Leigh.
I cannot recall discussing the inverted spining of the ASK21 with you at all.
I think, in general discussion with A W I mentioned, with qualified
reservation, I'd had a discussion with Wolfgang Lieff some time ago and
he'd related some nasty qualities the aircraft had displayed during an
inverted spin while he was attending an aerobatic test flying course. My
memory of his advice was so hazy that I intended to re visit the topic with
him but have'nt done so yet. To get the record straight I'll ask Wolfgang
to provide us with the facts thru aus-soaring.
I've not had any serious concern at all about the ASK21 inverted stall
turning into a spin. I have however used the manouevre (inverted stall)to
illustrate and amplify its value in recovering from scary situations when
an aircraft is inverted e.g. minimum height loss and low speed recovery
from inverted.
Regards,
Noel.
At 05:58 PM 3/25/01 +0830, you wrote:
> Hi All, This email was to go to Noel Roediger, but as I don't have his
>email address and know he reads this list, you can all see it. During the
>Gawler World Comps, the Balak club had one of our K21's on site and I was
>doing some demonstration flights. <G> Noel expressed serious concern that
>the inverted stalls we were doing could turn into inverted spins. I knew
>from other sources that the ASK21 was not prone to doing these. However, as
>some of you know, the Balak club is pursuing approval to use tail weights
>to enable the K21 to be used for spin training. Having got the
>airworthiness aspects signed off, it is now in the hands of the GFA Ops
>panel. Another K21 owner, having heard of all this, has also put his 2
>cents worth in by obtaining a report done by the USAF at Edwards Air Force
>Base in 1989 on the K21's spin characteristics and especially using tail
>weights. This document is 118 pages long and is a _fascinating_ read. In
>relation to inverted spins, they discovered that it can be done and I have
>lifted relevant sections and reproduced below. But first, this was the
>response to the USAF that the manufacturer gave on what they had found
>about spinning the K21: "ANNEX (5) Enclosure to our letter dated Dec.19,
>1988) TRANSLATION BY A.SCHLEICHER Dipl.Ing.Stich. Braunschweig, 21.03.1980
>Aerobatics flight testing with the glider ASK 21. OO-ZLN. carried out as
>Braunschweig on March 20, 1980 With five aero tows to FL80, two test pilots
>with a co-pilot in the second seat could terminate the aerobatics flight
>testing. The in flight c.g. was in the forward to middle range. The Pitot
>probe was made longer by 7 cm (by an insert). For middle to
>forward c.g. positions spins are not possible. Inverted spins could not
>be achieved neither by static nor dynamic entry. There is a strong wing
>dropping tendency, but by one half positive loop level flight can be
>regained at 130 to 150 km/h without a major loss of height." With my
>experience I can only but support this. Certainly, I have not added to any
>inverted stall with a boot full of rudder. Solo, depending on how severely
>one wants the blood to rush to one's head, a wing can be made to drop and
>certainly two up, it is very unstable at the stall and you can feel one or
>the other wing just itching to drop even at -1g. The following is the
>section from the USAF report on inverted spinning: "INVERTED SPINS The
>main purpose of Phase IV testing was to verify if an inverted spin mode
>existed. This verification was important to operators at USAFA since their
>aerobatic training in the ASK-21 involved extended inverted flight
>maneuvering. Manufacturer test data (see Appendix 1) indicated inverted
>spins were impossible. For these tests, manufacturer's flight manual
>instructions required the pitot probe extension be installed and no tail
>ballast be used. Without tail ballast and with the lightest weight test
>pilot, the most aft c.g. achieved was 15.84 inches. However, since other
>ASK-21 gliders have a more aft empty cg and other pilots are lighter in
>weight, aerobatics could be flown in the ASK-21 with more aft cg's.
>Therefore, the aircraft was configured with special ballast in the front
>seat and flown solo from the rear seat. There were four flights flown with
>this front seat ballast A fifth flight was flown solo from the front seat.
>These combinations produced cg's between 15.84 inches and the aft limit of
>18.46 inches (see Table Al). Inverted spins were achieved at each of these
>cg's. Susceptibility
> For the most forward cg tested at 15.84 inches, an inverted spin could
>only be achieved if lateral stick was maintained opposite the yaw (cross
>controls). In reference to the equations of motion in Appendix H, lateral
>stick against the spin was a prospin input for inverted spins. Aft of 17
>inches cg, sustained inverted spins were possible without maintaining
>lateral stick against the spin. This indicated that inverted spins were
>less likely to occur at cg's forward of 15.8 inches since control positions
>were more critical. Overall, the ASK-21 was extremely resistant to inverted
>spins since only Phase D inverted stalls resulted in inverted spins,
>regardless of cg. Although results indicated increased resistance forward
>of 15.8 inches, this does not imply inverted spins at more forward cg's are
>impossible. Characteristics:
> Inverted spin entry was not achieved forward of 17 inches cg from static
>maneuvers such as straight ahead, inverted, and wings level stalls. The
>dynamic entry maneuver in Appendix A (Table A6) was designed to simulate a
>student's overcontrolled inputs when attempting a slow roll aerobatic
>maneuver. The aircraft was pulled to a 20-degree nose high attitude from a
>90 KIAS cruise condition (shallow dive). A roll was then initiated with
>full lateral stick and rudder pedal. When passing through 90 degrees of
>bank, forward stick pressure was applied to keep the attitude above the
>horizon. This produced negative g, requiring opposite rudder pedal for
>coordination. These inputs were intentionally exaggerated to full forward
>stick, full lateral stick, and full opposite rudder pedal, which produced
>significant yaw rate at the 180-degree bank angle point in the roll. This
>also produced some roll coupling which assisted spin entry. Full
>cross-control inputs were held until spin entry was achieved. The ensuing
>departure and spin entry was similar to the upright spin. The nose fell to
>approximately 60 degrees nose low and then hesitated. Cockpit g forces
>built up to -2g and then the nose oscillated up to 40 degrees nose low. The
>spin was developed after approximately 180 degrees of rotation and was
>oscillatory. Altitude loss was 200 to 300 feet per turn and the rotation
>rate was one turn every 3 to 3 1/2 seconds. At the cg's tested, the
>inverted spin oscillations occurred every 3/4 to 1 turn. Once the spin
>developed, g forces oscillated between -1 and -1.5 g, although as much as
>-2.3 g was observed during the departure for one spin entry before that
>spin developed. Airspeed oscillated near 40 KIAS and remained stalled
>throughout. Cockpit g forces were uncomfortable but other spin
>characteristics were comparable to the upright case. Inverted spin
>recovery was immediate (1/4 to 1/2 turn) when controls were neutralized.
>Altitude loss from initiating recovery to level flight was 400 to 500 feet
>Since the spin included a component of roll rate as well as yaw rate, the
>aircraft rolled to an upright attitude, without further pilot input. This
>resulted in a more pleasant recovery than was expected. Pilots felt that
>high-speed dives would be a problem if a roll to upright were required
>since this would force a delay in initiating dive recovery. Since the roll
>to upright occurred as a natural response to neutralizing controls in the
>inverted spin, high speeds during dive recovery were not encountered.
>Airspeeds were typically between 90 and 100 KIAS during inverted spin
>recoveries. Because these spins have not been previously documented and
>occurred in a typical training scenario, the manufacturer's flight manual
>should be revised to include the text provided in Appendix G.(R1)" In
>summary, one of their eight major findings from their investigations into
>the spin characteristics of the ASK-21 was: "8. Inverted spins were
>possible and occurred during aerobatic maneuvers if cross-controlled inputs
>were maintained sufficiently long." This little black duck has no interest
>in crossing the controls during an inverted stall and waiting to see what
>happens. Cheers,
>Leigh Bunting
>Colonel Light Gardens
>South Australia
>Find out more about Col. Light Gdns. here -
>http://www.cobweb.com.au/~pknight/clghs/
>
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