Coordinated turning stall and spins

Posted this to the discussion on spinning Blaniks from a coordinated turning stall.

November 9, 2003 Turning Stalls and Insipient Spins

As promised, apropos to this discussion on spin entry from coordinated turning stalls, I took a tow this morning to 5000 feet agl and performed a series of coordinated and cross control turning stalls.

The aircraft used was a Ventus 2bx, delivered this year. I have approximately 75 hours in this aircraft and about 525 hours total in the model. I flew the glider at approximately 70% of the aft cg limit. Wing loading was 7.8 lbs per square foot. All stalls were entered in the first positive flap position.

My intention was as follows: to perform a series of turning stalls, both coordinated and cross controlled, to determine the departure and post departure characteristics of a modern fiberglass sailplane. Stalls were entered gently and in a shallow bank (lower wingtip on horizon). Whether coordinated or cross controlled, I fixed the controls in the pre-departure position for three full seconds after departure (that is, no attempt was made to recover immediately after the stall break).

Once off tow I completed two clearing turns, then stalled the glider wings level twice to establish attitude. I then entered a coordinated shallow left turn and gently eased back on the stick. The stall broke cleanly. The glider initially yawed about 30 degrees to the left, dropped its nose through the horizon, then began to increase its bank angle and gain speed. G forces accumulated and I recovered from the spiral dive at about 80 knots and roughly 70 degrees of bank. (As noted above, the elevator was held firmly aft and aileron and rudder neutral until recovery was initiated.

I repeated the same maneuver to the right. The stall break was less clean (more mushy). Development of the ensuing spiral dive was slower, but airspeed and bank angle both accumulated until I released the controls and initiated a recovery.

I repeated this sequence with like results.

I then entered a shallow bank turning stall (left) while skidding slightly. As the low wing began to drop, I applied about ½ stick travel to the right, ostensibly to raise the dropping wing. Entry into the spin was immediate and dramatic. The glider yawed approximately ninety degrees while dropping it nose to about 60 degrees below the horizon. I left the controls in this position for a count of three (one one thousand, two one thousand…) The glider completed approximately 1.25 rotations before I initiated a recovery (stick forward, ailerons neutral, opposite rudder, pull up from dive).

I repeated this process to the right. However, this time, I gently accelerated the stall (achieving a slightly higher nose attitude before departure). Once again, I skidded the turn (10 to 20 degrees), and tried to pick up the low wing as it stalled, this time with full deflection of the aileron. The ensuing spin entry was even more dramatic. I was unable to measure rotation rate (even roughly) because the glider's nose went immediately past vertical. As I lost the horizon I became disoriented, until I looked out at the wingtip and found the horizon again. I nonetheless fixed the controls for a count of three. There was no noticeable g build up until I initiated a spin recovery. Max speed during the dive was just above 120 knots, about 20 knots more than I typically see for a recovery from a fully developed spin.

It should be noted that my glider has a flap redline of 80 knots. In all cases, if airspeed exceeded 80 knots, I moved the flap handle to the first negative position.

My interpretation: while the glider exhibited a yawing motion during the coordinated turning stall, it did not auto rotate, nor did it show any such propensity. Some pilots may find the dropping wing, yaw motion, and reduced g force of a coordinated turning stall disquieting, but when compared in sequence to an actual autorotation leading to a fully developed spin, the prior is patently docile. Height loss after an immediate recovery from a coordinated turning stall using a release of back pressure and coordinated ailerons and rudder could be measured in 10s of feet. The spin, however, from entry to the bottom of the dive recovery was measured in hundreds. Loss of height for the first spin, from entry, through development, to the bottom of the recovery dive was 475 feet. The second: 750 feet.

Conclusions: draw your own.

Good test card Chris.

Thanks for the great info.

9B

At 00:00 12 November 2003, Chris Ocallaghan wrote:

Excellent post, I do have a couple of questions. Did the glider start to recover from the spin with positive flap selected? My understanding is that in a flapped glider the first action is to zero the flaps. Do you think that by selecting a negative flap setting this accelerated the glider to a greater velocity than selecting zero flap would have done? (I do not know offhand the limiting speed for a Ventus in zero flap)

You quote figures of 475ft and 750ft for the fully developed spiins. Do you have any figures for the spiral dives off the co-ordinated turns? (I do note that you delayed recovery for 3 seconds) It seems to me that any stall in the final turn will result in hitting the ground before recovery can be completed which bears out what I have always said, teaching people to recognise the approach of a stall and/or spin, and take appropriate preventive action, is more important than teaching spin recovery.

One final question, if a spin is entered at 300 feet should recovery even be attempted? Are the chances of survival greater if the glider hits the ground spinning than if it is part recovered and 'tent pegged'? Interesting what?

At 00:00 12 November 2003, Chris Ocallaghan wrote:

Chris,

Thanks for taking the time to post this report.

It seems that the test aircraft is very unforgiving of poor recovery technique. Is this typical of modern 15m ships? How would it have behaved in the same situation with a full ballast load?

When I transitioned from the ASW-19 to the ASW-28 I explored its characteristics in turning stalls at the aft cg limit and found, just like the 19, it was benign even with abused control inputs.

Andy (GY)

Similar tests were also done in South Africa on a fully ballasted Ventus 2b after a fatal accident there last year. The findings were that the ship was generally very stable, but could snap into a spin if not properly controlled - sometimes going inverted. Recovery heights were alarmingly large. The fatal accident occurred off tow from under 1,000 feeet, as I recall.

Mike

ASW 20 WA

My resosponses in-line below...

Correct, I should have added that as part of spin recovery I moved the flap to the first negative position. Though this is not expressly dictated in the flight manual, the ensuing dive will certainly exceed the flap redline (including 0 degrees). And, of course, dumping the flap will immediately decrease AOA.

Approximately 200 to 250 feet, including the 3 second delay prior to recovery. Unfortunately, my trace from the flight is not particularly instructive. My FR was set at 4s intervals, so it doesn't show much detail. Height loss is interpolated from the pressure altitude trace as rendered in SeeYou.

That's a tough one to answer. I see your point: better to hit the ground at 70 knots than 100 knots. In either case I suspect the results will be the same. I suppose it a matter of whether you expire at the scene or several hours later in an ICU. To that end, I'd always try to recover

Thanks for your responses. My reason for posing the final question is not so much speed as deceleration. I once saw a tiger moth hit the ground spinning, both pilots survived albeit one is now a paraplegic. It is not just forward speed but a matter of what strikes the ground first, a matter of pure chance I agree. What is certain is that in a dive the most likely thing to reach the scene of the accident first is the nose of the glider where the soft part sits. This may indeed happen in a spin but forward speed is not the main factor, it is the rate of descent and will this be less in a spin than in a dived and accelerating condition? It's not speed that kills you it's stopping. I really don't know and I am not eager to find out either Whatever the answer the best solution is to avoid the spin in the first place but sadly it is not a perfect world. The only difference between a fatal and non-fatal accident is the dead body and that can also be a matter of pure blind chance.

At 17:06 12 November 2003, Chris Ocallaghan wrote:

As a matter of interest in this subject let me provide the following. In the early 1960s I rebuilt a Pratt-Read glider. This glider was used by the USA Navy during early WW 2 (1941-42?) for flight training. During the rebuild I obtained an original Navy Flight Manual for the glider. In the manual, in bold print, was a sentence that stated ' If entering a spin below 1000 feet DO NOT attempt recovery.' The reason for this was that the Pratt-Read tended to spin flat. Recovery from a spin was near vertical for several hundred feet at a speed of more than 100 mph. Vertical speed in the flat spin was something like 500 ft/min. Therefore it was deemed safer to hit the ground in a flat spin rather that nose down in a vertical dive. I would think that most (all??) modern gliders will not spin flat. Therefore, whether one allows the spin to continue or attempts recovery, the attitude of the glider will be nose down. With a recovery attempt there is at least a chance of survival.

Don,

I underestand completely your concerns. It's a subject that's troubled me for a long time, and I seem prone to flip flopping. The problem isn't so much a question of energy... you'll have less in the spin than in the ensuing dive after recovery (both of which are nose down), but having a 'procedure' that you can apply without thinking. When close to the ground, you simply don't have time to observe and react to more than a few inputs. For example, if I were to cross-control the aircraft into a stall below 300 feet, if I were over trees, I might just lock up the controls, close my eyes, and get ready for the hurt. But to do this I would have to overcome my rote training... that is, if I sense a departure, I recover immediately. I'm not sure that type of switch would be valuable. The lesson I've taken away from this discussion is that in the pattern, the yaw string stays bolt straight. An unexpected stall can be handled if the aircraft is coordinated. If not, the bottom falls out quickly.

If you accept as axiomatic that a stall can happen at any speed and at any attitude, then I have to place priority on coordination fist, airspeed second, though both are clearly primary concerns in the pattern.

It is a virtue, or perhaps a nuissance, of our sport, that when near the ground, the envelope narrows significantly. Between 1000 agl and 10 agl is like climbing solo. Falling is not an option, and we need to attune ourselves to that.

I must mildly object to this general characterization of the Lark, although it's possible that they vary considerably from one IS28 to another. I have never flown one that behaves as you describe.

The IS28B2 that I just sold (SN 102) behaved more or less as you described the behavior of the Ventus 2bx. As such, I think of the Lark as a wonderful advanced trainer for pilots making the transition to glass. My time in the Lark made my first Nimbus flights very comfortable.

Bill Daniels

During my recent instructor course (last September) I had some spin training in a PW6 (no other glider allowed to spin was available). I found it very easy to enter as well as to recover the spin. After that the (meta) instructor said 'I guess that even if you release all controls, it will recover', so we tried it and it recovered, then I said 'But we had the trim set for flight at max L/D, I guess that with full aft trim it does not self recover', we tried it and it did not self recover.