

Edited By Erfolg on 04/09/2009 13:07:16