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Calculating vertical time and distance in a stall turn (US Hammerhead)



 
 
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  #1  
Old November 12th 03, 04:25 PM
Dave
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Default Calculating vertical time and distance in a stall turn (US Hammerhead)

I've been teaching aerobatics for years but last weekend (after a session of
stall turns) a student said he was surprised at how long we were vertical.
Afterwards it occurred to me that it would be fun/interesting to look at the
sums, later I realised that I didn't know how to DO all the sums!!

If the propellor thrust and form drag are initially ignored it's easy
(forgive the elderly units):
induced drag = 0 (in an ideal stall turn the CG track is vertical so the AOA
needs to be zero, therefore the induced drag should be close to zero)
initial velocity u = 120kts (200 fps)
final velocity v = 40kts (67 fps)
acceleration f =-32 fps/s

v**2 = u**2 + 2fs
so distance s = 555ft.
v=u+ft
so time in the vertical t = 4secs.

Now to bring in form drag and thrust.....
If we assume the deceleration is linear the form drag could probably be
based on the average
speed. And if we assume the engine is producing 200BHP with a propellor
efficiency of 75% we should be able to work out something useful.

Thinks - but Thrust is proportional to torque!..... elderly brain gives up.

How can I factor-in the propellor thrust and estimate the form drag to give
a realistic estimate of time and distance?

In practise I think it would be possible to estimate the net acceleration
due to thrust and form drag by flying a ballistic push (zero AOA) from some
angle above the horizon to the same angle below (to eliminate the effect of
gravity), starting at 60kts and then timing the acceleration to whatever
speed is reached. I'll try this next weekend and then see how the final
estimate matches the reality of a stall turn.

....but I'd still like to know how to calculate the approx thrust....??

Dave S


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  #2  
Old November 19th 03, 11:32 AM
Dave Pilkington
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Default

Dave,
I have a simple Qbasic program to do just that - it can be downloaded from:
http://groups.yahoo.com/group/ozaeros/files/Technical/
You'll need to register with that Yahoo group to get access.

Regards,
Dave Pilkington

"Dave" wrote in message ...
I've been teaching aerobatics for years but last weekend (after a session of
stall turns) a student said he was surprised at how long we were vertical.
Afterwards it occurred to me that it would be fun/interesting to look at the
sums, later I realised that I didn't know how to DO all the sums!!

  #3  
Old November 19th 03, 04:04 PM
Dave
external usenet poster
 
Posts: n/a
Default

Thanks Dave - I've downloaded it. How do you estimate thrust and drag?
It's many years since I wrote any Basic, what do I use to run it?

Dave S

"Dave Pilkington" wrote in message
om...
Dave,
I have a simple Qbasic program to do just that - it can be downloaded

from:
http://groups.yahoo.com/group/ozaeros/files/Technical/
You'll need to register with that Yahoo group to get access.

Regards,
Dave Pilkington

"Dave" wrote in message

...
I've been teaching aerobatics for years but last weekend (after a

session of
stall turns) a student said he was surprised at how long we were

vertical.
Afterwards it occurred to me that it would be fun/interesting to look at

the
sums, later I realised that I didn't know how to DO all the sums!!



  #4  
Old November 20th 03, 11:48 AM
Dave Pilkington
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Default

There's a QBasic compiler to download there as well. One you've got
that you can see all the equations in the program.
I estimated thrust from prop data I obtained from one of the
manufacturers - you're close enough with prop efficiency.
Approx drag deduced from a rough drag polar analysis of published info
for max level speed and climb. All the data is in the program for
several aircraft types.
Have you got MS FlightSiM? It works pretty well and should give you
what you want too. We've set it up at our flying school - if it gets
much better I might give up real flying.
You can contact me direct via www.ozaeros.com

"Dave" wrote in message ...
Thanks Dave - I've downloaded it. How do you estimate thrust and drag?
It's many years since I wrote any Basic, what do I use to run it?

Dave S

 




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