Lawn dart upon launch? - Page 2 - RC Groups
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Oct 02, 2012, 01:05 PM
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The only time I've thrown with the flaps down, I was lucky it didn't hit me in the back of the head. Instant loop.

Don't understand how deployed flaps would cause a dive??

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Oct 02, 2012, 01:30 PM
Walter Roos
ROOSWH's Avatar
Last edited by ROOSWH; Oct 07, 2012 at 11:05 AM.
Oct 02, 2012, 01:32 PM
Walter Roos
ROOSWH's Avatar
My settings will cause a very flat horizontal release with full down flaps.
I think it depends on your CG and how much down elevator you have mixed in.

That's why I disabled my flaps in launch mode several years ago. It was a safty issue, not a crash problem.

I figure I've lawn darted as many or more DLGs than just about anybody, and I ain't bragging about it. Usually (many times) it's because I left the switch plug in while launching. 2nd most common reason for my lawn darts is changing planes during a DLG contest round and not getting the new model set in the transmitter. Only once when my Horz-stab mount broke loose on launch,that was just a really bad crash about 30 feet in front of me, and not a real lawn dart.

And then my one other type of lawn dart. I was flying my new Predator II and the trim was just right, so I set my transmitter on the ground to show-off just how stable my new Predator was. As I walked away from my transmitter, it fell over and the 72 MZ antenna lay flat on the ground. The Predator turned straight down from 20-30 ft up and made a perfect lawn dart. The fuselage was almost perfectly vertical.

Last edited by ROOSWH; Oct 02, 2012 at 01:39 PM.
Oct 02, 2012, 01:37 PM
Registered User

Ballast up your plane, get it to good altitude, and dive it down steeply in speed mode. Get it going very fast. Level it, then very abruptly pull flaps.

If the compensation curve is off, then there can be a notable pitch response.

If the flow disruption due to dropping the flaps messes with the flow around the horizontal stabilizer, then there can be a notable pitch response.

There can be a difference between the transient response from suddenly dumping flaps, and the normal response from more sedately dumping flaps. Moving flaps slowly keeps the flow closer to steady-state conditions. Very rapid motion though won't necessarily let the flow stay in the steady-state pattern. We see an example of this all the time with DLGs - the vertical tail. It would stall with the loads we are subjecting it to, were the flow constrained to have to fit the steady-state behavior. But the yaw is so quick compared to the distance traveled that the flow never reaches steady state. The flow stays attached better, and much higher lift coefficients can be achieved than would normally be the case.

There is another reason which a tuck can occur. If one uses weak flaperon servos, or the flaperons are not stiff enough, then they give way under the high air pressure. The elevator though wouldn't give way. So with a proper compensation curve for the elevator, rapidly dropping flaperons at high speeds can result in a dive because the elevator is compensating for normal flap travel but the actual travel is reduced in this case.

Most people though don't have the compensation curve very close to correct anyway. It is worth taking the time to fix it... At relatively normal flight speeds, one should be able to cycle the flaps slowly anywhere in the range of travel, or over the whole range, and the glider should keep flying sedately along with no manual elevator correction required. If this is not the case, more work should be done. Landings are so much nicer when flap compensation is properly set up. BTW, a linear mix will not do it. It takes at least a bi-linear mix to come close, and that tends to be close enough in practice for normal flap travel ranges. If one sets flaps up to have extreme travel (for a DLG) then bi-linear won't be a good enough approximation and one needs to use a full curve mixer. The compensation rate is greater in the upper region of the flap travel and lesser in the lower region. The built-in mixers often available with modern programmable transmitters are usually not sufficient for the job. Just set it to zeroed out, and use a programmable mixer.

Oct 02, 2012, 01:49 PM
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Gary please do not do a GT you will more than likely strip one or both of your servos. If you do that low and one strips the plane will flip over and total it out.
Oct 02, 2012, 02:58 PM
Oleg Golovidov
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If the model flies straight and level from a toss, or better yet from a very strong toss, and seems to climb normally for about 20ft, then abruptly noses down - most likely you are catching the stick on your clothes or body when bringing the radio back after launch. I had this happen myself. It is very difficult to notice or feel, and it is totally puzzling. A very light bump on the elevator stick is enough to send the model into the ground.
Oct 02, 2012, 03:26 PM
Registered User
Lots of good thoughts to probe. As followup to some of the questions/comments...
No jacket involved. Pretty darn sure that the elevator stick was not bumped.
Tx was definitely in speed mode. I did not actually see the elevator move, but I definitely heard the elevator servo when I pressed the launch button.
When in speed mode, the throttle stick (landing) flaps are unavailable.
The Osiris was carbon fuse with whiskers. Pull-pull on both rud and elev.
The Flow fuse is Kevlar nose with antennas inside. Pull /spring on both rud and elev - servo pulls for up. Pull string on elev was still secure after crash.
No switch on either plane. Battery plugs in - to an extension on the Osiris and into the rx on the Flow.
Did not see the flashing light on the Spektrum rx that would indicate loss of signal.
Servo tray was still solidly glued (except where it was broken) .
The Flow has Hs5045s on the flaps... Futaba 3114 on elevator.
4.8v /300mah nimh was fully charged 30 minutes prior to disaster. Have not yet recharged to see how much capacity was used during flights prior to crash.
On one flight 15 minutes prior to the crash, I did try a catch-and-launch and did so without getting into launch mode. I threw her with the flaps fully deployed... She went up very slowly at about a 45 degree angle.
Nothing on the fuse appeared to have moved. One 3gram piece of lead in the nose secured with foam and with the battery wedged in. All quite secure.
I had just thrown the plane 10-15 times with no issues on launch or during flight... No hard landings.

My flying buddy has the plane right now ( he's much better at repair work than i am) but all of the radio settings will be checked thoroughly as soon as I get a chance.
I was still in test/trim mode so not sure if cg was dead on, but it seemed close.

So.... I am questioning...
battery capacity and load handling....
I continue to wonder if something in my launch setting created a 'disturbance' during a hard launch that resulted in a hard tuck...
I wonder if it's possible that my 'hard' launch created a situation where the elevator servo was overpowered...
Cg setting ....
Carbon wings shadowing receiver...
Thanks to you all . I will post again if I find anything more definitive and will watch to see if there are any additional thoughts out there
Last edited by olythom; Oct 02, 2012 at 03:49 PM. Reason: Added detail
Oct 02, 2012, 03:31 PM
Registered User
Also considering launch technique again as related to bumping the elev stick. Both of these incidents occurred when I thought to myself... "this will be a big throw" ... Maybe my technique gets sloppy when i try too hard???? I have tried to re-enact the hard launch but who knows?
Oct 02, 2012, 03:38 PM
Registered User
And... Speed mode elev trim as GT mentioned. I did get some good trimming done during those test flights but most of that testing was in cruise and thermal modes. I never trimmed the speed mode at truly high speed.
Thanks again.
Last edited by olythom; Oct 02, 2012 at 03:52 PM. Reason: Typo
Oct 02, 2012, 04:06 PM
Registered User
I presume the flap stick is not disabled when not in launch mode? Did the dive occur when the preset was released? It could then be flap stick or trimming. I don't release my preset until I see the trajectory I want, but many I think release it on timing. Since I throw fairly upwards I don't need as strong a preset as many use anyway - so it is easier for me to time it. YMMV, and I don't recommend this for people who haven't flown DLGs for a couple of years at least, so they have become pretty consistent. I've also used the preset to dodge planes in mass launches a couple of times over the last several years. It is the one control I still have available before my right hand can get to the right stick.

At the CASA contest we had rain. My plane got soaked, and my transmitter got wet. Thankfully I throw decently upwards, because my beater plane did a notable downwards arc, while I was still holding the launch preset. I kept holding the preset until my hand was on the sticks again. It was quite reproducible. I dialed in a lot of trim but never got the plane really flying. I considered it too unsafe and abandoned the contest. In my case I'm fairly sure it was water in the transmitter, but it could equally well have been water in the plane. There was also a slow roll that I had to correct, but the down pitch was nearly deadly.

Oct 02, 2012, 04:11 PM
Registered User
One thing to consider that I don't think has been mentioned yet.
I personally have had launches in the past that have been similar to what you described and I found that there were electrical circuit faults created when launching because of the increased forces on the connections. So I suggest to you maybe to investigate your wiring at harness and receiver. It is extremely difficult to see a minute solder or wire fault because it appears closed during inspection but can open during launch. I know from experience.

Good luck,
Oct 02, 2012, 05:38 PM
Registered User
Thanks Ray. In these two cases, the electronic systems were completely different. After the first disaster, I put all new electronics into the second plane. No clear connection issues after crash.
GT, my launch condition is; speed mode (to set flaps and elevator) with a touch of up elevator that is added in while the button is held. I throw at about 25 degrees above level and release the button when I get to a good angle of climb. In the latest crash, my finger was still on the button. I don't think I even got my right hand on the tx but am still considering that as a cause.
Is there any way that bumping or holding rudder could cause this sort of problem?
Oct 02, 2012, 05:44 PM
Registered User
I've thrown planes where the vertical tail broke on the throw, or the rudder horn ripped out. I'm not that strong a thrower, but I've flown in some very strong wind! What happens is the plane tries to roll inverted.

Oct 02, 2012, 06:47 PM
Registered User
Hey Gary, I did the same thing this summer, but in my case it was definitely user error. After flying for a while I brought the plane in, adjusted some setting on the radio, turned the radio off (for some unknown reason), then proceeded to throw the plane right into the ground as hard as I could. That was not a good feeling at all Lots of lessons learned on that one. I now almost obsessively trigger the launch preset a couple of times before each launch an listen for the elevator noise.
Oct 02, 2012, 07:37 PM
Registered User
I'm casting one vote in favor of the pulling-the-transmitter-into-your-body-after-the- throw theory. Your first post discounted that possibilty but your later posts are admitting the possibilty, so I suggest thinking a bit more about it and trying to observe that aspect of your throw technique a bit more in the future. I destroyed a model a few years ago by that mistake. I had a throw where the model pitched down and almost impacted the ground, so I checked everything over and found nothing wrong. The very next throw, the same thing happened again, except the model did impact. It was after that when I finally figured out what had happened, confirmed by using a little corner of my brain to monitor where my transmitter hand was going after the throw. I had in fact developed a habit of drawing that hand in close to my body, to the point where the transmitter stick was hitting my chest and getting a pronounced down elevator impulse.

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