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Old Jul 24, 2012, 07:43 PM
Arizona Fun
Casa Grande Arizona
Joined Mar 2010
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Tight loop snap roll... why?

I have an E-Flite Pulse XT 25e. When doing a loop with D/R on low (70%), it loops just fine. D/R on high and whoa it does a mild snap type rollover at the top of the loop. Upon inspection I finally noticed the elevator halves are not connected firmly, the servo side seems firm enough where the control horn is, but the other side of the elevator flexes pretty easy while the servo side is still. There is only a wire connecting the two halves. It seems elevator flex would act like a small set of ailerons on the tail causing the tight loop rollover. Do you guys think my diagnosis is correct? With high up elevator input I think the air forces the weak side down but the servo side stays firm causing an aileron effect. I had another airplane do this but don't remember which one, might have been my Mini Pulse, I just didn't do tight loops with it. I want to firm up the Pulse's elevator but there is no way to connect the two halves other than the wire. I read about running a bead of CA along the wire but can't see how that would do much good, any ideas? Thanks...
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Old Jul 24, 2012, 07:47 PM
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Wintr's Avatar
USA, MO, Florissant
Joined Nov 2010
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Yes, differential elevators will cause roll; advanced jet fighters use this as part of their control logic. Can you replace the wire with a small carbon fiber tube?
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Old Jul 24, 2012, 07:49 PM
Fly it like you STOL it!
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United States, FL, Daytona Beach
Joined Jan 2011
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That's probably why. You could pull it all apart and put in a stiffer wire, run a 2nd elevator servo, or maybe add a CF rod as a torque rod....all of those require some work and mods though, so if you aren't comfortable doing that just leave it and avoid tight, high-G stuff.
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Old Jul 24, 2012, 08:17 PM
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scirocco's Avatar
Australia, ACT, Kambah
Joined Feb 2001
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Or on the higher rates you're getting enough pitch to reach the stalling angle of attack for one wing before the other, and the model autorotates - basic asymmetrical accelerated stall, doesn't have to be going particularly slow. I have a couple of models where injudicious full back stick on high rate is pretty much a guaranteed snap roll, although in my case on Mode 1 it's probably exacerbated by some inadvertent rudder.

ps: take a look at the similar threads at the bottom of this page
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Old Jul 24, 2012, 08:22 PM
I broke it
David Hogue's Avatar
USA, NC, Clayton
Joined Feb 2001
5,773 Posts
Dont worry about the joiner, the problem is too much elevator throw coupled with a too aft cg. Move the battery up or reduce the throw on high rate. A little at a time till its stops snapping out of the loop. Great airplsne...wish I still had mine.
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Old Jul 24, 2012, 08:58 PM
Arizona Fun
Casa Grande Arizona
Joined Mar 2010
303 Posts
Thanks for the input guys. scirocco I think your theory might be more of what's happening. In thinking about it more, that Pulse loops pretty tight on high rate with a higher angle of attack and seems to also slow down enough for a partial wing stall and elevator flex might magnify the effect helping to flip it over. In the tighter loop the plane seems to start it's rollover just before the top of the loop. With lower D/R the plane likely has less angle of attack and keeps more airspeed over the wings in a slightly bigger loop therefore not causing the 'asymmetrical accelerated stall' I have also tried to make to tight a turn at low speed with this Pulse and it starts a corkscrew which is the same thing that could be happening with the tight loop at lower speed. That's some fine knowledge and thinking scirocco, I appreciate it. I'll keep the low rate on the elevator and call it... 'avoiding the scirooco loop'
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Old Jul 24, 2012, 09:14 PM
Arizona Fun
Casa Grande Arizona
Joined Mar 2010
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Quote:
Originally Posted by David Hogue View Post
Dont worry about the joiner, the problem is too much elevator throw coupled with a too aft cg. Move the battery up or reduce the throw on high rate. A little at a time till its stops snapping out of the loop. Great airplsne...wish I still had mine.
Now there's another good idea thanks. I didn't think about CG but I always leave mine a bit nose heavy. I remember reducing my elevator high rate until it stopped loop snapping. I just checked my radio and high rate is 80% It probably stopped snapping at 85% but I went a little further to be safe because that's quite a surprise when you do a simple loop and that snap happens, a slight panic could cause a bad crash. Thanks again for the input and yes, it is a great airplane. I also reinforced the main wing joining point with plywood and epoxy after reading about some wing folding problems.
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Old Jul 25, 2012, 01:54 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Aberdeen
Joined Mar 2006
11,594 Posts
Yep, snapping out of loops is usually a sign that you are trying to loop too tight and are stalling the wing. You can pretty much forget the elevator issue because if that had any effect it would be a very slow gradual roll, not a sudden snap.

Stalls are often misunderstood. Many think they happen only because the plane flies too slow, but in reality a stall occurs when the wing exceeds it's critical angle of attack, and that can occur at any flying speed if you haul back on the elevator hard enough and if the elevator has enough authority.

The simple solution is to not haul back so much on the elevator
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Old Jul 25, 2012, 04:21 AM
Low'n Slow is safest, right?
KaiWE's Avatar
Norge, Østfold, Fredrikstad
Joined Jun 2011
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Quote:
Originally Posted by David Hogue View Post
Dont worry about the joiner, the problem is too much elevator throw coupled with a too aft cg. Move the battery up or reduce the throw on high rate. A little at a time till its stops snapping out of the loop. Great airplsne...wish I still had mine.
Can you please elaborate on this? I find that those of my planes who snap on top of tight rolls tends to behave better when I move the CG more aft, e.g this behavior is more pronounced with high elevator trow and too front CG...
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Old Jul 25, 2012, 07:09 AM
Electric Coolhunter
Thomas B's Avatar
United States, TX, Fort Worth
Joined Jun 2000
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Quote:
Originally Posted by KaiWE View Post
Can you please elaborate on this? I find that those of my planes who snap on top of tight rolls tends to behave better when I move the CG more aft, e.g this behavior is more pronounced with high elevator trow and too front CG...
Some models can loop a little tighter with an aft CG and show less tendancy to snap out of the loop, but other factors come into play: aircraft configuration, wing leading edge hygiene, structural stiffness, etc. I would not assume that moving the CG aft to reduce snapping would hold true for all models. Moving the CG forward usually reduces the tendancy to snap out of a loop for most models.

I have seen a foamie do as you describe due to the fact that the more aft CG allowed a faster reaction to the pitch input, which allowed aerodynamic loads to hit a higher level faster, which caused the wIng tips to wash out a little more due to flexing, reducing the critical angle of attack overall.
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Old Jul 25, 2012, 07:23 AM
Low'n Slow is safest, right?
KaiWE's Avatar
Norge, Østfold, Fredrikstad
Joined Jun 2011
553 Posts
Quote:
Originally Posted by Thomas B View Post
Some models can loop a little tighter with an aft CG and show less tendancy to snap out of the loop, but other factors come into play: aircraft configuration, wing leading edge hygiene, structural stiffness, etc. I would not assume that moving the CG aft to reduce snapping would hold true for all models. Moving the CG forward usually reduces the tendancy to snap out of a loop for most models.

I have seen a foamie do as you describe due to the fact that the more aft CG allowed a faster reaction to the pitch input, which allowed aerodynamic loads to hit a higher level faster, which caused the wIng tips to wash out a little more due to flexing, reducing the critical angle of attack overall.
Thank you! - learning more each day

BTW: The planes I have that behaves better with the CG a little bit aft is a Durafly Micro Slick, lower stall speed/lands slower and no snap-out-of-loop - and the 1055mm EPP Yak 55 from HK, which tends to snap-out-of-loop when flown with a bit more forward CG and a bit heavier due to a larger battery (2200 vs. 1800).

Greetings from Norway,
Kai W-E
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Old Jul 25, 2012, 08:27 AM
Redacted per NSA "suggestion"
dedStik's Avatar
United States, VA, Virginia Beach
Joined Feb 2012
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Quote:
Originally Posted by Wintr View Post
Yes, differential elevators will cause roll; advanced jet fighters use this as part of their control logic. Can you replace the wire with a small carbon fiber tube?
This, my PZ 109 has split elevators. When I got it the original owner augered it, doing my pre flight I noticed one was slightly higher than the other, I was worried about induced roll due to it, and spent some time eyeballing them to get them even.
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Old Jul 25, 2012, 09:22 AM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
12,788 Posts
What it sounds like your model is doing is actually an aerobatic maneuvre - an 'Avalanche', AKA a regular inside loop with a snap roll at the top.

From your description of the inadequate elevator joiner, when you apply elevator, the side the pushrod goes to will move as demanded, assuming its linkage is slop-free. The elevator on the other side of that floppy wire will, in all circumstances, move less due to airflow blowing it back against that too-thin wire.

Though there will be a little roll coupling off this mis-matched pitch, I doubt its enough to make the model roll 360 at the top of a loop.

Likely causes of what sounds like a slow snap roll are, as above, an aft CG and/or too much elevator throw. Sounds like the model is nibbling on the edge here - if you got a really fast snap off up elevator, you'd likely be out shopping to buy another model by now.

Try moving your battery pack forwards a little, if possible, and accurately measure your elevator throw to ensure that either they match the model's specs or that the model's manual could be wrong and it needs either or both of these corrections.

A properly designed and set-up aerobatic pattern style model should be capable of staying out of spinning and snap rolling elements until the pilot wants to go there. Mine, though I don't compete any more, only snap or spin from the appropriate speeds with full inputs of aileron, rudder and elevator.

Good luck with it

D
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Old Jul 25, 2012, 10:22 AM
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Joined Mar 2010
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Not likely...
The most likely culprit is your RIGHT hand moving the right stick roll control (left-right) as you apply right stick elev control (up-down)... It's why Expo was invented I do believe...

Doesn't take much
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Old Jul 25, 2012, 11:05 AM
Electric Coolhunter
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United States, TX, Fort Worth
Joined Jun 2000
14,515 Posts
Quote:
Originally Posted by rtbates View Post
Not likely...
The most likely culprit is your RIGHT hand moving the right stick roll control (left-right) as you apply right stick elev control (up-down)... It's why Expo was invented I do believe...

Doesn't take much
Accidentally adding a little aileron as you do a tight loop usually does not cause a snap roll. It simply creates an off axis loop with some roll.
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