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Jan 06, 2006, 05:27 AM
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Discussion

Supra F3J winch launch "agressive" settings


I've noticed an interesting message from Mark Drela on RSCE, concerning winch launching Supra settings, which could apply to any glider. I wanted to answer to this, but unfortunately i can't manage to subscribe to RSCE. By the way, I'm starting this thread on RC group. I hope Mark Drela, Jo Grini, Larry Jolly and Daryl Perkins and others will join the discussion !

Here is Mark message on his Supra settings:

"This is rather far aft hook position which requires
special action during launch. On my Evo program the
left slider is an elevator (speed) trim with a small gain.
For the launch throw I push the slider all the way forward
which feeds in a slight amount of down-elevator.
Once the glider rotates and settles down in the climb,
I pull the slider back to its normal center position
for maximum load just short of stall. I have 100% Ail.Diff
and lots of Ail->Rud coupling during launch, so I can
steer with might right thumb, leaving the left thumb
free to work the slider.

I've done some sims of the initial pitch dynamics
immediately after the throw. There is a very significant
CL overshoot at the end of the initial nose-up rotation.
So if you trim the glider for maximum pull during the
climb and zoom, and throw with this trim, then you are
guaranteed to stall at the top of the nose-up rotation.
So for maximum launch performance, it is necessary to add
some initial nose-down trim to safely get past the
initial pitch transients after the throw."


Then i've a couple of questions:

Mark, are you working on a simulation model for the winch launch ? Such a tool would allow us to optimize this very important flight phase in various conditions. In real world the air is different each time you launch, so it is very hard to know after a good launch if you did better, or if the wind or a thermal helped you, even with an alti logger... With a math model you can set the atmosheric parameters and play with different settings...
About your elevator preset during launch and rotation: i wonder if additionnal flap deflection would'nt help in this phase. This moves backward the center of pressure of the wing which help stabilize the plane,. No matter the drag, it gives you maximum lift which is, for sure, necessary in this phase. Then, when "critical" altitude is reached, less flap may help to find a better compromise "lift/drag" (I mean here "still hudge lift/ but not to much drag") so the glider climb fast... Maybe both elevator and flap presets are necessary for the transition phase, what do you think ?
But while saying this i'm going for this question: what is the optimal setting for the flaps ? For a given airfoil, between launch at let's say, 10 m/s, and the end of the climb (before zoom) at 40 m/s 50 m/s, or more, the Reynolds number varies a lot ! Appart from handling qualities, is the optimal flap setting the same between 10 m/s and 50 m/s ?? Do we know if we want the higher Cl, no matter the drag ? Or a compromise ? If we can identify this compromise, is Xfoil able to predict the airfoil performance accurately ?
Thanks for your work, wish you many interesting flights for 2006.

Loc
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Jan 06, 2006, 11:49 AM
4 wheels move a body; 2 a soul
I saw that post as well and found Marks launch technique interesting. May give it a try but personally, I prefer to use my right thumb for any ele adj on launch. I dont like tying up my left "rudder thumb" during launch. I'm not at those guys level and need to be ready to use the rudder now/then during launch.
Walter
Jan 06, 2006, 03:24 PM
Registered User
Slightly off topic but, what if you couple the flaps to the elevator so that as you transition to the top of the climb and feed in some down, you get less flaps. Then turn it off the flaps for the zoom. This would also allow you to get put more flaps (get more lift) by pulling back during the climb-out.

Gemarl
Jan 07, 2006, 07:45 PM
Registered User
I'd think that adding more flap deflection, in the situation where avoiding the stall is critical, would be the wrong way to go. It would just increase the likelihood of the stall, wouldn't it?
Jan 07, 2006, 08:16 PM
Registered User
Quote:
Originally Posted by loic_debisschop
Mark, are you working on a simulation model for the winch launch ?
... With a math model you can set the atmosheric parameters and play with different settings...
Some example outputs are here:
http://groups.yahoo.com/group/Allegr.../Winch_Launch/
winchlaunch_sim.pdf

Yes, you can "play with different settings", but this is a huge task.
Just look at the number of adjustable parameters on the side of the plots.

The best you can do is get some sense of what's most important and work on that.
Jan 08, 2006, 07:00 AM
Registered User
Quote:
Originally Posted by GaryO
I'd think that adding more flap deflection, in the situation where avoiding the stall is critical, would be the wrong way to go. It would just increase the likelihood of the stall, wouldn't it?
I agree, but if you have favorable conditions, like a stronger headwind, why not take advantage of some extra flaps. This is all speculation, of course. I'm still working on the basics of winch launching. Lost 3 wings so far.
Jan 08, 2006, 11:58 AM
4 wheels move a body; 2 a soul
Strong headwinds, more flap? How hard do you want to pull anyway?
Youll just start breaking lines, you cant get a launch if you are always breaking line.
Walter
Jan 08, 2006, 11:24 PM
Team WC2013 F3K
oakman7004's Avatar
Not necesssary.
If you use your own winch (If the contest organiser does not provide winches, like on a F3B comp) and own lines (like mono not braided) then you upgrade a size in diameter pull harder on the line to stretch and gaining more power from the line in the zoom...

my .02

Cheers Jonas Ekman

ps. Using a braided line provided by organizers is another story/technique! I think that was what you ment, Walter...
Jan 09, 2006, 06:00 PM
Registered User
Quote:
Originally Posted by glderguy
Strong headwinds, more flap? How hard do you want to pull anyway?
Youll just start breaking lines, you cant get a launch if you are always breaking line.
Walter
So I buy a stronger line. I want to pull as hard as my wings will let me and get as much lift as possible without stalling. That's the whole point right?
Jan 09, 2006, 10:59 PM
4 wheels move a body; 2 a soul
Tell that to the CD at next windy 2 day you go to. Im sure he will stop the whole event to change to a stronger line.
Walter
Jan 10, 2006, 05:22 PM
in Maryland, that is.
Roger_n_MtAiry's Avatar
Quote:
Originally Posted by Gemarl
So I buy a stronger line. I want to pull as hard as my wings will let me and get as much lift as possible without stalling. That's the whole point right?
Ahhh... no. The point is to realize a consistant, smooth, high launch to a good position/attitude/altitude, without breaking anything, so you can then concentrate on flying the task (fun or contest).
I am reminded of a similar situation in auto racing, to wit; To drive fast, you must be smooth.

Live long, fly well and prosper,
Roger
Jan 11, 2006, 12:43 AM
drCarter
I believe that many of us fly through a stall on launch and don't even realize it. On my TD planes I set my cg at of very close to neutral. My towhook is also in the same position. I set my elevator preset to where the plane will track straight while in launch mode. Any more elevator and the plane acts like it wants to tip stall on the way up. I also have noticed the over rotation on launch and like to stand on the pedal at least the first 100 feet. The line tension is about the only thing keeping the plane on tow at this point. I see this alot from watching others as well. At the point of over rotation I believe we are seeing the plane enter a stall for just a second or two. The momentum of the plane is what keeps it straight. I do know that any more up elevator in my preset would lead to a tip stall.

Flaps and Wind: I have a slider on my TX that controls my launch camber preset. I vary this depending on the wind. I own a winch and use the same type of line as the club. I do not like line breaks and probably shoot for 90% of what the line will take. I want the same pull regardless of the wind conditions. There are times in contest when the wind will pick up to over 20 and I have even launched with the flaps reflexed. This proceedure has worked well for me and I get a pretty consistant launch.
Jan 16, 2006, 12:17 AM
Registered User
Quote:
Originally Posted by markdrela
Some example outputs are here:
Yes, you can "play with different settings", but this is a huge task.
Just look at the number of adjustable parameters on the side of the plots.

The best you can do is get some sense of what's most important and work on that.
I somewhat agree with Mark. I put together a launch sim about 15 years ago (!!!, i've been doing this a long time...) when I was just getting serious about F3B. A few years ago I updated it to capture variations in settings during the launch. The danger with sims is that you can spend a lot of time playing around...

What is important is to optimize the "L/D" of the system. There is a whole sub-genre of optimizing line size and drum diameter, but I'm not going there in this msg. The fun part is that the "D" part of L/D is a bit messy. During the initial part of the launch, the weight of the airplane is functionally equivalent to drag. The line drag is a big factor, which is a large part of why the F3B folk want to use the thinnest line that they can get away with. The initial portion of the launch does want higher Cl, which hints towards higher TE deflections for the initial portion of the launch. Also, the initial portion of the launch needs less up elevator due to the towhook being lower than the cg and center of drag.

For the typical plane that I fly, I've standardized on a single TE setting for launch. I realize I'm giving up a little bit, but I like reducing pilot workload...

My towhook position is dependent on what event I'm flying. For F3J, I use a conservative (forward) position because the penalties for errors during the tow are pretty high in F3J. For F3B, I go for a more agressive towhook position due to the relative importance of launch height compared to making a mistake on tow (aka, tipstall). You can always (...almost always) relaunch in F3B, but in F3J a relaunch is very painful. The conservative towhook position is such that after the plane stabilizes after the throw I can add a bit of up elevator without too much fear of asking too much from the plane. The agressive towhook position might need a touch of down elevator during the initial portion of the tow, but will want a shade of up elevator during the latter portions of the tow unless there is no wind/tension.

Big things of note: Line tension before the throw, and throwing the airplane are both very important. If you are one of those guys that just kinda lets go of the plane, you are giving up a bunch of height. Partly due to the amount of energy that it takes to initially accelerate the plane, and partly due to the conservative setup that is required for airplane longevity. If you don't get the plane up to a reasonable flying speed on the throw, the plane will accelerate horizontally (line tension and all that) and then will rotate to a vertical orientation. But the plane will overshoot, and will likely stall with an agressive setup. You have all seen this behaviour in normal flight. A start without much of a throw is kind of like just dropping the glider from a tower. It will nose down, pick up speed, then the nose will come up above the horizon, the plane will slow down (too much) then stall, and repeat until the oscillations damp out. On the winch similar things happen, with the difference that the effective gravity is aimed towards the turnaround, as well as the oscillations are more damped due to the effects of the line.

Back to lurking,
JW
Jan 31, 2006, 08:46 AM
Registered User
Thread OP

Thanks Joe ! I wish Philip Kolb would join this conversation !


Thanks Joe for all those details ! For people who haven't found it already, i recommend your excellent article concerning F3B launch analysis:
http://www.quietflyer.com/pdf/01-03F3BLaunch.pdf
I also recommend the articles on http://www.nesail.com/articles.php
"Maximizing your launch potential" - Joe Wurtz- and "launch meister" from Daryl Perkins. It would be very interesting to compare these techniques to German techniques. I went to F3J EC last year in Croatia as an helper for the French team. I was impressed by Philip Kolb launches and i noticed a few things:
- German people launch differently than any others: thes hold the fuselage in front of the wing. See picture attached. I wonder why they do that, maybe the portion of the line between the hook and the hand act as a "catapult" during the launch, giving more speed to the model. Any gesses ?
- Philip Kolb is climbing fast (thanks to his launch), starts the zoom before anybody, and climbs vertically for a long time after the release. It means he is very fast at the output of the zoom, which also means he build a lot of tension before the zoom. I noticed he was doing a hudge arc of circle to do that, like a slow motion pitch down compared to the others...
At the end, most of the time he launches higher than the others in the round and was the first to release ! Even for the launch, he well deserve his european first place ! I know he uses dataloggers to measure heights, i gess he did optimize a lot his lauches too...

Loc
Jan 31, 2006, 12:17 PM
Registered User
There are several reasons to hold your model at the cg or forward of the cg. One is to enable a high angle of attack at release. The other is to be able to throw your model effectively. I suspect the German throwers you mention hold their models in front of the wing so they can get a better grip. I don't think there is much of a catapult effect - at least I haven't noticed any. . .

Tom


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