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.
[As an aside, perhaps, Andrew Wright might care to elaborate on how _he_, as appointed navigator (I was just flying the thing), managed to get us hopelessly lost within the Gawler circuit area. He is supposed to know this airfield better than the back of his hand.]<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).
Spins: For middle to forward c.g. positions spins are not possible.
Inverted flight: 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/
