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Old Oct 09, 2012, 12:25 AM
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Tuomo's Avatar
Jyvaskyla, Finland
Joined Aug 2003
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You can test your launch setup by flying around in launch more. Adjust elevator to make plane fly nicely without stalling or diving.

Then go adjust hook to get nice steep climb.

This method is not absolute truth, but it will take you in the ballpark. The most difficult thing to find in launch setting is correct combination of tow hook and elevator positions.
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Old Oct 09, 2012, 12:26 AM
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Francesco's Avatar
United States, CA, Mountain View
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Quote:
Originally Posted by Tuomo View Post
You can test your launch setup by flying around in launch more. Adjust elevator to make plane fly nicely without stalling or diving.

Then go adjust hook to get nice steep climb.

This method is not absolute truth, but it will take you in the ballpark. The most difficult thing to find in launch setting is correct combination of tow hook and elevator positions.
Beware, this is only true if the CG and the towhook positions are actually quite close to each other. Can lead to disaster if towhook is well aft of the CG.
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Old Oct 09, 2012, 01:31 AM
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Regarding flap blowback, I assume you have tied the servo to both top and bottom skins?
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Old Oct 09, 2012, 07:45 AM
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Dallas, TX
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Originally Posted by Tuomo View Post
You are right. I am very conservative on running servos at higher than recommended voltage. And regulators are just one more thing to go wrong... When I find a good high voltage servo setup (all 6 servos), I will go immediately to LiFe.
I use 4x JR DS378HVs in the wing and 2 JR DS3717HVs in the fuse on my Supra Pro Comp with a 2s Hyperion LiFe. Works great! Also, in a Sprite I have DS188HVs for ailerons and DS285MGHVs for flaps and tail.



Alan
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Old Oct 09, 2012, 01:21 PM
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Jyvaskyla, Finland
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Originally Posted by lesterpk View Post
Regarding flap blowback, I assume you have tied the servo to both top and bottom skins?
It is the most simple way to improve flap servo performance. Never bothered with aileron servos.
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Old Oct 09, 2012, 01:27 PM
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Jyvaskyla, Finland
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Originally Posted by Francesco View Post
Beware, this is only true if the CG and the towhook positions are actually quite close to each other. Can lead to disaster if towhook is well aft of the CG.
Using the method described, I end up hook very slightly forward from CG (to keep things easy I set CG it slightly stable from neutral stability).

There are no magic bullets or secret information. Launch setup is mainly work work work - and secondly about understanding the launch dynamics.

The problem with setups given by manufacturers etc. is that you never now the thinking behind them. Often they are over-safe, but sometimes quite hot. And there are also other factors like launch style and tension that may affect the way how one wants to set up his plane.
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Old Oct 09, 2012, 01:33 PM
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Originally Posted by Tuomo View Post
Using the method described, I end up hook very slightly forward from CG (to keep things easy I set CG it slightly stable from neutral stability).

There are no magic bullets or secret information. Launch setup is mainly work work work - and secondly about understanding the launch dynamics.

The problem with setups given by manufacturers etc. is that you never now the thinking behind them. Often they are over-safe, but sometimes quite hot. And there are also other factors like launch style and tension that may affect the way how one wants to set up his plane.
That's exactly the problem with that routine: you end up with about the same static margin in launch as you use in normal flight. Which, if you understand the launch dynamics, is not the peak performance point.
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Old Oct 09, 2012, 02:39 PM
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Jyvaskyla, Finland
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That's exactly the problem with that routine: you end up with about the same static margin in launch as you use in normal flight. Which, if you understand the launch dynamics, is not the peak performance point.
In practise I probably have plenty more stability in launch than in normal or thermal flight modes. This is due to launch camber.

But please elaborate. I think the method described is very common among F3J/B flyers. And sensibly used, it leads to modern middle of the road F3J launch trim. I learned it first from F3B community where the pilots seem to follow it very rigorously, ending with hook more rearward position than what is common in F3J.
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Old Oct 09, 2012, 02:46 PM
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In practise I probably have plenty more stability in launch than in normal or thermal flight modes. This is due to launch camber.

But please elaborate. I think the method described is very common among F3J/B flyers. And sensibly used, it leads to modern middle of the road F3J launch trim. I learned it first from F3B community where the pilots seem to follow it very rigorously, ending with hook more rearward position than what is common in F3J.
Overstability in launch is generally bad, it causes oscillations which cause the plane to overshoot the optimal angle of attack and enter a pre-stall condition. To overcome this you either use your thumb or settle for a less than optimal trim.

Reducing longitudinal stability via the towhook position is an easy fix. A plane like the Supra also gets some residual stability in launch from the pylon-mounted wing.
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Last edited by Francesco; Oct 09, 2012 at 02:52 PM.
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Old Oct 09, 2012, 03:32 PM
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Jyvaskyla, Finland
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Originally Posted by Francesco View Post
Overstability in launch is generally bad, it causes oscillations which cause the plane to overshoot the optimal angle of attack and enter a pre-stall condition. To overcome this you either use your thumb or settle for a less than optimal trim.

Reducing longitudinal stability via the towhook position is an easy fix. A plane like the Supra also gets some residual stability in launch from the pylon-mounted wing.
I think we started this thread from that - too much stability causes funny things as plane accelerates in line. But it is not black/white. If you trim your plane is absolutely neutral stability in launch, it is also very critical to proper throw (tension & trajectory). I think in practice we want something in between (depends on your throw / your helpers). Also in F3J I see no problem in sacrificing a wee bit of tension for stability and speed (look for slightly lower lift coefficient in launch, just a little forward towhook).

Your point about Supra pylon is good one. I did not think of that, but common sense tells me that Supra pylon gives some leverage for rotation after less than perfect trow? My point was to have 2 launch phases. First one trimmed for getting the quickly to optimum climb (plenty of stability), second phase for steady climb in longer launch (and neutral stability).

The last point we have not discussed is wing flex. It is always bad for consistent launch trim, and specially so if wing twists torsionally. If plane suddenly gets unstable in strong wind launch, it might be also due to structural reasons. With adequate structure, there is no need to change launch trim even in very strong winds, just go to thicker line.

Finally, we get large % of total altitude from zoom. The time we spend in line is preparation for high zoom - plenty of tension and absolutely straight up.
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Last edited by Tuomo; Oct 09, 2012 at 03:39 PM.
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Old Oct 09, 2012, 06:31 PM
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Originally Posted by Tuomo View Post
I think we started this thread from that - too much stability causes funny things as plane accelerates in line. But it is not black/white. If you trim your plane is absolutely neutral stability in launch, it is also very critical to proper throw (tension & trajectory). I think in practice we want something in between (depends on your throw / your helpers). Also in F3J I see no problem in sacrificing a wee bit of tension for stability and speed (look for slightly lower lift coefficient in launch, just a little forward towhook).
Don't know... tension IS speed, there is no such thing as sacrificing one for the other, except for very short transients.
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Your point about Supra pylon is good one. I did not think of that, but common sense tells me that Supra pylon gives some leverage for rotation after less than perfect trow? My point was to have 2 launch phases. First one trimmed for getting the quickly to optimum climb (plenty of stability), second phase for steady climb in longer launch (and neutral stability).
More than that, two 500N forces opposing each other 6-7 cm apart are more than enough to stabilize the attitude even if aerodynamic longitudinal stability is nil. You can setup the plane more aggressively with less risk.
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The last point we have not discussed is wing flex. It is always bad for consistent launch trim, and specially so if wing twists torsionally. If plane suddenly gets unstable in strong wind launch, it might be also due to structural reasons. With adequate structure, there is no need to change launch trim even in very strong winds, just go to thicker line.
Correct.
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Finally, we get large % of total altitude from zoom. The time we spend in line is preparation for high zoom - plenty of tension and absolutely straight up.
Correct, too.
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Old Oct 10, 2012, 03:08 AM
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This is what has worked for me for the SUPRA PRO launch setup

1. Firstly test on a bungee to get the right elevator setting at the SPEED setting (which for me is a flat bottomed underside..probably not optimium but easy to set)..the CG is at 103mm.

2. Then while I am at it on the bungee get the right elevator trim for CRUISE ( + 2 degs down) and THERMAL (+ 6 degs down) at the 103mm CG setting..(I must admit I don't use the THERMAL setting that much)

3. Now.... to tune for LAUNCH setting I use a winch with Speedline..150mtrs to turnaround. For this tuning I set the hook as forward as possible for the first couple of launches...I keep the elevator for LAUNCH phase at the SPEED setting...then for each successive flight I then move the hook back a notch at a time until the airframe looks uncomfortable going up the line....when I have consistently got that condition where it is difficult to control up the line I then start moving the hook it forward a notch or two to make it feel comfortable as you go up......also good idea to throw with your wings level. TE for LAUNCH is at +15 deg

It takes a bit of a structured approach...as I always tend to get distracted at the task at hand by chasing thermals and rising air

I am sure there are just as many other ways to do it...however this seems to work for me.
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Last edited by cobeng; Oct 10, 2012 at 03:14 AM. Reason: Added bit more info
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Old Oct 10, 2012, 09:45 AM
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United Kingdom, Holbeach Saint Johns
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Hello Everyone -

I have now brought a proper balancer to get the CG right and also i have had a play with different distances,this is what i have done.

103 mm = 15 grams
105 mm = 10 grams
108 mm = 0 grams

On the last test flight it was 96 mm with 50 grams.

I've now got it at 103 mm and reduced the flaps up by 5 mm.
As for the tow hook i have moved it all the way to the front of the slot.

Any Comments Appreciated
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Last edited by RC-JG; Oct 10, 2012 at 09:48 AM. Reason: Updated
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Old Oct 10, 2012, 09:49 AM
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As for the tow hook i have moved it all the way to the front of the slot.

Any Comments Appreciated
That doesn't seem good to me but, hey, whatever floats your boat.
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Old Oct 10, 2012, 09:51 AM
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That doesn't seem good to me but, hey, whatever floats your boat.
It's just a start point i know it will need moving - i will have another look tomorrow and maybe move it just 2-3 mm in front of the cg point.
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