Nerd Fever

Saturday 2011-10-22 saw the first three flights of the rocket glider.

(Most of this post is adapted from this thread where there are more details.)

Only one of the flights was really good, but we learned a lot from all 3; and the glider is in good shape.

Boris and I decided to name it the “Autonomy”. Our goal for the day was to get some flight information about the proper pitch setting for a nice stable glide.

There is still a lot of data analysis to be done, but you can look at the videos below. The whistling noise on the audio track is a good indication of relative airspeed, I think.

Flight 1 – Roadrunner G80 (115 N·s)

Video from the ground:

Video from the on-board camera (looking out the bottom of the glider):

Immediately after apogee the glider went into a sharp roll to the left. This was because I forgot to turn off the navigation system, which hasn’t yet been calibrated for the roll sensitivity of the glider. It commanded left yaw and put it into the roll. While spinning it lost lift and dived toward the ground.

The parachute deployed at 200 feet AGL as planned. No damage other than a very tiny zipper (similar to the one on Friday).

Flight 2 – CTI/AMW H54 White Longburn (168 N·s)

This was our best flight of the day. I remembered to turn off the navigation system.

Video from the ground:

And from the on-board camera:

The glider arcs over a little prematurely. Looking at the video, I think this is because the “straight” boost position for the elevons actually is slightly pitch-up, so the glider tends to arc up (toward the rudder) during boost.

I think we can reduce this in future flights by (a) adjusting the boost position a little bit pitch-down, and (b) adding a little bit of yaw command to the boost position to induce a slow roll during boost to even out any pitch-up/pitch-down tendency.

By chance the glider was right-side-up at apogee, and the transition of the elevon position from straight (for boost) to the glide position is very clear in the video. When the elevons go to glide position the glider does an Immelmann turn (due to the high airspeed at apogee – this was a pure accident) and then settles into a steady glide.

The really good news here is the glider does seem to be self-stabilizing due to the dihedral; regardless of what attitude it was in at apogee, if left alone it seems to settle into a stable glide.

During the glide, Boris was manually adjusting the pitch setting via the telemetry system. The effects of this aren’t too obvious in the video, but I’m hoping to learn more when I look at the logged data (servo position vs. climb rate, airspeed, etc.).

Three seconds prior to parachute ejection, as planned the elevons go full-up to induce a flare for airspeed reduction. This is hard to see from the ground video, but the pitch change is pretty clear from the on-board video.

I plan to look carefully at the GPS data from this flight (also correlating that with the two video streams). I hope to get a sense of the forward airspeed of the glider (at various pitch settings), whether or not we have enough control authority to do a full-stall flare (and how long it takes to do that), sink rate, etc.

Flight 3 – CTI/AMW H100 Imax (286 N·s)

On the last flight of the day we tried a somewhat larger motor, aiming to get more glide time and a chance to try manual yaw control.

This time I worked the radio controls (probably a mistake) and Boris ran the camera.

Ground video:

On-board video:

Again you can see the glider pitching up during boost, causing a premature arc-over and too high an airspeed at apogee. It’s more pronounced here than in the prior 2 flights.

It’s hard to tell from the ground video (Boris, you need to zoom in more!), but you can see that at about 0:13 on the on-board video, as soon as I switched on the manual yaw control, I massively over-controlled it and the glider went into a sharp left roll, and dived that way toward the ground, much like in flight 1.

Clearly the glider is VERY sensitive in roll (as predicted by some posters here).

On flights 1 and 3 (the bad ones), the parachute ejection was triggered by the failsafe “TimeToImpact” calculation, well above the 200 foot (60 m) deployment altitude. The rocket decided it was less than 4 seconds from impact while descending at > 30 MPH (13 m/s actually) so it popped the chute, despite still being well above the deployment altitude.

Only on flight 2 (the good one) did the 200′ altitude trigger a flare and then deployment.

Unfortunately, on flight 2 it looks like the GPS signal dropped out partway thru the glide. At first I thought this was because the camera (wrapped in foil) shadowed the GPS antenna, but looking at it again I don’t think that’s it. I’m not sure what it is – I haven’t seen this on prior flights (under a parachute).

For our next flights (November 5 is the schedule), I’ve made the following changes:

• Reduced parachute deployment altitude from 60 m (200′ AGL) to 45 m (150’ AGL) to get more glide time.
• Reduced YAW_FACTOR from 1.0 to 0.1 (100 to 10) to make it less sensitive. This makes the servo response less sensitive by a factor of 10 in yaw – this is the most I think it can be reduced and still have positive control in yaw.
• Tweaked the port elevon 3 half-turns down to try to trim out gentle left-turning tendency seen on flight 2 of 2011-10-22.
• Adjusted YAW_STOWED_POSITION from 0 to -1 (50 to 0) to induce a slow roll during boost to counteract any pitch tendency during boost.
• Adjusted PITCH_STOWED_POSITION from -0.4 to -0.54 (30 to 23) to counteract pitch-up tendency during boost seen in 2011-10-22 flights. I think AstronMike is correct – this is caused by differential drag on the rudder (hadn’t thought of that – thanks for pointing it out!).
• Lengthened the flare period from 3.0 to 4.0 seconds.

Maybe other things once I’ve looked more carefully at the logged data. We’ll see.