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Knife edge Problem with Yak

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Knife edge Problem with Yak

Old 05-21-2010, 07:43 PM
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BusterBust
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Default Knife edge Problem with Yak

Howdi

I have a Yak 54 with a 50cc on it...every time i try a knife edge, as soon as i apply the rudder (with the top of the plane toward me) it wants to curl away from me fairly hard. in order to keep it going down a straight line i have to use a fair amount up elevator to counter the curling away from me.

Any ideas as to what is might be causing this abrupt turn away when i try the knife edge?



Thanks in advance
Old 05-21-2010, 09:40 PM
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BarracudaHockey
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Default RE: Knife edge Problem with Yak

CG or it needs mixing.

Also might check the incidence of the stabs in relation to the incidence of the main wing.

Does it need a lot of elevator trim to fly level?
Old 05-22-2010, 05:43 AM
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Default RE: Knife edge Problem with Yak

Yaks tend to pull to the landing gear at knife edge. The best thing is to make sure it is set up correctly ie cg is right - incidences are right - lateral balance is right - it is trimmed flat and true. Then use the mix to make it fly straight on knife edge and to keep it from rolling out on knife edge.

It takes 2 mixes to make it work rudder to elevator and rudder to aileron

That should get you going straight again
Old 05-22-2010, 10:20 AM
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JOHNS3D
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Default RE: Knife edge Problem with Yak

Buster what radio are you using?.
Old 05-22-2010, 12:25 PM
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BusterBust
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Default RE: Knife edge Problem with Yak

Thanks for the replies...JOHNS3D, im using JR 9303.

The CG thing has me worried, long story short, i have a Revolution engine..which is kind of long, to make it fit properly inside the cowl i had to cut the engine box back...having said that, it is very sensitive on the elevators but i dont see the tail draging to say its tail heavy..and when i come in for landing it floats in very nicely...no nose drop.

Having cut the engine box back means i cant use the CG thats in the book anymore correct? The engine being in a different place would change the CG...im not sure which way.
Old 05-22-2010, 03:05 PM
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rmenke
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Default RE: Knife edge Problem with Yak


BusterBust:

If you moved the "moment" (distance from the wing L.E. to prop), the CG would change slightly in relation to the change. Sounds like you moved the engine location within the cowl a little, should not make much of a change assuming the design engine weight and your engine weight is close. When ever something does not fly as the design has proven with other models, best check out motor, wing and elevator thrust lines. Not really that hard if you have a incidence meeter. You need to hold the airframe from moving if possible. I use a crutch made from particleboard shelving, 1" X 12". Its straight. Each end is adjustable up and down. I adjust this to hold the airframe in place as the measurements are made, otherwise a little bump and things change. Another advantage is that the wings will be held in place by each side of the crutch. Assuming your work bench is level, start by getting the wing level (hardest part to modify) then you can check the elevator and motor incidence using the wing as the base line.

One of the problems with this is getting a true reading on the wing and elevator due to the moving ailerons and elevator sections when airfoil shaped. I rubber band two balsa strips at each side of the wing something like a over sized clothes pin. The aileron can be adjusted a little to center and be held in place. Same for the elevator. Seems like a lot of work, but it's the only way I have found to keep the base line accurate.

If you have a good digital incidence meeter, you can try to work it out, two meters make life a little easier. When a design calls for 1/2 of 1% like several pattern ships built through the years, you have to be accurate to get things right. Before the digital meeter, life was hard with the bubble level or plumb bob. Yes, can be a lot of work, but when you get things right, you are rewarded with a exceptional flyer, well worth the work in the long run. Good luck, hope this helps a little.
Old 05-22-2010, 04:12 PM
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Panther45
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Default RE: Knife edge Problem with Yak

Buster

We sort of skipped your question on the CG. Moving the motor position will not change the book proper CG location. That will not change but your actual CG likely would change. It is key to get in the range shown in the book (measured properly no eyeball stuff) and then fly the thing.

Trim flat into the wind at cruising speed and then roll inverted and see how much down it takes to fly flat. A little down is taken by most IMAC folks to be just right. Lots of down and you are nose heavy while having the plane climb with no down in it would be tail heavy. That must be done right after thrust checks and before anything else is checked or changed. This will make a difference to everything else you are doing with the plane.

Give it a try then let us know about the knife edge performance you see.
Old 05-22-2010, 08:40 PM
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Default RE: Knife edge Problem with Yak

Everyting points to a very aft CG, check also aileron neutral position (not "flapped", nor "spoileroned"), when you finish with these, mix the rest out

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