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JW on F3B speed task

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?

Craig Greening.
Marina, CA.

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.