aerodynamic or thrust problem?

[C-MIC]

Hi,

I have a 2m e glider, which is powered by a GS BL3630 ( a bl similar to mega's 2230/2, with kv=3750) +2,8:1 GB +APC 10x6 folder, the whattmeter reads 32A 230 W with 7cells, and 36A 280W with 8cells.
(the stock motor is a 550 + 8x 4,5 folder)

At its maiden,I used 8cells, it went straight up after being launched with full power, so I cut the power and it ended up stall turn but controllable and landed safely.

Then I have it trial flight today, and changed to a 7 cells pack, shimmed the t.e 2mm higher so the angle of incidence is definitely reduced (chord length=200mm). I launched it again it flown NORMALLY and then gained height at about 30 ft , then it went straight up as before. I cut the power again at 100 ft and get stalled, I rolled it out and begin to glide, very WELL.

So can anyone help to tell me what to do with this glider? Is it still overpowered? I checked the cg is right at both flights and I have added more down thrust when I replaced the stock power set up.
Do I need to add more downthrust?
Thank you for your attention and reply in advance.

:confused:

David:(

[Ollie]

If the glide is right then the only thing to do is keep increasing down thrust until there is minimum change in pitch attitude with throttle advance or retard.

[C-MIC]

Thanks Ollie, I will try as your advised. Hope I can get it done and fly this Sat..
By the way, I would like to know if the angle of incidence is wrong set at factory, say too much of it. Whether it can be corrected just with the flapperon, because I am not sure if the shimmings I added should be removed?

Thanks alot.

David

[BMatthews]

You say that on the second flight it flew normally for a short ways and THEN nosed up abruptly? If so that sounds more like it may be a radio problem. Or was the distance so short that the model was just accellerating up to speed and then nosed up.

Since you've powered this model with what shounds like a lot more than originally intended you may have two things at work here.

First off the higher power WILL require a bit more downthrust. I would suggest one or two degrees additional.

Secondly you may want to just use enough down trim to get it to some sort of height and then do the usual gliding dive test to check for an optimum CG placement. A forward CG placement and the resulting higher decalage angle will produce a trim that is more sensitive to speed and power just as you are describing.

Oddly enough shimming the trailing edge up changes the thrust to wing to tail relationship in the same way as removing downthrust and reducing the decalage all at the same time. It helps in one way but hurts in the other. Adding positive incidence to the tail is the preffered option since it preserves the wing to thrustline downthrust angle.

[Ollie]

BMatthews has a good point. You should do ground range checks with th antenna down or off an at various throttle settings including off. If there is significant range difference under any conditions the motor noise problem should be corrected by filtering before flying.

[BMatthews]

Originally posted by C-MIC
....if the angle of incidence is set wrong at factory, say too much of it. Whether it can be corrected just with the flapperon....David

This is not the way to do it. Adjusting the flaperon setting changes the airfoil shape at the same time it alters the incidence angle. If it turns out that the angle between the wing and tail is not optimized then the correct technique is to add down elevator trim and live with the deflected elevator or, if the aesthetics of a deflected elevator bug you, remount the whole stab to the proper angle that you determine based on those dive tests.

Of course all this is based on not finding another simpler solution to your problem. Either the radio being glitched by the motor or perhaps your motor isn't actually mounted at the angle you think it is. Have you double checked that?

[C-MIC]

Thanks guys,

I have ground radio checked clear, so it shouldn't be that caused the weird flight characteristics.
I tried to change the power set up back to brushed 550 but with geared,so that the power can be reduced and weight can be increased in the head, but unfortunately such set up only give 105W with a 12x8,5 apc folder. ( that is the biggest folder I can buy in Hongkong. I don't like to waiting for overseas' mail order)

So finally, it seems that I can have only option to use the above said mod. power set up with increasing the DOWNTHRUST ANGLE.
I will also remain the aerodynamic set up unchanged as stock and see WHAT happened indeed so I hope I can tack down the problem eventually. Let me hope that it can survive in next trial flight tomorrow.

I will report the result here. Thank you for all your kindly concerns & opinions.

David

[vintage1]

It does sound like the CG is rearward.

That can have the effect of instability in pitch - a little up and then it pulls way up into a stall.

I remember this from my buddies one and only attempt to buld a FF contest duration type plane in the 60's. VERY difficult to get it to climb correctly, and glide correctly all at the same time.

That had a rearward CG for best glide too.

One day he got it right and it flew away :)

I'd check CG first, then trim it almost zero zero. Oce up at height you can trim for teh glide. May be worth mixing some down with the throttle too...

[C-MIC]

That glider has finally made successful flight today. Thank you all for your inputs.

I can now conclude that the weird flight characteristics is mainly caused by the insufficient downthrust angle of motor.

I had increased the downthrust angle from 1deg to 3-4 deg. , and removed HALF of the shimmings at t.e., which I will completely remove in next flight cos I needed to hold a bit of up to maintain horizontal today.

It flew really brilliant though it is only a cheapie, I really enjoy it more than some of higher priced ships I owned.

Regards,

David