Regis-de-giens wrote: ↑
Fri Jul 13, 2018 3:17 am
Very nice topic that may slide to deep technical arguments. I have been investigating a long time on L/D ratio measures, here is my current conclusion :
For a Hydrofoil
1-You can use a boat to pull the rider , put a load cell (like luggage weight sensor) on the traction rope. Using the speed and weight of the rider you can derive the LD ratio.
2- without sensor you can just let the boat traction rope go and you can derive an from the distance you can do without boat you can get an average LD ratio among a range of AoA variation.
We can discuss the formulas for each if you want. By the way these measure of LD ratio is depending on bar stroke
(For kite) and rider'weight (For HF) you need to built a graph LD vs AoA (= vs bar stroke Or Weight) actually.
Correct, and good thoughts.
But, a few but's regarding number 1:
Must say I dont think it will be a good way, in my experience with measuring (occupation), as I think there are too many variables in this affecting the result, compared to the desired "accuracy" needed to compare foils.
You can make a L/D curve yes, but you can not get the AOA into the equation, unless you somehow know the polar beforehand IMO.
You have to see the full foil assembly front wing, fuselage, stabilizer, part of the mast, connections, somewhat as a "black box".
It will be possible to make rough estimates of the AOA, but thats it.
I think a water tunnel (made for it) is the only way to make comparable tests regarding this, with the benefit you get the AOA measured also, and much easier to plot the behaviour when over the stall point
Regarding #2, it will be almost solely dependant on how many tries you got, as I dont believe anyone can ride a foil at max L/D for long. Which you also imply by "a range of AOA variation".
For going max distance you dont need to change AOA, and the speed should be held constant, IF you have let go at max L/D speed or lower, otherwise you are correct.
Remember, max L/D is at a lower AOA than max "time" above the water before you touch down, two different AOA's.
You can ride for longer time before touchdown, at a bit lower L/D (higher AOA), but you will not ride as far when you touch down.
For everything else than best VMG, the full polar is needed if you have to compare speed, as it is close to zero AOA (constellation wing + stabilizer) we ride at high speed.
I might be pleasantly surprised of course, but I doubt any of these methods will work really