Simple geometry questions...

[Zak]

Hi,

I am trying to understand why...

Mountain Models Switchback:
The tail and wing are aligned, but the motor has down thrust.

Mountain Models DuskStick:
The tail and motor are aligned, but not the wing (LE is up maybe 3/8").

Both models have Clark Y wings.

What if, on the DuskStick, the tail, wing and motor were all aligned ?

I'm trying to understand the relationship between those things...


Thanks!
Sylvain

[lincoln]

The thing that matters is the motor vs. the wing. The tail is adjustable via elevator trim. If the elevator ends up being a bit up or down from "neutral", that's no big deal.

I'm skeptical that the Dusk Stick had a real Clark Y. People will call anything that's flat on the bottom a Clark Y, even though the real Clark Y curves up a little in the front, which makes a difference. The real one is quite a bit faster.

[JetPlaneFlyer]

Sounds like the Duststick is trimmed for slow sedate flying, so the set up is aimed at flying with quite high angle of attack on the wing and planty of stability.

The Switchback is preobably designed to be faster and so fly with the wing at a lower angle of attack and less stability for better aerobatic potential.

CG position governs how stable (in pitch) the model is and the angle between wing and tail sets the trimmed flying speed... but the two variables do inetract.


Steve

[Brandano]

motor downthrust and horizontal stab decalage are different cures for different problems. You normally have downthrust on a high wing model, since the center of drag is high and increase in throttle would cause the plane to pitch up. Conversely low wing planes may have some upthrust. The horizontal stab on a high wing plane will probably be more influenced by the wing downwash than on a low wing, but that's only a gut feeling of mine, I might well be wrong. In any case the horizontal stab decalage is generally adjusted so that the plane will ideally have no elevator deflection when trimmed for cruise, to try and obtain minimum drag and make full use of the deflection available.

[Zak]

OK, thanks!

[BMatthews]

The need for downthrust is not really tied into the location of the wing vs the thrust line. The idea of downthrust is to help counter the airplane's tendency to want to climb as speed is added. The drag of the wing does play a part in this but it doesn't define the need for downthrust.

Where does this climbing tendency come from? That is more the key to why we add downthrust.

It comes from the CG location and the resulting amount of angular difference between the wing and tail. On flat foamie 3D models where the CG is at or very close to the overall aircraft neutral point the need for this angular difference (sometimes called decalage) is zero. And with this there is a total lack of the tendency for the model to want to nose up if you dive it. When the model reacts like this or near to it there is no need to add any down or upthrust. You can try this by doing test dives where you push the nose down to a 60 degree dive and then let go of the Tx stick then watch what the model does. Models with a strong reaction to lift the nose and reverse to a stall need downthrust. Models like flat foamie 3D designs will not try to pull out at all. They will happily continue until you add some up or they lawn dart themselves.

On high or low wing models where the CG is near the neutral point the drag of the wing does become a much bigger portion of this reaction. It may even become the major portion. But only when there is no angular difference between the wing and tail due to this being the final trim position due to a strongly rearward CG location.

It gets more complicated when you toss in a "clark Y" airfoil that we all know is a "lifting" airfoil. Actually it's no more lifting than a symetrical airfoil. It's just that due to the camber of the shape a curved top with flat bottom airfoil will lift when the flat bottom is sitting at a zero degree angle of attack. The zero lift angle for any cambered airfoil is actually a negative angle. And a model with such an airfoil can only be said to be truly 0-0 in its setup when the wing is noticably angled downwards to this zero lift angle when the horizontal tail is sitting level. When it's set up like this and the CG is located very near the neutral point this oddball model will fly just like a flat foamie or like one with a symetrical airfoil. It'll do some things different from upright to inverted due to the odd airflow that the cambered airfoils causes. But it'll fly in a neutral manner with no downthrust or upthrust unless the wing is located well above or below the thrust line.

[Zak]

Wow, thank you!

I will print your post for future reference.

Sylvain

[BMatthews]

If that was for me then you're welcome.

A lot of the info in my post came from having flown and fighting to understand a wide variety of models over the years. It wasn't that long ago that this was all voodoo mumbo jumbo to me. About 6 or 7 years ago it all became clear thanks to the wide variety of models I'd trimmed out over the years combined with a rather heated discussion on one of the forums about the neutral point, stability requirement and a couple of other threads about airfoils.

Who says model forums don't help? :D