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Jun 12, 2019, 03:38 PM
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Discussion

Proper technique at the apex of the launch


I have studied the videos, read the posts, and learned much about launch techniques over the years from various folks on rcgroups (including, but not limited to Jun's launch clinic and Bruce Davidson's launch videos , etc.), and they have been immensely helpful, but the discussions primarily centered around what to do (and not do) during my rotation (with my feet, with my arm, how to properly "coil" and "uncoil") to get the most power and energy out of my launch.

What I cannot find much info on is what proper techniques to employ (using my radio) at the apex of the launch.

If I am going to be heading upwind, then I am pretty clear on when to push over (down elevator) without stalling, based on the energy that I believe I have left in my climb out.

But when it comes to a "turn and burn", exactly when and how do I rotate the ship during its climb out?
Specifically, what do i physically do with the sticks (just use ailerons?) and what series of trajectories should the model specifically take so that I stay efficient and not sacrifice altitude as I attempt to put my plane in a position to head down wind?

If there are threads that discuss this already (and hopefully a few diagrams showing the path the plane should take during this maneuver?) I haven't found them yet.
If you know of these threads, can you post the links here?

If you can describe what YOU do during the turn and burn (with the sticks, and what the model should look like, etc.) I would be most appreciative.

Thanks
Roy
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Jun 12, 2019, 04:27 PM
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Good question and I’m looking forward to what others do , personally I don’t so much turn as go into a half loop and roll out from inverted without any real loss of momentum. I go inverted when I still have enough momentum so I don’t stall on the downwind leg

I often get better , read higher, launches like this than a standard pushover?

Hope that’s started the conversation off , feel free to rubbish this technique as I’m still relatively new !

Dave
Jun 12, 2019, 07:06 PM
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I'm clearly no expert and launch low, but I do it the other way to Dave, I do the Bruce style, I'm a leftie too (not that it makes any difference) I just hold onto the peg a fraction longer and the plane arches off to the right on the climb. It almost kinda levels itself out too on the way up so I don't have to pushover all that much, just level the wings out at the top a bit as it will be banked over and off downwind it goes.

In practice I don't think I give much, if any elevator to level out, just a stab of ailerons to level the wings once the nose is pointing downwind at the the top of the arching climb. This way was more natural for me as I hooked my throws a lot in the beginning by trying too hard so I threw a natural turn and burn lol.
Jun 12, 2019, 07:27 PM
Time for me to Fly...
Mr. Wiz's Avatar
If conditions are such that I’m going to be doing a lot of “turn and burn” launches, I trim the elevator such that I don’t climb as steep as I normally would and I also trim my rudder such that the plane banks slightly to the left. I’m a right handed launcher... Then I throw very slightly across the wind in that same direction that I want the plane to bank. It takes some fiddling but once you get the launch mode set up right, the plane will climb out nicely and be turning in the direction you want to go while maintaining its forward momentum. I learned this from a good friend that happens to be one of the best F3K pilots in the country. It works well...
Last edited by Mr. Wiz; Jun 12, 2019 at 07:38 PM.
Jun 12, 2019, 07:42 PM
Time for me to Fly...
Mr. Wiz's Avatar
Quote:
Originally Posted by DizzyDuck
Good question and Iím looking forward to what others do , personally I donít so much turn as go into a half loop and roll out from inverted without any real loss of momentum. I go inverted when I still have enough momentum so I donít stall on the downwind leg

I often get better , read higher, launches like this than a standard pushover?

Hope thatís started the conversation off , feel free to rubbish this technique as Iím still relatively new !

Dave
Sometimes, Dave, I do this too. What Iíll do is simply hold my launch preset longer and force the plane to loop back up over my head and then roll out at the top. Itís not as good as the method I described above because a roll creates a lot more drag than you might think. Still in a pinch, it works well. But.... and this is a big one. Donít do this in a contest where you have people standing behind you. Doing so is just asking for a midair.
Jun 13, 2019, 11:07 AM
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Quote:
Originally Posted by DizzyDuck
Good question and Iím looking forward to what others do , personally I donít so much turn as go into a half loop and roll out from inverted without any real loss of momentum. I go inverted when I still have enough momentum so I donít stall on the downwind leg

I often get better , read higher, launches like this than a standard pushover?

Hope thatís started the conversation off , feel free to rubbish this technique as Iím still relatively new !

Dave
Thanks Dave!

I will try that.

Just to clarify, when you perform your roll out from the inverted position, are you JUST using Ailerons to initiate the roll,
or are you also dialing in any rudder, or down elevator to control and stabilize the roll?


Roy
Jun 13, 2019, 03:04 PM
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So generally, thinking about what I do , just hold the launch preset longer so it’s moving over my head and with the breeze. This seems to give it energy (maybe why I seem to launch higher) when it’s at its apex I gently blip the elevator to level out inverted , still being driven by the breeze and then initiate a roll just with ailerons.

Now it may just be my storms but it takes a tiny amount of aileron for about 1/2 second to roll upright. The energy it carries is ridiculous. I haven’t figured out why or how.
Jun 13, 2019, 04:02 PM
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Roy_N's Avatar
The best turn-n-burns I've seen, the pilot holds the rotation just a bit longer. This puts the plane into a slightly inverted climb. During the zoom portion, use aileron to turn the plane upright, finish the launch as normal except headed in a different vector. There is no rudder/aileron coupling in "zoom" mode.
Jun 13, 2019, 04:49 PM
Time for me to Fly...
Mr. Wiz's Avatar
Does anyone think gently turning climb style turn in burn I described above works well? The reason I ask is that I’ve done a lot of the loop back over my head turn and burn launches. They are effective and very impressive looking but in the wind, I prefer this other method. It not so amazing looking but I think it preserves more forward motion than the loop and roll method.
Jun 13, 2019, 05:09 PM
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Quote:
Originally Posted by Mr. Wiz
Does anyone think gently turning climb style turn in burn I described above works well? The reason I ask is that I’ve done a lot of the loop back over my head turn and burn launches. They are effective and very impressive looking but in the wind, I prefer this other method. It not so amazing looking but I think it preserves more forward motion than the loop and roll method.
I do the same (or similar) to you Whiz. I don't see the benefit of being upside down at any point of a launch .

I may have a bit of aileron input - but will have to check next time I do it....
Jun 13, 2019, 06:45 PM
Launch high. Fly low.
I try to keep this simple as well. Launch with the same trajectory as my regular launch; the only difference is that I'll pull the stick earlier at 3/4 of of the way to go inverted, usually with enough speed to correct the plane and head to whatever direction I need to. I like this coz I can get there quickly.
Jun 13, 2019, 09:07 PM
G_T
G_T
Registered User
Part I, windspeed gradient.

In the altitude range we have for DLG launches, there is a windspeed gradient. The windspeed relative to ground is very low right above the ground, moderate at our head height, and increases with greater altitude. The rate of increase decreases with increasing altitude, providing one is in a very large clear area. If there are trees around, the ground region could be somewhat sheltered and the gradient extending well above the trees.

Let's use this picture as an example:

https://www.researchgate.net/publica...d-gradient.png

That would be a windy day of course, but the same principle applies regardless of the values.

I'm going to make up numbers here so don't take them literally.

Suppose you can launch to 60m height, in dead conditions, which requires a launch velocity of at least 34m/s. Suppose you can achieve the same launch velocity relative to ground when the wind is blowing (really you'll lose some launch speed relative to ground if the wind is blowing much but let's ignore that for simplicity).

From that example graph, the windspeed at 60m is about 13m/s, and at head height it is about 7m/s. Most people don't fly DLGs at those windspeeds so let's cut those numbers in half - to 3.5m/s at head speed and 6.5m/s at 60m altitude. That is not unusual conditions to encounter at a contest.

With a typical throw of (let's pretend) close to horizontal with immediate rotation to vertical, the airspeed of the DLG leaving the hypothetical thrower's hands is 34m/s groundspeed + 3.5m/s windspeed = 37.5m/s airspeed.

If one achieves a 37.5m/s airspeed at launch and it is all converted to vertical motion and the plane has no wind resistance or yawing etc to rob energy, it now achieves an altitude of near 72m. The launcher has gained 12m altitude due to the wind.

But that was with immediate rotation to vertical. Suppose instead the launcher were to choose a trajectory that had a 60 degree climb angle instead? That's relative to wind, not relative to ground. The pilot will see the plane going up closer to vertical from standing on the ground.

What happens? The plane gets the same initial airspeed from leaving the thrower's fingers, and rotates the same, just not as far. Now once it gets to, say, 40m altitude, the wind is 6m/s (half of what that graph showed, as per our example). But that is 2.5m/s FASTER than it was at ground level. On a 60 degree climb angle, the vector component in the plane's flight direction gives a boost of about 0.83m/s to the airspeed. So it would go even higher.

Of course the airspeed boost is progressive with altitude increase, due to the gradient. It isn't a 2.5m/s wind sheer hitting suddenly at 40m! So the plane is getting a light boost all the way up the climb. So in the frictionless scenario it would go higher.

In fact, in the frictionless scenario, one would really opt to climb out very shallowly to extract the maximum possible airspeed boost from the windspeed gradient. Then by the time the plane got up to 60m it would have acquired a cumulative airspeed boost of 6.5m/s - enough to go quite a bit higher!

But planes have drag, unfortunately. So a super shallow launch in real life gets you upwind a ways but not up a ways! Then the plane runs out of speed and is stuck gliding down.

The optimal trajectory is not with the plane pointed vertical (would appear to be blown backwards on the climb) as no advantage is taken of the windspeed gradient only of the windspeed at near ground level. The optimal trajectory is not going out shallow, as that might maximize extraction for the portion of the gradient the plane gets to see, but drag kills it before it gets to see a large chunk of the gradient!

Optimal trajectory is going to be when you see the plane moving upwards and forwards perhaps somewhere near 80 degrees. That won't change all that much as the windspeed changes, within reason. What this means is the greater the wind, the sooner one lets off the launch preset and the shallower the plane climbs relative to the wind - and therefore the more wind speed gradient the plane gets to use to boost it's altitude.

Yes, I did a lot of handwaiving and don't feel like running through all the math. IIRC, a lot of it Dr. Drela did years ago and the topic has come up more than once.

Part II will be on the turn and burn.

Gerald
Jun 13, 2019, 09:21 PM
G_T
G_T
Registered User
Part II, turn and burn.

There are at least two common methods of performing turn and burn. One is to throw the plane into a climbing bank and do a half-circle to head downwind. The other is to make a quarter loop in the climb and roll out. Both will get you flying downwind.

In either case, to take advantage of the windspeed gradient the plane needs to be flying into the wind! If one did the half turn example and was flying perpendicular to the wind halfway up, then consider what happens. The plane does get a boost in airspeed low down from the wind, gets nothing halfway up (flying perpendicular to the wind gradient) and actually loses airspeed from the gradient as the plane turn s progressively downwind. It should be clear that the majority of the turn downwind should occur up the top of the altitude, where the cost of climbing the gradient downwind is reduced, and most of the benefit of the gradient has been taken from lower down by flying into it. So it should look more like a J-hook than a half circle.

Suppose one uses the quarter loop and roll method? Again it should be clear by now that one should delay arcing downwind until most of the gradient has been profitably utilized for an altitude boost. Then quarter loop and roll into horizontal flight over a relatively short altitude. Because the maneuver is a loop and roll simultaneously, one ends up cross controlling the rudder and ailerons for best efficiency. It takes a little practice.

Starting the looproll too low costs potential altitude of course, just like turning downwind too low with the circling technique costs potential altitude.

Both approaches can result in getting to a point downwind at a little higher altitude than you can get just launching forwards and then turning after leveling out, unless you are launching into lift. You can certainly get there several seconds faster. Often that is the greater advantage, if that means several seconds less time spent not in lift!

Gerald
Jun 14, 2019, 02:19 AM
Registered User
Thankyou GT

That explains a lot about why the glider behaves the way it does, also the reason I try not to touch the controls once the climb angle is set until the very last moment when I push over and roll. Low speed low drag.

Really appreciate the time it must have taken to type all that. Thankyou

Dave h
Jun 14, 2019, 12:53 PM
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Thank you Gerald.

I am looking forward to getting out there this weekend and trying all this out!

Roy


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