What does this imply?

[C-MIC]

Hi,
I have two 2m e-gliders, they seem to have one flight characteristic in common---when power off gliding, THEY ALWAYS FLY WITH THEIR TAIL IN DROPPING positions, ie, HEADS UP ...fly in a HIGH angle of attack.

Their CG should be slightly nose heavy, so I think they should fly in HEAD DOWN posture? I just wonder if the provided CG position is wrong? Or did I miss something ?How should I correct this?

Hope some expert can help me out? :confused:

Thank you for your attention and reply in advance.

David

[Sparky Paul]

The description is that the gliders are -trimmed-, by you, to the minimum sink speed.
If the flier, you, wants them to go faster, then you must alter the trim.

[vintage1]

The gliders will fly at wahatver angle of attack teh main wings need to generate the lift. If the main wing has a low angle of (geometric) incidence then the fuselage will fly tail down to compensate. Usually this will mean a fair amount of up trim will be needed to kaeep teh tail down as well.

If you find that the elevator is not in line with the tailplane for stable gliding flight, raise the front of the wing till it is, which should make the fuselage more horizontal in flight.

I don't think the CG will affect the 'sit' of the plane in flight very much, but the angle of rigging of the flying surfaces will - hugely, in teh cae of the wing. I'd check the rigging angles on the wing carefully.

OTOH maybe its designed to fly that way anyway?


TYry and measure teh wing and tailplane incidences.

[C-MIC]

Thank you for your answers.
Paul, yes, I have to put a lot of UP trim in order to make them FLYABLE, but it is not my desire to do so or they will sink like hell.

I want to know if there is a way to make it fly (GLIDE) horizontally, like the power planes do?
I will try to adjust the incidence of wing to see what happen, the tail is fixed so it is not able to adjust. But like I said , I have already ADJUSTED the tail with quite a lot of angle, the ELEVATORS, at around 5~10 degree UP positions. They look strange in that way.
Keep on giving me ideas to correct it please.

David

[Sparky Paul]

Experiment with c.g. position. Move it back in 1/4" increments.
Do a "dive test" at each change to see how much margin of stability remains.
When you have the plane gliding the way you want it, enjoy! :)

[NigelD]

The elevators in those positions will be causing quite a lot of extra drag. Does the model fly horizontal with the motor on? If it pitches up a lot with the motor on (or if you have a lot of downthrust), this could indicate that the CG is too far forward.
If we consider the model flying at a constant speed with the CG forward, up elevator is required to keep it level (to counteract the moment produced by the CG). Now if the speed is increased, the force produced by the elevator increases, and the model will pitch up. This is the stability test.
In gliding flight there are basically 4 things which affect pitch:
1. Wing incidence
2. elevator input or tailplane incidence (these are the same)
3. CG location
4. airspeed

With power on, the engine thrust line also has an effect.
forces due to wing incidence and elevator input change with the models airspeed, causing the model to pitch up or down.

If the model is trimmed to glide at its most efficient speed (glides for longest), and the tail is noticeably down, then the problem is probably the wing incidence, the trailing edge must go down.
im not entirely sure of what you describe- does the model stall when gliding, does it fly level with motor on? etc. does it just fly too fast without the up elevator?

If you can answer these then i think the problem can be solved!!

[vintage1]

If you have that much up trim, the CG may be too far forward.

[Sail 'n Soar]

If you have that much up trim, the CG may be too far forward.

Flying nose up/taile dropped is just due to a combo of wing incidence relative to the fuselage water line and most likely trying to fly at min sink/close to stall in a glide. All the other discussions relative to CG position, etc., impact the stability and how much elevator trim is required to position the wing at that incidence relative to the wind free stream. They will not cure the tail down glide. If the planes fly fine under power with a nose lower attitude, take out some up trim and just glide faster.

[vintage1]

Agreed, but the CG may be out anyway. I didn't mean to imply it was the source of the tail down problem per se.