Lawn dart upon launch? - RC Groups
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Oct 01, 2012, 11:09 PM
Registered User

Lawn dart upon launch?

I have had this happen to me twice now. Two different airplanes. Two different radio systems. Upon completion... Gentle test tosses leading to sal ... 20-30 trim flights to get things settled .... Both planes flying pretty well and doing what is to be expected during the trimming cycle... Then...

On a hard throw... The planes leaves the throwing hand, goes about 20 feet and dives straight into the ground (at very high speed) as if it had received a full down elevator command from the tx. In both cases, it happens so fast that my throwing hand has not even made it to the transmitter. The tx is in my left hand and is still well clear of my body in the follow-through.

In both cases, all parts of the plane were located at the crash site (I.e. Nothing fell off prior to impact). Except for crash damage, all radio gear appeared to functioning normally during post-crash inspection.

I do not have a lot of high-end dlg experience so I'm asking the pros out there... Is there something in my launch technique that can cause this? Any other experienced-based thoughts on the problem?
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Oct 01, 2012, 11:47 PM
Launch high. Fly low.
What plane?
What radio?
What kind of control setup-- pull spring, push rod, pull-pull?
Do you have a launch preset?
Oct 01, 2012, 11:57 PM
Registered User
If everything seems to be working normally prior to your SAL and the plane behaves as if a strong "command" has been directed to it that's inconsistent with your inputs, I have a strong suspicion from recent experience that your launch angle is blocking your antennas and causing a grey out (brown out, etc.). When this happens, lots of radios default to what I call a "radio neutral": basically what the servos would do with no subtrim or trim. If this includes a touch of down elevator (read: a LOT at high speed), then your plane is in pieces before you regain connection.

I had something similar happen on launch with a carbon D-box Stobel, The "radio neutral" position caused a smooth roll, giving me the very peculiar feeling that someone else was flying. When I landed (verrrry luckily my receiver antennae came back into "view" in time), I repositioned the antennae and set up a more suitable fail safe.

Hope this helps, and yes, the questions from Jcats would be extremely helpful to know.

Oct 02, 2012, 12:10 AM
Registered User
Cats - Osiris and fireworks flow5. One was a 2.4 ghz Futaba 10c w 6 ch rx . The other was a spektrum dx8 w a 6255 rx.

Barry. You have me thinking seriously about that. Both planes are mostly cf and the wing could easily blank the rx at the right attitude. I will look into that more closely.
Oct 02, 2012, 12:15 AM
Registered User
Btw. I cannot say enough about the survivability of the FW5Flow Strong.... The only real damage to the plane was some crushing of the Kevlar nose and a broken pivot rod on the horizontal. Not so the Osiris.... She was totalled from her incident....
The FW is near bulletproof.
Oct 02, 2012, 12:22 AM
Registered User
It can be a technique issue where you might be bringing the transmitter closer to yourself or even a jacket sleeve and bumping the elevator stick.

It can be a CG/trim issue. In the previous test flights, did that include trimming out speed mode at a pretty good speed? Is that the same speed mode as used for launch climb? For launch preset? If the CG is rearward, then the plane will dive at higher speeds. A dive test will indicate if this is likely to be an issue. Also, check the direction of your elevator throw for launch preset. If that is reversed you get a dive.

But the most likely thing is bumping the flaps down some with the thumb during the throw. Particularly if the camber->elevator mix is not yet tuned in for speed (launch) mode, or for launch preset mode, then the plane can tuck. I recommend disabling flaps for launch preset mode as it eliminates this possibility. Some DLGs can high speed tuck this way in my experience, but not all.

Oct 02, 2012, 12:36 AM
Registered User
I just realized something I should have included in my response that will help you test my theory-- bind up the radio and get everything running normally, then turn off your TX. This should put the RX in fail safe mode. If you get a down elevator movement, then I would bet strongly that you've sourced your problem. Again, I was lucky that my fail (not so) safe was a beautiful axial roll...

As for Gerald's response, I now also set up my DLG's so that the throttle stick doesn't move the flaps for the launch condition. Every time I've bumped them I've known it was my fault, which is why I didn't posit this as the cause of your incidents. Now it just gives me peace of mind. It's violent when if happens!

FWIW: I have set up my failsafe to be same as launch mode (touch of right rudder, up elevator) with the exception of a camber addition (somewhere between cruise and float). My theory is that if it browns out during launch, I'll know, but it won't destroy the airframe or completely kill the launch. In normal flight, I'd hope this would prevent a full out speed dive with the up elevator and positive camber, and the rudder inducing a "stay close to home" turn. Frankly, that is probably a thread of its own...

Good luck,
Oct 02, 2012, 12:43 AM
Launch high. Fly low.
Hmmm... even on the 10C, huh? I'm using the 10C with no issues. Currently have a kevlar nosed plane and it's all good.
Launch technique shouldn't cause it to dive unless the control setup is faulty or the servo is stalling or failing. Or getting caught on you shirt or something.
Not too sure about the radio neutral position-- This is the Fail safe feature, which I don't use.

Oct 02, 2012, 12:58 AM
Will fly for food
davidjensen's Avatar
You did not tell us your battery/switch of choice? I have seen many a switch fail. Also do you secure your radio and battery so they cannot move around in the nose?
Oct 02, 2012, 01:36 AM
Registered User
1900 Driver's Avatar
could it be double centering of the elevator or cg too far back(=tuck at speed)? were you able to dive test?

Oct 02, 2012, 02:34 AM
Sure it'll work
Flyextreme's Avatar
I've lawn darted once because I didn't flip my switch back to proper mode. I had some flap (with down elev.) going on at launch. Mine only made it out 10', and stuck the nose about 4" into hard dirt. The only damage was a little delam on the wing, and ripped an inch or two inboard aileron hinge. That's how tough the old LS-1 is.

Is it to obvious that you switch could have possibly been in the wrong mode? It can, and does happen sometimes.
Oct 02, 2012, 08:07 AM
Scott R
Sdr's Avatar
As soon as I read the original post, Gerald's theory of having flaps deployed on launch came to my mind... Because I've done it. When it happened, as soon as the plane left my hand it immediately went into the ground.

Was timing for a guy at the Bruce this past year when the exact same thing happened. He was quite baffled, understandably so, having been down that road before myself I told him likely his flaps were deployed. Sure enough the stick was pulled more than half way.
Oct 02, 2012, 08:33 AM
Registered User
I've had that happen to myself, and like Sdr, I had flaps deployed at launch. Which is why I now spring-load the flap control on the transmitter. Wind up and release, then WHAM!

I've also had it happen that the airplane (upon release) didn't rotate to climb, but head straight out and low. I forgot the pre-set.

And I've also expereinced the scenario of catching a control on a floppy jacket or sweatshirt, but that usually resulted in a loop. So, I always zip up the jacket, now.

Yours, Greg
Oct 02, 2012, 09:33 AM
Registered User
Last week I had my elevator servo slide forward on a rough landing and when I relaunched it had slight down elevator. Fortunately, I threw at a slight upward angle (which I don't usually do, but I just might from now on) so I had time to pull up before it hit. I landed it quickly, packed up for the day and went home to change my underwear.

I had the servo screwed to a plywood tray in the pod and when I installed it I didn't realize that if a screw loosened even slightly that the servo could slide about 1/8" forward. A lightly dorked landing was all it took to do that. I consider myself VERY lucky to still have my brand new Predator II in one piece.

So make sure your servos are installed such that they can't wiggle at all.

Oct 02, 2012, 10:45 AM
Registered User
1900 Driver's Avatar
I've had this happen after my plane had sat in the sun for a while and the conditions changed so I swapped planes. To my surprise my linkages had drifted from the heat! luckily I always throw at an angle and was able to get to the controls in time. So, after that happened I always do a light throw before I ever do a serious launch and try to keep my planes in the shade!


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