Okay, trip report. This was, as planned in the Robin R2160. Actual weight was approx. 730kg, vs. an MTOW of 900kg. QNH 1027, OAT zero C @ 3000' which was my "close throttle" height for most tests. OAT on the ground 4 C, dewpoint 3.
From the POH: Vs 63 kt, Vbg 78 kt, Vy 78 kt.
(i) From height, establish the aircraft in a Vbg stable, trimmed descent. Mark attitude with some sticky tape on the canopy. Write down/time V/S.
(ii) From height, establish the aircraft in a Vbg stable, trimmed descent at 45 degree bank left and right. Mark attitude again with some sticky tape. Write down/time V/S.
(iii) From height, establish the aircraft in a Vbg stable, trimmed descent with the mixture closed. Write down/time V/S.
V/S at Vbg, zero AoB, with throttle closed was 800-900 ft/min.
V/S at Vbg, 45 AoB with throttle closed was 1100-1200 ft/min. Stall warner was going off, but no noticeable pre-stall buffet was felt.
V/S at Vbg, zero AoB, with mixture closed was 1000-1100 ft/min.
The sticky tape on the canopy was really useful. It allowed me to set the proper attitude in the subsequent tests immediately.
The difference between throttle closed and mixture closed is between 10 and 20%. So we need to correct all subsequent measurements for that.
All V/S were from the VSI. I did not want to spend a lot of time descending through 2000 feet or more and timing the descent with a stopwatch. Maybe next time.
All items start from a stable, properly trimmed Vy climb along a line feature, into the wind as far as possible. When passing through a certain safe height (3000' most likely, weather permitting) I'm going to pull the power and:
a. Hold the nose up to see how long it takes for the aircraft to stall. Then recover the stall and see how much time/altitude I lose before in a stable Vbg glide.
b. Hold the stick in the same position, furthermore as in (a).
c. Completely release the stick, let the aircraft sort itself out, furthermore as in (a).
d. Fly the aircraft in a half-G pushover, furthermore as in (a).
e. Fly the aircraft in a ballistic arc (zero G), furthermore as in (a).
A couple of surprises here.
- The fuselage angle between Vy (full throttle) and Vbg (closed throttle) is actually only about 40 degrees or so (20 degrees nose up vs. down). Because of this, it really doesn't matter whether you do a zero or half G pushover, or simply let the nose drop all by its own. In all cases (except a and c) I was fully established in a Vbg glide while I dropped through the "close throttle" altitude.
- Holding the nose up as in (a), it took 4 seconds for the stall warner to sound, and 7 seconds for the aircraft to be properly stalled. But even then, I was able to recover to a stable Vbg glide 100' below the "close throttle" altitude.
- Completely releasing the stick (as in (c)) got the aircraft in a huge pitch/speed oscillation. I let the aircraft descend over 1000' but by then it had not yet recovered from the oscillation so I ended that test prematurely.
- Despite Vy being equal to Vbg for this aircraft, there was a *huge* difference in trim. With the R2160 the all-flying tailplane is in the propellor slipstream and that will be the primary reason. I expect this to be different in different aircraft.
Subsequent tests include one second of "oh ****" time after closing the throttle, then:
f. Establish Vbg according to the best technique from (a)-(e), then turn into the crosswind 225 degrees at 45 degree bank, turn the other way 45 degrees at 45 degree bank.
g. Same as (e) but with 60 degree bank
h. Same as (e) but with 30 degree bank
i. Like (f) but start with a left turn 45 degrees away from the crosswind, then turn 225 degrees into the crosswind (teardrop thus reversed). Altitude lost should be the same as (f), but you never know.
I actually used two seconds of "oh ****" recovery time after closing the throttle.
With (f) I was able to consistently complete the maneuver in 450'. At the end I was in a stable Vbg descent, and all I would need to do is flare. That flare obviously takes a few feet. The (limited, see further down) GPS data I have suggested that the 225/45 degrees works quite well to regain the centerline in the no-wind situation. I was too high up to confirm this overhead a line feature though.
(g) and (h) gave me 800' and 700', respectively. I only did this once but the data suggests another confirmation of Rogers.
(i) was also done in 450'.
Both (f) and (i) felt really comfortable. Especially during the 225 degrees turn, there is time to do something else, like issuing a very quick mayday call to make sure the runway is cleared.
j. Like (f) but this time roll into a 45 degree bank while pushing the nose down (at half or zero G as appropriate), ready to pull as soon as the Vbg attitude has been set.
This did not make a significant difference. Dropping the nose from Vy climb to Vbg is a very quick process, and so is rolling to 45 bank. I did this test only once and lost 450', just like (f) and (i).
k. Like (f) but use the Mark1234 technique of immediately rolling to 60 degrees bank, pull to the stall warner while the nose drops, play angle of bank against stall once the nose is at the Vbg attitude.
I tried this a number of times. It feels like a very aggressive maneuver which requires very precise aircraft control. I would not be happy doing this for real - it feels like it's too easy to lose control. But it did give better performance: The full maneuver was completed in 400' both times I tried.
I will also run an outdoor-type GPS which has track recording
The track recording of the GPS turned out to have an interval of approx. 5 seconds. It doesn't really help me with the analysis. I also had a camera installed in the cabin (which was indeed not even half-decent) but unfortunately it was not aligned properly and the instruments are not visible.
To summarize, I was able to complete the turnback consistently in 450', with only moderate technique. Obviously this was a very limited test and that number should be corrected for a number of things:
- I was not able to confirm that I would be able to make the runway. Just that the turnback would be completed in that altitude. Properly reaching the runway might require a further glide and thus further altitude. And obviously all this depends on climb performance anyway: If your climb performance (angle) is less than your best glide performance (angle) you'll never reach the runway even if you were able to turn back without altitude loss.
- I flew all maneuvers with the throttle closed. As my initial tests showed, we need to correct the numbers with approximately 10-20% to compensate for the residual idle thrust.
- All figures still need to be corrected for OAT, altitude, QNH and the fact that I was well under MTOW.
- All figures are specific for the R2160
- All figures are specific for *my* pilot technique. YMMV.
Personal lessons learned:
- You have about 2-3 seconds to recover from the "Oh ****" reaction. It doesn't matter all that much how fast you react and how aggressive you pushover
as long as you don't stall. Whatever energy you lose on the way up, is recovered on the way down. (Obviously the longer you dally, the further you end up from the runway.)
- You have to fly the aircraft to the Vbg stable attitude. Just letting the plane sort itself out doesn't work, especially considering the huge trim change.
- Knowing the proper Vbg attitude is very important. The faster you can set this, the earlier you can start the turn and concentrate on other matters. And the Vbg attitude was less nose down than I thought.
- The turnback (225/45 or 45/225 as appropriate, at 45 AoB), with the engine windmilling, will take *at least* 500 feet, and possibly closer to 600-700 feet under the wrong circumstances. You then might need some additional altitude to reach the threshold. For me personally, this means I'm going to keep on using 1000' as minimum turnback altitude, assuming a straight-out departure. (Turning crosswind at 500' obviously greatly improves your chances to make the runway though.)
Note: All the usual caveats apply. Do not change the technique you've been taught just because some anonymous bloke dumped a few numbers plus some mumbo-jumbo in an anonymous internet forum. If you decide to deviate from the technique that's been taught to you by a qualified instructor, you do so at your own risk, and I suggest you first validate your prospective technique yourself. Yada yada.