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inverted spin recovery explanation



 
 
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  #11  
Old August 17th 04, 12:21 AM
justin
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--
"Andrew Boyd" wrote in message
m...
wrote:

So if I normally commence a loop at 100 knotts but get the
entry speed wrong and start at 105 knots ....

Anyone care to formulate what happens when speed ( or "G")
are not constant?


Your speed and G are NEVER constant during a loop. A
vertical maneuver is always low and fast, then high and slow,
then low and fast again, etc.

You continually convert your kinetic energy at the bottom,
to potential energy at the top, then back to kinetic energy
on the downline. A hhead (aka stall turn) is a perfect example
of this. You go straight up until you stop, then pivot, and
fly down and gain airspeed again.

Given a constant density altitude, additional entry speed
implies additional G to make the same radius, assuming you
fly at (or near) the stalling AOA which generates Clmax.

Think of it this way: given that you fly at Clmax:

1) the radius of the vertical maneuver is a function of the
aircraft stall speed (Vs), and

2) The G you must pull or push is a function of the entry speed.

Does that make sense? It's not completely true - it will not
withstand a rigorous proof, but practically speaking, it's
what you really need to know to yank and bank down low.


Thanks for the informative posts. I wish we had more discussion on this
newsgroup about the physics of aerobatic flight. Here is an interesting
article in Air Force Flying Safety regarding optimizing the pullout in an
altitude-critical situation. I found it very interesting reading and
perhaps relevant to this discussion.
http://afsafety.af.mil/magazine/htdo...98/pullout.htm

Cheers!

-justin
yak52 driver


  #12  
Old August 18th 04, 05:50 AM
Peter Ashwood-Smith C-GZRO
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" An interesting fact, which is not obvious to many folks,
including
some aerobatic pilots (judging by the number of loop into the ground
accidents) is that the radius of any turn, up, down, sideways or
whatever, is a function of the square of TRUE airspeed, which is of
course a function of density altitude and calibrated airspeed.

So, if the density altitude increases your true airspeed by 5mph,
you get a 5mph^2 impact on your radius. This kind of change in radius
can ruin your day if you are playing down near the dirt.

This velocity^2 thing is also why the reverse cuban or loop down is
a real killer. If you start the pull with X knots too many, you will
use X^2 more radius for the 1/2 loop, throw in an increase in TAS of
say Y due to density altitude and you are into (X+Y)^2 more radius ...
not good. If you have not left margin either in terms of available G
or altitude you are either gonna high speed stall on the way down (and
hit the ground) or hit it on the arc.

I think this may need a little more explaining even if only for my
understanding. I am very new to aerobatics.

So if I normally commence a loop at 100 knotts but get the entry speed wrong
and start at 105 knots then my loop (assume horizontal plane and constant
speed for simplicity) will be

New_Loop_Diameter=Old_Loop_Diameter x New_Speed ^2 / Old_Speed ^2
i.e. a factor of 1.103.

A bad entry of 15 knots over speed would have a factor of 1.323.
But a target speed of 200 but entry of 215 would have a factor of 1.156.

If my understanding is not correct then please explain why. I prefer to
understand the physics/maths before I attempt some of these manoeuvres.

Anyone care to formulate what happens when speed ( or "G") are not constant?


Yes unfortunately while the radius is a function of velocity ^ 2, it
is also a function of a great many other things. The main point is to
recognize the non-linear relationship between speed and radius, and to
then recognize that density altitude changes produce non-linear
effects on radius.

I don't recommend you do anything with the math except make a big
'ahhhhhhh' sound and let it cause a sensible sense of dread in you
whenever you pull towards the ground.

By the way, a minimum radius pull is at a bit above the stall speed
so if you pull to just before the buffet you are pretty close
regardless of airspeed.

Cheers,

Peter
  #13  
Old August 18th 04, 02:47 PM
DaBear
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"Peter Ashwood-Smith C-GZRO" wrote in
message om...
" An interesting fact, which is not obvious to many folks,
including

I don't recommend you do anything with the math except make a

big
'ahhhhhhh' sound and let it cause a sensible sense of dread in you
whenever you pull towards the ground.

By the way, a minimum radius pull is at a bit above the stall

speed
so if you pull to just before the buffet you are pretty close
regardless of airspeed.


Hopefully someone isn't flying the bottom of a loop near stall
speed. Isn't the minimum radius turn going to be accomplished at
corner velocity and max-G pull?

Al


  #14  
Old August 18th 04, 03:30 PM
Dudley Henriques
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"DaBear" wrote in message
...
"Peter Ashwood-Smith C-GZRO" wrote in
message om...
" An interesting fact, which is not obvious to many folks,
including

I don't recommend you do anything with the math except make a

big
'ahhhhhhh' sound and let it cause a sensible sense of dread in you
whenever you pull towards the ground.

By the way, a minimum radius pull is at a bit above the stall

speed
so if you pull to just before the buffet you are pretty close
regardless of airspeed.


Hopefully someone isn't flying the bottom of a loop near stall
speed. Isn't the minimum radius turn going to be accomplished at
corner velocity and max-G pull?


Yes, turn rate (or time in the pull) is also a factor and max turn rate
and min turn radius are maximized if married with max available radial g
at corner. You should actually throttle up if you're behind corner!!!!
But you should never be in this situation going through a topside gate.
The airspeed should be married to the known radial g profile for your
airplane or a roll exit initiated IMMEDIATELY!!
The trick with the gate apex is not to go through at a higher airspeed
than the radial g profile you are using for the airplane will allow
through the backside commit. If the airspeed is too high through the
gate, roll should be used to exit instead of pull.
In low altitude demonstration work, the time to do your math is on the
ground. All you need to know in the air are your commit numbers. You
have them at the top or you don't...period! If you are going through a
top gate and ANY of the numbers are off and you don't execute a roll
save, you are well on your way to becoming a statistic!
I've attended more than a few funerals in the fifty years I've been
associated with low altitude acro. It's a totally unforgiving
environment!
Dudley Henriques
International Fighter Pilots Fellowship
Commercial Pilot/ CFI Retired

For personal email, please
replace the at with what goes there and
take out the Z's please!
dhenriquesZatZearthZlinkZdotZnet


  #15  
Old August 18th 04, 07:23 PM
Andrew Boyd
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(Peter Ashwood-Smith C-GZRO) wrote:

minimum radius pull is at a bit above the stall speed
so if you pull to just before the buffet you are pretty close
regardless of airspeed.


I think everyone agrees that you want to generate max lift
for minimum radius pullout, which by defintion implies Clmax
aka stalling AOA.

On the subject of airspeed, yes theoretically the minimum
radius will be achieved at the max G at which stalling AOA
may be maintained because the maneuver is completed in less
time, and thus there is less time for earth's gravity to pull
the aircraft down, ergo less altitude loss.

However this is potentially a very dangerous procedure if
not performed perfectly.

Many people like to wax eloquently upon what they would do
in various emergencies. They say that they would stop the
prop after an engine failure to extend their glide, or that
they would turn back after takeoff if they experienced an
engine failure.

Theoretically speaking, they would be correct. However if
they tried that fancy stuff in real life without considerable
practice they would quite likely kill themselves.

The critical, over-riding factor in a pullout is to get to
the stalling AOA. Yes, adding power can theoretically
somewhat improve things, but it will also kill you if you
aren't already at (and maintain) the stalling AOA - remember
radius is a function of velocity SQUARED - and you might over-G
and break the aircraft, or black out.

Increasing the G in a vertical pullout is a very dangerous
trick, especially in an ad-hoc, unpracticed scenario. Are the
benefits worth the potential cost?

Choose a gate altitude for your radius, with margin for error.
Choose a gate airspeed to keep your G within limits.

--
aboyd
  #16  
Old August 18th 04, 11:07 PM
Ed
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Every aircraft is slightly different, and the Pitts is most surely different
from the Decathlon. Looking in the NTSB database from 1983 to 2001, I see19
fatal spin accidents in the Pitts and only 4 in the Decathlon. Source data:
http://www.iac-chapter11.net/Fatal_A..._Accidents.htm

The Decathlon POH states that both rudder and stick input are required for
spin recovery. Can't remember the exact wording, but the basic gist was to
apply opposite rudder and anti-stall elevator at the same time.

Dave Swartz wrote an article on "unrecoverable spins", based on some spin
modes he encountered in a Decathlon. He described several instances where
he encountered rudder blanketing and was unable to slow the rotation. Both
pedals were "soft". Moving the stick back to neutral increased his rudder
effectiveness and allowed him to recover. As I recall, the spins he
described were inverted, flat, and accelerated. The link to the article,
and to Dave's website, doesn't work anymore; not sure why.

In my own limited experience, I found my Decathlon to be a bit reluctant to
spin inverted, and to quickly recover as soon as pro-spin inputs are
relaxed. For upright spins, I use the standard opposite rudder to stop
rotation, then stick to break stall, and it has worked every time so far.

Obviously the original poster shouldn't be seeking inverted spin instruction
over the internet. However, if he is just nervous about learning positive-G
acro without having had inverted spin instruction, that's not really too big
a worry. For any kind of blown maneuver in the Decathlon, just neutralize
all controls and let the nose fall through. Neutral controls will stop most
every incipient stall/spin in the D.



"Rick Macklem" wrote in message
om...
(Guenther Eichhorn) wrote in message

...
One important comment: DO NOT pull the stick back before the rotation

has stopped.
This is very dangerous since first you will accelerate the spin, and

then it may
get you in a cross-over spin, meaning you transition from an inverted

spin to an
upright spin, with the airplane rolling in the same direction, but

yawing in the
opposite direction. To recover from that, you now need to use the

opposite
rudder. If you don't notice that you have crossed over, you will more

than likely
not get out of the spin. A good friend of mine friend of mine died in

exactly
that situation.


My only concern with the about statement is that I'm not sure all

airplanes
will stop spinning without pulling the stick aft. (I recall in an inverted
spin in a Decathlon, I had to give it a little extra pull through neutral

to
get it to pop out. I've been told that Beggs-Mueller doesn't always work

for
the inverted spin in a Decathlon, which suggests some aft stick is

required
to stop rotation.)

I believe Sammy Mason's version is:
- Full rudder opposite to the yaw (he says opposite to the spin, which

I've
always found confusing for inverted spins)
pause
- then pull the stick smoothly back through neutral

In the three types I've done them in (Pitts, Decathlon, Extra), I never

needed
full aft stick. Of the 3, the Decathlon seemed to need the most aft stick

to get
it to pop out. (It's been a long time since I flew a Pitts, but I recall

it
recovering almost instantly and the Extra seems to come out easily,

although I
haven't sat there will full opposite rudder and full forward stick to see

if it
stops before any aft stick movement.)

ALWAYS first stop the yaw, then use the elevator however necessary.


Yes, I think that is a better way to put it. (And I think you'd agree that

it's
not the same as waiting for the spin to stop before moving the elevator.)

rick



  #17  
Old August 18th 04, 11:10 PM
Ed
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Hopefully someone isn't flying the bottom of a loop near stall
speed. Isn't the minimum radius turn going to be accomplished at
corner velocity and max-G pull?


Yes, turn rate (or time in the pull) is also a factor and max turn rate
and min turn radius are maximized if married with max available radial g
at corner. You should actually throttle up if you're behind corner!!!!



Question: how does one compute or determine cornering velocity for a
specific aircraft? Must that be done through flight testing, or is there a
mathmatical relationship to the stall speed/etc?

Specifically, does anyone know the cornering V for a Decathlon 160-CS?

Thanks!


  #18  
Old August 19th 04, 02:14 AM
john smith
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Ed wrote:
Hopefully someone isn't flying the bottom of a loop near stall
speed. Isn't the minimum radius turn going to be accomplished at
corner velocity and max-G pull?


Yes, turn rate (or time in the pull) is also a factor and max turn rate
and min turn radius are maximized if married with max available radial g
at corner. You should actually throttle up if you're behind corner!!!!


Question: how does one compute or determine cornering velocity for a
specific aircraft? Must that be done through flight testing, or is there a
mathmatical relationship to the stall speed/etc?



Do a Google search for 14cfr23.335.pdf

Specifically, does anyone know the cornering V for a Decathlon 160-CS?


With the above, you can calculate it for max gross weight.
For low speed aircraft I think it is just Va.
Look in the aircraft manual, it should have a V-n diagram.

  #19  
Old August 19th 04, 03:32 PM
john smith
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Thanks for getting this discussion pointed toward the mathmatics, Andrew.
A Google search of "V-n diagram" pulls up some really good articles on
recovery from a vertical dive that explain in easy to understand terms.

 




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