Incidences vs Altitude (Air density)

[hmedina]

Hello

I'm flying at 7800ft over the sea level. I assembled the Electrify Fokker DVII. I measured the incidences and I found the following. The reference is the fuselage line at cero degrees: Horizontal Stab +3 degrees, Upper wing 0 degrees, and bottom wing +1 degree.

I balance to get cero degrees referred to fuselage line, but the airplane when flying need a lot of up elevator to set the straight flying pattern, other wise the plane pitch down. I play with CG and the plane is about to be stalled, I notice this because at high speed the plane pitch down, but at low speed the plane pitch up. I think that the wings and horizontal stab incidences cause this phenomenon. Also I think that those incidences are ok when flying the model at sea level, but not at 7800ft over (my assumption is that the air density is the responsible of this phenomenon.

Because the air density is lower at 7800tf, I'm planning to set the horizontal stab to cero degrees (the ARF comes to build it at +3 degrees).

Is it true?

Do you think that it is going to work?

Thanks
Humberto Medina

[Brandano]

If you fly consistently at higher altitude you would be better off increasing the size of the aerodynamic surfaces, or finding a way to lower the weight, than altering the wing angle. Even if you were to adjust all the angles to trim the plane better you would still have a higher stall speed, and would still fly in a more limited speed range. Since this is a scale plane, try to trim the weight wherever you can. WWI planes were meant to be lightweight fragile things anyway

[Tom Harper]

Humberto,

I think you have identified the problem. Set the horizontal stab at zero or even a bit negative.

Altitude should not change the rules on stability.

Tom

[hul]

my glider's elevator trim doesn't need to be changed when flying in the mountains (>6000ft) vs. the flat field (at 1000ft)

Hans

[JetPlaneFlyer]

As Tom points out the root of your problem is nothing to do with altitude. The need to apply up elevator is due to the stabiliser being set at a higher incidence than the wings (negative decalage). You need to find a way to adjust the angle of the stab.

Are you sure that something has not been assembled incorrectly?... it is unusual to find such a fundamental error in an ARTF model.

Steve

[hmedina]

As Tom points out the root of your problem is nothing to do with altitude. The need to apply up elevator is due to the stabiliser being set at a higher incidence than the wings (negative decalage). You need to find a way to adjust the angle of the stab.

Are you sure that something has not been assembled incorrectly?... it is unusual to find such a fundamental error in an ARTF model.

Steve

Steve

I was looking into technical data (greatplane site) and there isn’t any information about incidences for the Biplane Fokker DVII. So I just follow the assemble instructions. I think that +3 degrees on the horizontal stab is ok at sea level but not at 7800ft

I also fond on other thread the following:

The main reason for positive tail incidence is due to the downwash from the top wing of the biplane. Nearly every biplane I've ever built required positive tail incidence. All wings produce downwash as a function of producing lift. Adding positive incidence to the tail essentially gets it flying "neutral" with respect to the air coming from the wings.

Charlie

Because I can't get it flying neutral at 7800ft with +3 degrees on the horizontal stab, I will set the horizontal stab to 0 degrees. The worse that could happen is to trim down the elevator.

Thanks
Humberto

[JetPlaneFlyer]

Humberto,
Altitude makes no difference to decalage requirement... Altitude just means that your model needs to fly slightly faster than it would at sea level.

The point about downwash is true in some respects however for overall stability the wing still needs more geometric incidence than the tail... Remember the 'zero' incidence angle is not measured from the bottom surface of the wing but from the reference angle at which the wings airfoil produces zero lift...
Theoretical arguments asside your model clearly needs reduced incidence on the tail as demonstrated by the up elevator needed for level flight... and I'll say again.. this is nothing to do with altitude.

Steve

[markdrela]

At 7800 ft, the air density is 0.79x that at sea level. This means that all your airspeeds, stall, takeoff, etc. will all be 1/sqrt(0.79) = 1.12x that at sea level.

The only change you need to make is to increase the effective motor voltage by a factor of 1.12x , typically via more throttle.

The prop RPM will increase by 1.12x to match the higher velocity, but nothing else will need to change. Keep the same prop, same trim, same everything else. Even the current should be about the same.

[macboffin]

Tailplane should be at zero or slight negative incidence. The only effect that altitude will have is that the stall speed will be higher. Keep it light!
Rule of thumb ; density decreases 3% per 1000 ft altitude.