Author Topic: Loops - Round or look round?  (Read 1832 times)

Offline Mike Wood

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Loops - Round or look round?
« on: 10, August 2015, 02:43:30 PM »
Moved from: http://www.gbrcaa.org/smf/index.php?topic=4417.msg24924#msg24924

This thread got me thinking - If you fly a perfectly round loop from the bottom of the box to the top then from the ground i.e. the judges position, it will look anything but round. So do you fly manoeuvres that are

geometrically correct but look wrong to the judges OR do you fly ones that look right but are actually wrong?

Simon
« Last Edit: 10, August 2015, 02:45:44 PM by Mike Wood »

Offline Richard Sharman

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Re: Loops - Round or look round?
« Reply #1 on: 13, August 2015, 01:12:23 PM »
Surely, the loop has got to look round from the Judge's point of view ? So, it isn't actually round, just seems so.  And probably explains one reason (among many)  why my loops don't look good. 


Think of the geometry of the situation:  you fly a perfect circle vertically above the ground,  but the judge is looking along the axis of a sort of cone at an angle to the ground.  Consequently he/she sees a sort of ellipse which will be flatter than a circle.  But if you fly the right ellipse (taller than a circle) it could appear as a circle. If we know some measurements we can calculate how different the circle and the ellipse are.


 Quick calculation (back of envelope) suggests that if you fly a vertical figure 150m out from the judge, who is looking at a base 15deg above horizon and a top 60deg above horizon, then the error (the amount by which the actual figure deviates from the apparent figure)  on the lower part of the circle is only  about 15m whereas the error on the upper part is a massive 32m !  Hence, you have to extend the upper part of your loop more than the lower part by quite a margin to make it look round.         






Offline Peter Jenkins

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Re: Loops - Round or look round?
« Reply #2 on: 13, August 2015, 04:05:45 PM »
Since the judges are just 10 mtrs behind the pilot while the model is, or should be, 150 mtrs away from the pilot, I would have thought that any distortion of the shape being flown would be pretty much the same to both.  There is 0.6% difference in their viewing distance after all (10/150).  If you are flying in the Worlds then this might make a difference but for the rest of us mortals I wouldn't have thought it would.  I have yet to see a judge holding a circular wire frame to check for roundness of the loop or any part thereof.  Surely, it is all down to how it looks to the judge?

Offline Kevin Caton

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Re: Loops - Round or look round?
« Reply #3 on: 14, August 2015, 05:41:06 PM »
They need to look round to the judge. You generally see flat tops. My tip is to look at the relative height of the "clock points" - the 3 o'clock and 9 o'clock points (when the plane is tracking vertically) should be half way up the loop. Usually you see that the 12 o'clock point is too low.

Offline Stuart Mellor

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Re: Loops - Round or look round?
« Reply #4 on: 15, August 2015, 07:55:56 PM »
No Contest (if you excuse the pun!) - the idea is to present correctly shaped manoeuvres to the judges. As Peter says  - there is practically no difference in observed shapes between the judges & competitor, so the manoeuvres need to look round. Parallax, or the difference in what the pilot sees & the judge sees only occurs in relation to the box end markers, but a loop at the box end has to be distorted to look round from centre.

45 degree climbs at box ends (cubans etc) will always be distorted & look steeper than they are - i.e a genuine 45 degree climb as seen by an observer at the box end will appear steeper when viewed from the pilots position - so they need to be shallower to compensate. (Could be calculated with a bit of trig (cba))

You can demonstrate this by flying directly away from yourself & pulling into a shallow climb. To you, this would look to be vertical.