|Mar 09, 2014, 05:52 AM|
Build my first plane... But naturally aiming to the ground ?
I am not a good pilot but still decided to build my first plane for aerial photography.
I wanted something super slow and kind of auto_stable, some sort of a slowstick with a wide wing
2 meters wing, 70dm2 area, 1.6kg with battery, gravity center at 1/3 of the wing from the front of the wing, only 3 channels
Problem : once the engine is cut, plane goes downwards quickly, like nose heavy. Had to trim the elevator to the max to lift the plane nose.
I have identified a few things :
- elevator cable from servo not stiff enough, will insert it into a carbon tube
- elevator had to be trimmed like crazy to lift the plane nose otherwise plane would dive
- Center part of the wing not rigid enough, will reinforce it
- center part of the wing is truly horizontal. Should I put it at an angle ?
From that maiden flight, could you pls guys suggest stuff to check for a better balance ?
Maiden flight :
|Mar 09, 2014, 07:33 AM|
Joined Aug 2004
It is . to tell from the limited info you provided. Perhaps the CG is to far forward. Could also be that there is not enough difference between the angle of attack of the wing versus the elevator; you normally want the elevator to be at about 3 degrees less angle of attack than the wing. How is your thrust line, you probably need about 3 degrees of downthrust an maybe a bit of right thrust on the motor. You do need a good stiff pushrod on the elevator if you are using pushrods. My favorite is to use pull-pull with cables on both elevator and rudder on big planes as (if properly installed with a bit of positive Ackerman) there is no slop or loss of control when high forces are put on the surfaces.
|Mar 09, 2014, 10:18 AM|
Here's a good CG calculator -
Trim the model for a good glide, (no power), then trim the trust line to minimize trim changes between power on and power off flight.
|Mar 09, 2014, 02:28 PM|
Thank you so much guys.
you normally want the elevator to be at about 3 degrees less angle of attack than the wing
I did not know that, great tip.
Would you please direct me to good links / pics in order to educate me about :
- trust line
- pull-pull with cables on both elevator and rudder on big planes as (if properly installed with a bit of positive Ackerman)
I will google it anyway , must do my homework.
|Mar 09, 2014, 02:40 PM|
I think Rodney actually meant 'incidence angle'.
Assuming a datum line somewhere along the fuselage, (I prefer one parallel to the horizontal stab), the wing is generally set at a positive angle, (1 to 3 degrees), to that datum.
Hopefully the picture will help -
|Mar 10, 2014, 02:21 AM|
Even with that 3 degrees added you still need to trim the model while in flight.
The easy way to add the angle between the tail and the wing is to put a shim under the leading edge of the wing that raises it enough that you add two to three degrees of incidence to the wing.
I like to add incidence to the wing instead of angling the stabilizer down to achieve the angle needed because adding positive incidence to the wing also automatically adds a little downthrust to the motor. And that will aid slightly in avoiding a strong nose up tendency with power.
I would also suggest that you let the model climb higher before cutting the power so you can correct for any dive and work the trim into the model. As it was by the time you realized you had a problem the model was back on the ground. You had no time or altitude to do the initial trim to get the model gliding well.
Right at the end you had the model flying along with some stalling. Was that under power or in the glide?
The dive and skip off the ground during the launch and climb out suggests that you corrected for the dive and held some up elevator during the climb. Is that right? If so then adding the incidence angle to the wing will help with that as well.
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