HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Jul 09, 2012, 12:54 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
1,652 Posts
Help!
Why does my modified Jetiger flip over shortly after launch?

All seems to go right for a few seconds after launching, then it flips over and down it goes.
System: 64mm fan, 3s 1300mAH Lipo, 40A ESC.
Some of my thoughts:
1) CG is too high above the wing.
2) When slowed down, fan torque takes over.
3) Needs to fly faster on a 4s battery.
Any ideas appreciated.
Maiden 1 (0 min 27 sec)

Maiden 2 (1 min 15 sec)
djacob7 is offline Find More Posts by djacob7
Reply With Quote
Sign up now
to remove ads between posts
Old Jul 09, 2012, 01:01 PM
Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
451 Posts
Can you describe the control inputs your were giving as it exited the turn (right before it flips) in the second video?
DPATE is offline Find More Posts by DPATE
Reply With Quote
Old Jul 09, 2012, 01:11 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
1,652 Posts
Essentially right stick + a bit up elevons and a tad slow down.
djacob7 is offline Find More Posts by djacob7
Reply With Quote
Old Jul 09, 2012, 01:47 PM
Registered User
Joined Oct 2004
2,770 Posts
Snap roll? Looks like a nasty tip stall to me. You could try drooping the leading edge outside the dogtooth slightly. Right stick with an elevon setup means that the left elevon will be down, increasing the wing camber and AOA, making it more prone to stalling. You can add aileron differential, but this will mean pitch-roll coupling.
[edit] I think you can also reduce drag a lot by increasing the gap between the duct intakes and the tails. That narrow gap widening out all of a sudden seems like a major airflow disruption to me.
Brandano is offline Find More Posts by Brandano
Last edited by Brandano; Jul 09, 2012 at 01:52 PM.
Reply With Quote
Old Jul 09, 2012, 01:54 PM
Herk
HerkS's Avatar
Virginia USA
Joined Jun 2007
1,645 Posts
If it was mine, I'd probably try moving the CG forward.
HerkS is offline Find More Posts by HerkS
Reply With Quote
Old Jul 09, 2012, 02:04 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,352 Posts
It seems stable until you get the speed up and input the first control movements. So I'm going to go with the idea that it's the thin wing foam that is flexing and producing the tumble. If I'm right about the foam twisting at higher speeds and air loads then you need to either make the wing and likely the two vertical surfaces stiffer and stronger or you need to use a thicker piece of foam which you cut to a proper airfoil shape to control the amount of drag.

The easy fix would be to cover the existing wing and tail surfaces with a skin of paper glued onto the foam. It can be something light but still fairly strong like newsprint or printer bond paper. For a glue you can use something like spray adhesive or a water based glue such as thinned down white glue or wall paper paste. Or a possible option for an adhesive would be water based varnish. Try some options on some scrap wing material to see how stiff the foam by itself is and then how stiff it becomes when you add a top and bottom skin.
BMatthews is offline Find More Posts by BMatthews
Reply With Quote
Old Jul 09, 2012, 02:08 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
1,652 Posts
Thanks, Brandano. I will first try drooping the tips since it does look like a tip stall.
I also wondered about the small gap between the ducys and the tails. In fact I made a slightly larger gap than they have on the original.
Will report outcome.
djacob7 is offline Find More Posts by djacob7
Reply With Quote
Old Jul 09, 2012, 02:11 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
1,652 Posts
BMatthews, I think you have a good point there about surfaces flexing at higher speeds. I will stiffen them up somehow. Thanks!
djacob7 is offline Find More Posts by djacob7
Reply With Quote
Old Jul 09, 2012, 02:14 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
1,652 Posts
Thanks HerkS, I'll try that too.
djacob7 is offline Find More Posts by djacob7
Reply With Quote
Old Jul 09, 2012, 02:33 PM
12th Pursuit Squadron
TheAeronut's Avatar
Garland, Tx.
Joined Mar 2002
1,100 Posts
I am not an areodynamic genious, but:

It appears to go yaw unstable with more than the smallest control inputs. This may be interference between the duct and the fins.

I would shorten the nose a bit if possible. I would also move the vertical fins as far away from the duct as possible and shift them back or rake them back to get them as far aft of the CG as possible. That and/or adding a fin well back on the duct itself - it should not need to be at all large to considerably aid yaw stability.

As bmatthews stated, it might be the wing twisting, so stiffening it up could help but personally I would go with my previously indicated fixes first.

Best of luck and I hope that the easiest fixes do the job.

J.P.
TheAeronut is offline Find More Posts by TheAeronut
Reply With Quote
Old Jul 09, 2012, 02:41 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
1,652 Posts
J.P., I was trying to stick as close as possible to the original design. One small difference is I made an angle between the two tail fins whereas the original has them parallel.
I also did not flare the wing tips as seen on the original design.
djacob7 is offline Find More Posts by djacob7
Last edited by djacob7; Jul 09, 2012 at 02:57 PM.
Reply With Quote
Old Jul 09, 2012, 02:57 PM
Grad student in aeronautics
United States, GA, Atlanta
Joined Oct 2010
451 Posts
Quote:
Originally Posted by TheAeronut View Post
I am not an areodynamic genious, but:

It appears to go yaw unstable with more than the smallest control inputs. This may be interference between the duct and the fins.

I would shorten the nose a bit if possible. I would also move the vertical fins as far away from the duct as possible and shift them back or rake them back to get them as far aft of the CG as possible. That and/or adding a fin well back on the duct itself - it should not need to be at all large to considerably aid yaw stability.

As bmatthews stated, it might be the wing twisting, so stiffening it up could help but personally I would go with my previously indicated fixes first.

Best of luck and I hope that the easiest fixes do the job.

J.P.
You got me thinking that maybe the entrained/induced flow from the duct/motor is affecting the vertical stabs.
DPATE is offline Find More Posts by DPATE
Reply With Quote
Old Jul 09, 2012, 04:18 PM
12th Pursuit Squadron
TheAeronut's Avatar
Garland, Tx.
Joined Mar 2002
1,100 Posts
The issue that I see is that the duct intake cross section is much greater than the FSA. This large duct entrance makes the area between the duct and the vertical fins open up rapidly. I can only imagine that this really messes with the airflow and pressures over the fins. I would suggesting rebuilding the duct to make the entry only slightly greater than the FSA and certainly no greater than the total DF cross section. I don't know if making the entrance narrower or lower would be better - try one and see.

J.P.
TheAeronut is offline Find More Posts by TheAeronut
Reply With Quote
Old Jul 09, 2012, 05:59 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,352 Posts
I offered my flexy surfaces suggestion because as long as the speed is held down due to climbing the model seems stable for quite some time. It was only when it noses down to level or a slight dive that the issues occur. And to me that suggested speed related causes. And the most common by far speed related cause is twisting of the main fixed surfaces and flutter. And in this case I don't hear any fluttering or reed like singing from the model.

Something to try if you wish is to climb higher and make gentle CLIMBING turns where the model's speed is held down to a stable value. When it is well up and slightly out in front where you can see what is happening nose over to level flight and see how long it takes to go into a flip. When it does cut the throttle down to shutoff. If you are able to then damp out the flip flops and get the model to glide in a stable manner bring it back and land. Then go home and skin the wings and fins to make them more rigid.

Um... I'd suggest at least 300 feet up when you transition to full throttle level or slightly diving flight. That'll give you enough time to recover and go to a gliding flight mode.

If it won't recover then it may still be main fixed surfaces that are too flexible. But it also means you have the CG too far back or messed up when copying the size of the fins.
BMatthews is offline Find More Posts by BMatthews
Reply With Quote
Old Jul 09, 2012, 07:53 PM
Fremont, CA
United States, CA, Fremont
Joined Jan 2009
1,652 Posts
J.P., I'll look into the inlet area problem and see if I can reduce it. Thanks.
djacob7 is offline Find More Posts by djacob7
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Help! Why does my blade mqx fly like s@#t?! topforce Multirotor Talk 9 Jul 29, 2012 11:25 PM
Help! Why does my modified Jetiger flip over? djacob7 Foamies (Scratchbuilt) 0 Jul 08, 2012 01:33 PM
Discussion Why Does My Plane Need So Much Uptrim? krexken Electric Plane Talk 18 Jun 30, 2012 04:35 PM
Discussion Why does my smaller motor give more thrust ? DaN_Th3_MaN 3D Foamies 13 Jun 17, 2012 12:50 PM
Question Why does my heli not work so well with 8 cell over 7 cell? shark7000 Micro Helis 9 Jun 04, 2007 08:58 PM