1
$\begingroup$

I've tested my RC plane yesterday. It's a canard. After hand launching, the plane goes straight, then I change the elevator to go down and it's going down, but when I pitch up the canard doesn't work and the plane crashes.

  • The plane is at full throttle.
  • When diving the plane slightly rotates to right.
  • Controls tested before flight.

Edit:

  • Airfoil is a NACA 2412, but tips gets a little thick (all cutting, sanding is done by hand)
  • No incidences.
  • One servo for the canard and one servo for the ailerons.
  • No thrust angle.

The plane: RC plane with canard

CG is just in front of the vertical stabilizer (the little cross in the image)

$\endgroup$
8
  • 3
    $\begingroup$ I'm voting to close this question as off-topic because i think it belongs to robotics.stackexchange.com $\endgroup$
    – vasin1987
    May 28 '16 at 9:56
  • 6
    $\begingroup$ I disagree, if I read it correctly it is an elevator efficiency question, but needs polishing. $\endgroup$
    – Federico
    May 28 '16 at 10:21
  • $\begingroup$ What airfoils, incidences and control deflections were used? You leave us guessing at what happened; if you provide more details we could narrow the possible causes down. Right now I guess it was flow separation (stall) on the canard, but without more details I am just speculating. $\endgroup$ May 28 '16 at 12:05
  • 2
    $\begingroup$ Thanks guys. I've updated the question with requested information. I hope to be useful (I also have a short video from crashing time). $\endgroup$
    – SAMPro
    May 28 '16 at 18:48
  • 1
    $\begingroup$ What are the control deflections for the canard? IE: How far does it move in max up and down throw? Also, just to confirm, the back is not elevons, only ailerons? $\endgroup$ May 29 '16 at 21:45
5
$\begingroup$

Thank you for giving more information, but I still have to resort to speculation. Here are some things you might want to consider:

  • A rear-mounted prop stabilizes the aircraft. Full throttle limits your control power. You would had a better chance of recovery with engine idle.
  • No incidence means that the plane needs to compensate for the c.g. location with elevator deflection alone. If your plane is statically stable, it would need to be trimmed with some elevator-down deflection for straight flight. This limits the maximum possible canard lift and your nose-up control power. Better give the canard a few degrees of positive incidence.

However, from the picture it might well be that you use a full-flying canard. In that case, the incidence is set automatically by trimming, and a nose-up command means increasing the incidence further. Now the control commands do not change the camber of the canard wing which reduces both the amounts of maximum and minimum lift possible. Better use a canard with a stabilizer-elevator combination to increase control power.

You talk of cutting and sanding. Are the wings and canard made from styrofoam? Then @TipStall is right: The plane lacks stiffness. Commanding a descent (what you described as "going down") increased the dynamic pressure and thus the forces warping wings and canard.

For now, I think you suffered a canard wing stall, but I am not able to say why. Either it was aeroelasticity (twisting of wing and canard) or insufficient control power. In that case, use a stabilizer-elevator combination for the canard, throttle your engine and maybe consider a less stable aircraft by shifting the c.g. back a bit.

$\endgroup$
4
  • $\begingroup$ The stabilizing effect of the engine prop will depend on the thrust vector. if its above the center of gravity it has a nose down effect which would be destabilizing in the example given in the question. I agree that it sounds like canard stall. If the canard has a large available range of motion i.e more than about 15 degrees nose up the canard would definitely have stalled. $\endgroup$
    – DJ319
    Dec 5 '17 at 8:20
  • $\begingroup$ @DJ319: The thrust vector only adds a pitch moment. This can be used for pitch control by changing the throttle setting, but does not add stability. The prop by itself acts like an additional tail surface which stabilizes the plane. $\endgroup$ Dec 5 '17 at 12:51
  • $\begingroup$ @ Peter Interesting I have never heard of this effect before. I will look into it thank you. Do you by any chance have a paper or book I should look at to speed things along? $\endgroup$
    – DJ319
    Dec 6 '17 at 8:20
  • $\begingroup$ @DJ319: Try "Calculation of aerodynamic forces on a propeller in pitch or yaw" by Crigler and Gilman. NACA-TN-2585, downloadable here. Stability requires the forces to change with pitch and yaw changes. $\endgroup$ Dec 6 '17 at 10:00
5
$\begingroup$

My vote is that the surfaces aren't rigid enough and get twisted so much that the control surfaces can't overcome the effect. How floppy (non-rigid) are the wings and the canard surfaces? The wing looks to be sagging under its own weight! My thought too is that you're tail heavy, tail heavy will pitch both ways and you can get stuck in either positive or negative stall (or tuck). Tail heaviness causes the plane to tend towards going or falling flat like a leaf; it's usually strongest (leeast recoverable) when under the influence of negative G's. It doesn't matter whether the craft is upright or inverted; it's still "negative G's" including the caused airflows. Still, my best guess is weak surface rigidity. Good luck with your cool project!!!

$\endgroup$
2
  • $\begingroup$ Every wing reinforced with two 6mm fiber carbon pipe that connected to another wing with a 20cm 4mm steel bar, but tips gets a little twisted with hand. $\endgroup$
    – SAMPro
    Jun 4 '16 at 12:59
  • $\begingroup$ @SAMPro: The carbon pipe adds bending stiffness but not much torsional stiffness. Better to wrap the full wing in one layer of glass/epoxy. $\endgroup$ Dec 5 '17 at 12:53
3
$\begingroup$

I would suspect a combination of a very nose heavy CG, and not enough up elevator control throw, but it's impossible to determine based on the information given.

$\endgroup$
3
  • 2
    $\begingroup$ @Federico it's an acceptable answer given the information provided. If the question gets updated, I'm sure he'll update his answer. $\endgroup$
    – falstro
    May 28 '16 at 11:37
  • 1
    $\begingroup$ From experience I'd say the CG is a little tail heavy. It's a canard but with very big canards which makes it closer to a tandem. So I'd expect the CG to be somewhere in between the trailing edge of the canard and the leading edge of the wings. $\endgroup$
    – slebetman
    Jun 1 '16 at 13:28
  • $\begingroup$ @slebetman "I'd expect the CG to be somewhere in between the trailing edge of the canard and the leading edge of the wings." Well, that doesn't narrow it down very much! $\endgroup$ Jun 1 '16 at 18:11
1
$\begingroup$

To add to Greg's answer (which points to the two most obvious suspects), I'd say that a typical stable canard should have a higher incidence on the canard than on the wing. You can offset it with a constant pitch up trim, but apparently you don't have enough of it (for the chosen CG position).

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.