I'm working on some small (tiny) hypersonic aircraft and have run into a variable trim problem. At subsonic speeds the aircraft will nose down. As speed increases trim will improve until, at somewhere in the 760-1100 mph threshold, it will be perfect. But near the 2000 mph mark the aircraft will all start to pitch up. Any attempts to improve trim at one end of the speed range makes it all that much worse at the other end. Adjusting the trim in flight doesn't help due to the small size of the aircraft, with any changes being too little or too much.
I believe that variable trim is a problem that all aircraft have to some degree, but with hypersonic aircraft having such an extreme speed range the problem is more extreme that usual.
About the only idea I have is to put on a set of hinged wings that would allow the pilot to shift the CoL. Does anybody have any ideas on what I can do to address this?
Thanks.
You’ll always get nose droop at low speed for a realistic hypersonic craft. I simply made it controllable and limited drop as much as I could. As for why my canards don’t work on your craft:
You’ll need to make sure the wing part isn’t connected to anything else. If you continue having issues, I can help @atgxtg
@LiamW Thanks. The wings are almost perfect for one of my airframes. I see you still got some nose drop at low speeds too. BTW, I tried to swipe your carnards, but they just drop and do nothing unless I give them a angle and speed (then they work). What am I missing?
I’ll link a recent hypersonic WIP build of my own. I used all moving canards @atgxtg
@LiamW -That's hard to do within the the zero drag constraint. Although it's not high speed that is a problem so much as low speed. When flying at high speed I'm either maneuvering or at high altitude, both of which minimize the problem.
But...
I think I can combine DerekSP's and F104Deathtraps ideas and downscale 'em to fit. I'm going to try to add airbrakes at strategic locations (top/front, top/aft, bottom/front, bottom/aft) and have them activated by Trim. I can invert the activations for the "down trim" brakes and use activation groups to set the brakes for low speed or high. If I've got a clue, I could also use the brakes much like flaps to assist in take-offs and landings.
You need to get your centre of thrust perfectly centred with the centre of mass to have it fly straight without trim at high speed.
Some interesting ideas, thanks.
The two control surfaces seems the most applicable to my tiny aircraft (one design is smaller than most cars), especially since one design already has canards so it would be easy to implement without actually changing the design.
The problem isn't that applying trim (1/4/7) won't counteract the nose dropping or lifting, but that it applies too much. Rotators on the horizontal stabilizers might do it. I tried rotators on the main wing with some success.
Unfortunately a gyro isn't an option. What I haven't mentioned yet is that I got some fuselage shapes down to 0 Drag Points, so I'm limiting myself to options that won't increase drag. I tried nudging a gyroscope into the airframe but could not do so without eliminating the drag (best results were in the 110-135 drag range). But I will definitely consider it for some other builds on the same airframes.
In real life, the entire horizontal tail can rotate tiny increments in response to trim input. I would recommend you design your plane to do the same. Use a rotator with the unput set to "trim" and then set the range to something small, 5° or 10°. In real life, supersonic wings are symmetrical and generate no lift at all unless the tail is pulling the nose up a bit. If you're going above 300mph and 10° of pitch trim isn't enough to keep the nose from drooping, consider enlarging the main wings.
I would also recommend that you review the layout of the rest of your plane. Is the COM too close to the COL? Are the wings set to "semi-symmetric" or "symmetric"? Make sure your plane can land at a safe speed (preferrably under 150mph).
or have two dedicated trim control surfaces, and toggle them so one will work at low speed and the other at high
gyro