Joe's post is great...though I am totalyl lost on the climbing last leg
thing...got to try that one day :) perhaps that is why I go slow every
speed run.
Anyway, one thing that Joe did not specifically target is the task
itself, and it's requirements. The basic premise of the task is to fly 600m
as fast as possible. BUT the plane flies much further. So the basic
premise of hte task is to maximize GROUND speed, not necessarially air
speed! Hmmm. That really does take a lot of thinking and gets a lot of
really good people! Plane velocity is important, but a fast plane with slow
groundspeed does nothing for the run.
Many times you will see an F3b flier launch to the moon. Only to fly
1-2 seconds slower than someone that launched about half as high. What
happened? The lower guy maximized his groundspeed, and the higher one flew
faster, but longer. Joe started to touch on this with his last leg talk.
The shortest flight starts and ends on the same altitude! Can you do that?
Sure, but it will be slow. The idea is to maximize your speed initially,
and then maintain it through the whole run while talking in all the other
variables. Variables such as wind gradient, launch height, ballsat, your
plane's characteristics, turn capability, wind speed all come into effect.
Has anyone figured them all out? Nah...but that is why we keep trying!
Jason Werner
----- Original Message -----
From: Joe & Jan Wurts
To: Craig
Cc: RCSE
Sent: Sunday, July 08, 2001 3:55 AM
Subject: [RCSE] Re: F3B question
-----Original Message-----
From: Craig <[EMAIL PROTECTED]>
To: Joe & Jan Wurts <[EMAIL PROTECTED]>
Date: Saturday, July 07, 2001 9:52 PM
Subject: F3B question
Hi Joe,
I have a question from watching you and Gavin practice in San Jose a couple
of weeks ago if you have the time. Watching you guys fly speed (Rich and
Daryl also) I noticed that there is not a constant rate of descent during
the four legs. The first three appear to have about the same loss but the
last turn is consistently low with the final leg being almost level. Is this
normal (and why) or related to the strong wind on the day?
Regards,
Craig Greening.
Marina, CA.
Craig,
As there is evidently a desire for actual technical type content, I'm cc:ing
this to
RCSE as well.
This seemingly simple question has a lot of nuances in the answer.
I used to fly speed with the objective of keeping a constant speed
throughout the course, neglecting wind effects. After I made a speed
run simulation, I learned that this is not the optimal course.
First, I'll talk about no-wind speed, then later talk about the effects
of wind.
In an ideal world (constant drag coefficient, no Reynolds number effects)
the ideal speed run would be to fly at a constant speed throughout the
course. The entry dive would be timed to enter the course such that there
would be a constant descent through each leg, with the descent angle
such that the plane would hold its speed. Dr. Selig wrote a paper some
time ago that numerically proved this (uses calculus of variations). In
fact,
my senior thesis pretty much showed the same thing (back in '84.. I'm
getting old!).
But, in reality, Reynolds number rears its ugly head (or in this case, kinda
pretty...). The drag coefficient goes down with airspeed due to the
increasing
Re. The drag still goes up, just not as fast. It turns out that the
optimal course
has the plane at its maximum speed at the start of the speed run, with a
nearly constant velocity on the first leg, decelerating a bit more for each
of the additional legs. The last leg has a lot of deceleration in it. If
the ground
wasn't in the way, you would want to be climbing on the last leg! Think of
it this way, any energy left when the plane exits the course is energy not
used
during the speed run. This analogy is not strictly accurate, but gives one
a flavor.
There are some nuances in the turns as well. Somewheres around half of the
energy lost in the speed course is lost in the turns. Might even be more
than
this, I'd have to look it up in the sim. In order to regain the speed, it
is a really
good idea to split-S the turns, especially with the first two turns. This
does
two things for you. One, it recovers energy that is lost in the turn. The
other,
is that it gives you airspeed with no penalty. If you dive during the
straight
part, your course gets longer, as you are not taking the shortest path to
the
other end. The split-S manuever gives you the energy without lengthening
the
course.
Now, in the wind, things change a bit. There are two drivers here. One is
that
you start higher (launches are better in the wind). The other is that one
leg is
upwind, and one is downwind. It is better to bias the course such that you
end
up with more speed on the upwind legs compared to the downwind legs. One
good
way to do this is to do a big split-S when transitioning from downwind to
upwind
(typically on base A, although we've had a lot of experience flying downwind
speed, especially at team selections and WCs). Also, the upwind legs tend
to want a little more descent in order to maintain that increased speed.
The
wind carries the plane along on the downwind legs, so it does not matter
very much to dive there.
Thanks for helping out at practice last weekend.
Regards,
Joe
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