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Old 11-05-2012, 05:39 PM   #1
npowell28
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Default Question on Clark Y

Just a quick question on the Clark Y airfoil. How should this airfoil be orientated on the plane. should a line on the flat bottom be parallel to a line through the horizontal stab, or should a line from the te to the point on the le be parallet to the horizontal stab? Hope that makes sense

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Old 11-05-2012, 06:11 PM   #2
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Aligning the bottom of the airfoil with the aircraft datum line gives approx 3 degrees of true incidence. That works pretty well for many/most models.

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Old 11-07-2012, 04:32 AM   #3
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Its common to align the bottom surface with datum line through fuselage / tail stabs initially. This gives a relatively neutral alignment at moderate speeds and up, but depending on model type can be insuficient at low speeds, leading to excessive elevator input.

It is not uncommon for packing to be inserted under Leading or Trailing edge of wing at mount to adjust after flight. To correct ballooning of model when throttle closed or opened .... yes - it can happen either way depending on speed and angle of attack / incidence of the wing.

Biplanes often exhibit it worst ... along with trainers. Increase speed and away they climb !

Not often thought about - but high lift sections as Clark Y are more sensitive to engine thrust lines ... if they are poorly matched you can have situations where at cruise model is straight and level ... shut throttle and model pitches up sharply. Why ? Wing is at +ve incidence, motor is down thrust to counter. Remove motor thrust and wing before speed reduces now balloons up. Increase motor and model returns to equilibrium again. My big gasoline biplane suffered this recently and in the rebuild will have the semi-symet top wing lowered at Leading edge to cure. The motor already having been reduced on down-thrust.
That brings in another point ... some biplanes due to the amount of lift they have ... top wing can be at lower angle of attack than the bottom ...

Apologies if I veered of the thread path a bit !!

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Old 11-07-2012, 10:01 AM   #4
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Originally Posted by solentlife View Post
Its common to align the bottom surface with datum line through fuselage / tail stabs initially. This gives a relatively neutral alignment at moderate speeds and up, but depending on model type can be insuficient at low speeds, leading to excessive elevator input.

It is not uncommon for packing to be inserted under Leading or Trailing edge of wing at mount to adjust after flight. To correct ballooning of model when throttle closed or opened .... yes - it can happen either way depending on speed and angle of attack / incidence of the wing.

Biplanes often exhibit it worst ... along with trainers. Increase speed and away they climb !

Not often thought about - but high lift sections as Clark Y are more sensitive to engine thrust lines ... if they are poorly matched you can have situations where at cruise model is straight and level ... shut throttle and model pitches up sharply. Why ? Wing is at +ve incidence, motor is down thrust to counter. Remove motor thrust and wing before speed reduces now balloons up. Increase motor and model returns to equilibrium again. My big gasoline biplane suffered this recently and in the rebuild will have the semi-symet top wing lowered at Leading edge to cure. The motor already having been reduced on down-thrust.
That brings in another point ... some biplanes due to the amount of lift they have ... top wing can be at lower angle of attack than the bottom ...

Apologies if I veered of the thread path a bit !!

igel
The balooning that you describe is what i have experienced with bout of my scratchbuilt models using the clark y. As soon as i get some decent weather i'll have a mess around with the thrust lines first. My latest model is balanced correctly. Actually its more biased towards the nose. It takes off great but when you give it some throttle it really balloons. It'll cruise ok but it flys around with its nose in the air and doesnt really show any sort of authority. As soon as i get some decent weather i'll have a mess around with the motor thrust lines first.
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Old 11-07-2012, 10:35 AM   #5
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Originally Posted by npowell28 View Post
The balooning that you describe is what i have experienced with bout of my scratchbuilt models using the clark y. As soon as i get some decent weather i'll have a mess around with the thrust lines first. My latest model is balanced correctly. Actually its more biased towards the nose. It takes off great but when you give it some throttle it really balloons. It'll cruise ok but it flys around with its nose in the air and doesnt really show any sort of authority. As soon as i get some decent weather i'll have a mess around with the motor thrust lines first.
If you are fltying around at reasonable speed and nose is in the air - that can be due to tailplane being too low an angle to airflow alied to wing incidence... It is not only motor and wing ... but tailplane also plays a serious role in this.

Worth getting hold of an aeronautics book ... particularly model based ... I have an excellent book that was part of a series allied to UK RC magazines back in the 80's ... gives all sorts of advice on angles / rig etc.

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Old 11-07-2012, 10:56 AM   #6
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It's based on a C130 and that flys pretty good. Basically when it's given any more than 1/2 throttle it balloons. Anything lower than that it'll just put the nose up and flay with that sort of attitude. I'm more thinking that its to do with either the motor thrust line or the wing seating. I guess it's just a matter of trial and error.
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Old 11-07-2012, 12:37 PM   #7
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Originally Posted by npowell28 View Post
It's based on a C130 and that flys pretty good. Basically when it's given any more than 1/2 throttle it balloons. .
Likely to be caused at least in part by the CG being too far forward. I know it's counter intuitive but if you move the CG back (and re-trim the elevator to suit) then the plane will balloon less when you increase flying speed. Obviously there is a safe limit how far back the CG can go but usually on conventional looking planes 1/3rd way back from the LE is fine.

Counter to popular opinion 'Clark-Y type', flat bottom or even under-cambered airfoils arenít inherently any more prone to ballooning than a symmetrical airfoil. In fact taking the airfoil in isolation the opposite is true. A Clark-Y or any other normal cambered airfoil will tend to tuck nose down as speed increases. It's the CG position and trim of the tail that causes the nose up ballooning not the airfoil.

From Wiki:
Conventional cambered airfoils supported at the aerodynamic center pitch nose-down so the pitching moment coefficient of these airfoils is negative.[4]


Thrust line plays a part too but getting the CG and trim correct is the place to start, and then look at thrust line after.
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Old 11-08-2012, 04:11 AM   #8
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Originally Posted by npowell28 View Post
It's based on a C130 and that flys pretty good. Basically when it's given any more than 1/2 throttle it balloons. Anything lower than that it'll just put the nose up and flay with that sort of attitude. I'm more thinking that its to do with either the motor thrust line or the wing seating. I guess it's just a matter of trial and error.
Add a mm or so under the rear wing edge at the seat ... reduce its angle of attack. Keep adjusting till you find happy medium.
Initially you'd think it needs under leading edge .... but in fact no.
The characteristic you describe is often where the wing is too +ve and will climb as airflow increases. The reason it is nose up at low speed - is you are inputing elevator to keep her flying straight and level .. using that excess AoA to fly. I bet the controls are a bit 'mushy' as well at that ...

I agree that it may not be the answer - but it sure sounds like some of the models I've had over years that did similar and I altered wing angle.

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Old 11-08-2012, 06:00 AM   #9
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Originally Posted by solentlife View Post
I agree that it may not be the answer - but it sure sounds like some of the models I've had over years that did similar and I altered wing angle.
That may well work providing you move the CG back at the same time. If you dont move the CG then all that will happen is that you will need to add up elevator trim to prevent the model diving and you will just end up that the up elevator trim exactly cancels out the change in wing incidence.
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Old 11-08-2012, 06:17 AM   #10
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Problem with things like this ... all parts are working together and they all have to interact within limited parameters ... or else strange antics occur !

One of the best ways to get to a solution is to compare with a similar style model ... that flies well .... it can then surprise what the solution could be ...

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