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Old Feb 23, 2003, 06:01 PM
Heli Mad
stevebarrow's Avatar
Australia, VIC, Elsternwick
Joined Feb 2002
973 Posts
EDF snap roll on loop question

I have a Wemotec Alpha Jet that has had a couple of flights recently. I have noticed that it wants to snap roll when I try to pull into a loop or at the top of a loop. Does anyone have an idea why this model should want to snap roll, especially from a high speed level run being pulled up into the loop? It appears to have a lot of speed on loop entry. It rols fine. My solution so far is to not loop it but I would like to do more than rolls and turns. Any ideas would be appreciated.

The model is running a Midi fan & Mega 30/20/2 on 12 cells.

Thanks,

Stephen
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Old Feb 23, 2003, 06:23 PM
The blade numbers go up to 11
stumax's Avatar
Sydney, Australia
Joined Aug 2002
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Stephen, a few questions before anything can be nailed down:

1. How tight are you trying to loop? (and how high is the wing loading?)
2. How straight is the wing with regard to twist?
3. Does it always snap the same direction?

It sounds like a classic tipstalling scenario. My F104 Starfighter does it if I try and loop too tight because I built the wing without washout. If it always snaps the same direction I'd be looking at the wing twist to see if you haven't got washin on one side, or washout on one side. Also check that the model balances laterally ie. not one wing down. From memory the alpha Jet as a swept wing, which will aggravate tipstalling without washout, especially if the airfoil has the same camber at the tip as the root (the tip should have less camber). hope that helps a bit!

Stu.
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Old Feb 23, 2003, 06:31 PM
Heli Mad
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Australia, VIC, Elsternwick
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Stu

I am trying to do large loops.
Not sure of the wing loading.

Will have to take note of the direction of the roll when it occurs. Last couple of times I have been a bit busy making sure it stays aloft!

I also suspect that it might be stalling a wing.

As far as I can tell the wing is straight.

I'll get some more data next weekend when I fly it again.


Stephen.
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Old Feb 23, 2003, 06:33 PM
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Also make sure one elevator is not deflecting more than the other.

Daren
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Old Feb 23, 2003, 06:46 PM
I don't want to "Switch Now"
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Toronto (Don Mills), Canada
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Three words - High speed stall. Try reducing the elevator deflection. If there is a warp, the washed in wing will always stall first. You could also check the hinge gap (same on both sides) or better yet seal it.
Pat MacKenzie
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Old Feb 23, 2003, 06:59 PM
Heli Mad
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Australia, VIC, Elsternwick
Joined Feb 2002
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The elevator is taped so there is no gap. I did notice that the elevator was very touchy and I am going to reduce the rates to about 50% of what they are now on single rate and 70% on dual rate.


On a different track:
The differential aileron setting I am using works well.
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Old Feb 23, 2003, 07:12 PM
The blade numbers go up to 11
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Sydney, Australia
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Stephen, you could always come up to Sydney for a demonstration! . If you're doing large loops, do have enough power to get through them? What happens when you do a knife edge pylon racing turn? Try that turning left and right and note the difference.

Stu.
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Old Feb 23, 2003, 07:23 PM
Heli Mad
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Australia, VIC, Elsternwick
Joined Feb 2002
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Thanks for the invite if I am up that way I'll try to find room for the model.

Large loops - on the first flight I pulled a reasonably large loop and the model seemed to do pretty well until the top of the loop when it snap rolled. It does seem to have enough power though - mabye a little slow over the top to keep flying.

My T-33 (koysho) will do reasonably large loops with a ND-480 fan unit and the std fan on 10 cells.

I'll try the left/right turns you suggested and see what the effect is. Probably from a great height!

I'll see if I can get some video of it flying.
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Old Feb 23, 2003, 10:29 PM
Scott Black, Montreal
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You are using too much elevator travel for the cg location that you have chosen. Those two parameters must be properly in balance. A forward cg requires more elevator throw for a given response than an aft cg. If you stall going into or (more usually) recovering from a loop then either move the cg forward, or reduce the elevator throw.

Here is how I set up a model. Do the dive test in order to determine a cg location that suits the plane and your flying style. From a trimmed cruise condition up high, put the nose down 45 deg and stabilize for a second or 2, then release the stick. If the airplane continues straight or just pulls up slightly then the cg is how I like it. If it pulls up very sharply it is probably too far forward. If it tucks under and starts to dive steeper it is too far aft. Adjust the cg to get the response you want. Your comment that the elevator is twitchy indicates that the cg is too far aft, which would cause the problem you describe.

Now, set up the elevator throw. You should be able to slow down gradually to a stall with the stall occuring at around full elevator. That way during maneuvering flight where you are using say 1/3rd travel you will be nowhere near the stall. So if it stalls at less than full stick, reduce the rates until it stalls only at full stick. Then once you have it set, redo the linkage so that you get the same elevator through at 100% rates. This will give you better resolution. Setting up the airplane in this way should address the problem.

My little sport pylon model will snap at full elevator, but at 85% rates it can turn a pylon with full stick and no problem.

This has nothing to do with warps or gaps. If you have either, the airplane will not stall straight ahead, but you shouldn't even be getting near those AOAs when attempting a loop. The problem is that the elevator is too powerful. Either reduce its travel or move the cg forward. But moving the cg has an impact on overall handling, so you are better to set the cg first, then tailor the throws.
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Last edited by sblack; Feb 24, 2003 at 06:46 PM.
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Old Feb 24, 2003, 09:26 AM
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Ontario, Canada
Joined Nov 2002
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Agree...

Yup, sblack, hit it right on...


My old foamy Spitfire did exactly the same thing.

Upon starting a loop, I could pull too much elevator and cause the plane to violently snap, if I simply used less elevator the loops were fine.

It was almost like an on/off stability switch! Go beyond about 75% up, and boom, snap roll!


It was kind of a fun trait to have, but a little unpredictable at times!



Sometimes the easy solutions are overlooked...



Thanks.
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Old Feb 24, 2003, 05:07 PM
The blade numbers go up to 11
stumax's Avatar
Sydney, Australia
Joined Aug 2002
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If the elevator is too sensitive it will instantaneously increase the wing's angle of attack past the stall angle. If the wing isn't stalling then there is nothing to cause a snap - the elevator can't do it on it's own, it needs one wing to stall. With a heavily loaded jet with a thin airfoil, with swept wings, the tip can easily reach the stall angle by vitrue of the increased upwash at the tip induced by the wing sweep. If the loop is large, then chances are that the speed is bleeding off, the angle of attack is increasing and chances are the aircraft is not tracking dead true (ie corkscrewing slightly) so one wing is at a slightly higher angle of attack, so it stalls.

It's very easy to make any aircraft stall even under full power. When I was training for my commercial pilot's license we used to have to find the max rate of turn by banking to various angles, then pulling the turn as tight as we could. Usually around 70-80 deg angle of bank at full power we could keep pulling back, the stall indicator would go off, and shortly after you could feel the buffeting indicating the onset of stalling, pulling back harder led to the aircraft tipstalling out of or into the turn (depending on if you were in balance or not).

When I flew F3D pylon we used to see how tight we could turn by entering a turn (90deg angle of bank) and slowly pulling it tighter and tighter. This was at around 200mph. Eventually when the turn was too tight the model would stall into the turn because the lower wing was at a higher angle of attack. Quite a sight to see!

Stu.
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Old Feb 24, 2003, 06:36 PM
wheel chock
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NSW, Australia
Joined Nov 2002
293 Posts
I have to agree with stu,

I have done the same power on, stall training, but in a 152 aerobat which could do a full 90 degrea AoB, so we had to fudge it a little by reducing power.

I've seen videos of deep stall, which is similar to the loop scenario, but pulling out of a dive. The pilot pulls back to much, the wing stalls and the plane just 'mushes' it's way into the ground. Worst part is, they mostly happen at airshows.

How is the slow speed stall? If your going to get a wing drop or tip stall, that may be the safest time to see which wing is stalling.

mitch.
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Old May 24, 2004, 08:53 PM
Heli Mad
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Australia, VIC, Elsternwick
Joined Feb 2002
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Hi Guys

Thanks for the advice, I'll try moving the CoG forward and see if that makes a difference.

A bit of a delay in responding - I must be working to hard

Stephen
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