yclui

My Feedback: (1)

After receiving advices from experienced HLG pilots and Chris Adams in particular, I decided to
start HLGing from the very beginning and trimming the plane is step one. I have been told that I should get the plane gliding flat by itself at any air speed. The step I went through are as follows :

1) Launch the plane gently , trim the elevator for a flat glide
2) Launch the plane with all the muscle I have got and see how the plane goes
3) If the plane climbs by itself, move the CG backward, if it dives instead ( tuck under ? ) move the CG forward.
4) Go back to step (1) until the plane would keep the same flat gliding angle irrespective of the launching speed.

I know I should have adjusted the incidence of the stabilizer instead but at this stage, my objective is just to get a quick feel of the reaction of the plane to the process so I substituted stab shimming by elevator trimming.

I have tried the process on a free-flying toy glider ( 10 inch span, flat-plane airfoil ) and found it worked very well. The plane clearly told me what the correct CG position was. Too far forward, it would climb at high launching speed and too far backward, it tucked under.

The behaviour of my Highlight HLG ( 1.5m span, slightly under-cambered airfoil ) is not that clear cut. It would also climb at high launching speed if the CG is moved forward but it never tuck under. I have tried to shift the CG back to as far as the middle of the wing chord and what the plane did was still climbing but it would stall more abruptly and responded very poorly to controls. Finally, a hard landing that cracked the fuselage terminated the experiment .

May I know whether this is the kind of behaviors generally expected for HLG ? I am going to try stab shimming next week, will it bring any significant difference ? Finally, should I just use my feeling to judge what the correct CG position and decalage are instead of relying on the demarcation between climbing and tucking under ?

Thanks in advance for any advices,

Y C Lui

talybont

Hi Lui,

it depends on profile and the angle between wing and tail how the plane will climb after launch. Don't change the CG only for launch because you will get an instable javelin unable to glide well. Just try a smaller angle and maybe a profile wich is not as curved as the one you have in use. The CG will be set up while gliding.


MfG,
Armin

Ollie

The best information on CG adjustment is available is at:
http://www.polecataero.com/
Go to articles and select the article on CG adjustment by Dr. Mark Drela. Once you have the CG set for your flying style, you may or may not require different elevator settings to get the type glide and climb after launch that you want.

yclui

My Feedback: (1)

Thanks !Ollie, the link you have provided is really useful. :thumbup:

ilikeplanes

That XP3 looks very cool. I may have to get a poly version.

Mark Drela seems like one cool guy. I'd like to share a pitcher of beer with him and Joe Wurts.