REFLECTOR: Deep Stall Descent

Ron Brown romott at adelphia.net
Tue Dec 27 09:00:20 CST 2005


Here's a very interesting post re flat stall descent rates for canard 
aircraft!!!!  Perhaps NASA should do some more research studies!

By the way, hope everyone is having a very enjoyable holiday!  (You must be, 
no body seems to be building as evidenced by the extreme silence of the 
Reflector of late!)

Ronnie Brown



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There are 2 messages in this issue.

Topics in this digest:

      1. Re: deep stall
           From: Mark Spedding <spodman at megalink.com.au>
      2. Re: aileron hinges
           From: Gary Hall <gary.chris at comcast.net>


________________________________________________________________________
________________________________________________________________________

Message: 1
   Date: Tue, 27 Dec 2005 23:37:09 +1100
   From: Mark Spedding <spodman at megalink.com.au>
Subject: Re: deep stall

Hello,

Am sitting here typing with one eye shut as a result of damaging a
cornea at a bushfire, so had time to look at old CP's. The last is the
longest and most interesting.

CP34:
"The pilot of a southern California Long-EZ was seriously injured and
passenger suffered a broken hip when the airplane crashed into a dry
river bed. The eye witnesses to the accident reported that the airplane
was doing aerobatics. It appeared to enter the beginning of a loop, did
not have enough speed, fell out of the maneuver. The engine stopped,
(negative "g" will cause a carbureted engine to suffer fuel starvation)
the aircraft nosed over and spiralled down to about 100 feet, where its
wings were leveled and it descended until it struck the ground. The
aircraft hit a 20 degree embankment almost wings level and slid forward
only about two feet. There was no fire, although the right fuel tank was
ruptured."

If it only slid forward 2 feet it must have been coming down almost
vertical and at a startlingly low rate of descent. I think a BRS would
have done better.

CP39 (fatal):
"We have carefully examined the wreckage and it appears that the
airplane struck the beach with very little forward speed in a flat
attitude. There was no evidence of rotation."

Hmm. I reckon a BRS would have done better.

CP61 (fatal):
"It was obvious from 300 feet above the crash site that the VariEze had
impacted inverted, with little or no forward or lateral velocity. This
was evidenced by the mud splash marks radiating out from the center of
impact...it is our belief, based on our knowledge of the VariEze design
as well as our previous experiences examining several crash sites
somewhat similar to this one, that this aircraft fell essentially
vertically onto the surface of the salt pan. It struck the salt crust in
a nose low, wings level, but inverted attitude. "

Hmm. No better upside down.

CP68 (the one I was looking for):
"THE FOLLOWING IS AN ANALYSIS OF THE UTAH ACCIDENT
The Utah accident involved a deep stall, flat descent (angle of attack
of about 80 degrees). The fact that the pilot survived and that a
slower-than-expected sink rate occurred (confirmed by video tape
evidence of the last 2.3 seconds of descent) presents somewhat of a
dilemma. We are baffled as to why this can occur. A similar phenomena
has been experienced during several deep stall accidents with the
Velocity aircraft. All were survivable and one went into water with the
pilot experiencing no injury at all! (See article in July '91 Sport
Aviation.)
The Utah Long-EZ had a wing-loading of about 12.2 lbs./sq. ft. and,
considering all its area, including the wings, strakes, cowl and
fuselage, a "flat-plat loading" of about 9.2 lbs./sq. ft. (1150 lbs.
divided by 125 sq. ft.). A basic calculation of the predicted rate-of-
sink in a flat descent would use a flat-plate drag coefficient of about
1.2 and would predict a sink of about 4820 ft. per minute or 80 ft./sec.
This would definitely not be survivable.
Using two different methods, we have calculated that the Utah Long-EZ
probably had a drag of about 2.8 times that predicted by simple flat-
plat theory, i.e. a co-efficient of about 3.3. This results in an energy
at impact of only about 1/3 that which would result from the "calculated
prediction" sink of 4820 ft./min. Here's the two methods:
1) Analysis of the video tape shows a sink rate of about 48 ft./sec.
(2900 ft./min.). This required measuring the size of the airplane image
and may be off as much as 30 percent. The post-crash video data show the
rate of drift of dust from impact. Comparing this rate of drift of dust
(wind was about 20 knots) to the rate of sink of the airplane (on video)
confirms the approximate 48 ft./sec. estimate.
2) Assuming a 48 ft./sec. descent, the main landing gear would absorb 18
ft./sec. before the fuselage strikes the dirt - this is a relatively
accurate calculation knowing the gear's stiffness and strength.
Absorbing the remaining 30 ft./sec. over a total deflection of
approximately 6.7" (cushion, plus fuselage, plus dirt), results in an
average deceleration of about 25 G with a peak deceleration of about 40
G. Considering the support and attitude of the pilots back, this is
consistent with the injuries he sustained. An 80 ft/sec descent would
result in a fatal 150+ G impact of the spine.
Both these methods are very rough but (along with the deep stall
experience with the velocity) they tell us that an unusual phenomena is
occurring. It is likely that a large, trapped vortex forms above the
aircraft. It's relatively easy to see how this could increase the drag
by 25 to 50 percent, but it makes no logical sense that it could
increase drag by a factor of 2.8 - this would require the airplane to
decelerate a column of air that is more than 3 times the size of the
airplane! What is even more baffling is the report (not confirmed by us)
that the Velocity aircraft sinks at less than 1500 ft/min (15 knots!).
If that were true, it would have to have a "flat- plate" drag
coefficient of about 12! ! (A totally illogical result). We suspect that
the Velocity and Long-EZ have similar drag coefficients and that the
cushion of water landing provided the difference in pilot injury.
The Utah pilot had one thing going for him, he was sitting on seat
cushions fabricated from Tempa-Foam an excellent impact absorber."

Hmm. Did he figure out a way to build it out of helium?

If anybody wants to feel little depressed do a search for 'crash'
through the CP's.
-- 
Cheers,

Mark Spedding - Spodman
Darraweit Guim - Australia
Theoretical Cozy IV VH-UMI
Celebrating one year of not building anything.



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