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Old Nov 10, 2014, 04:28 PM
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Ted Strader Whirlwind surprising behavior

The Ted Strader Whirlwind is one of my favorite models. I just love the looks of it and built one back in 1964, flying it rudder-only with escapement, home made radio and Fox 15 power.
It flew really well and created very good memories. When the radio failed one day, it just flew away free-flight fashion one Sunday and was recovered intact in a wheat field two miles away the following Thursday.
I built another one recently with proportional rudder, elevator and throttle. Power is an MVVS Junior 2cc diesel engine which has proven to be quite a bit more powerful than the Fox 15 in the first one.
This new one exhibits the following behavior:
At full throttle, it will respond to rudder inputs by yawing very visibly right or left but keep flying in a straight line without leaning into a turn. the result is that it is almost impossible to steer it at full throttle.
Reduce throttle, pull a little bit of up elevator and it becomes controllable again, also the glide is no problem.
The problem is loss of rudder-induced roll effect at higher speed, recovering at slower speeds.
As the picture shows, it is a low-winger with little dihedral (1 1/2" under each wingtip over the 48" span), but I do not quite understand why it would behave as described.
I know that fitting ailerons would solve the problem, but I cringe at the idea when I remember the first one flying so well.
Any explanation?
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Old Nov 10, 2014, 05:00 PM
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My almost uneducated guess is that the increased power pulls the nose and corresponding wing AoA down. Since the dihedral-yaw effect of rudder-only depends on the angular difference of the AoA between the yawed wings, reducing that reduces the effect, in this case way too much.

First suggestion is to use less power or a lower-pitch prop. Next would be to move the CG forward and add up trim, either in the stab or elevator.

L
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Old Nov 10, 2014, 05:12 PM
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B.t.w., to my eye, comparing the photo on the right, which I guess is your new model, with the old photo on the left, as well as the dihedral brace/doubler in the plans .... your new model has significantly LESS dihedral than the old design.

It couId just be an obstacle delusion, but if I were you I would double-check that! IF true, it's time to get out the old steam kettle and slowly, carefully bend in more dihedral, letting the wing decide were it wants to bend. Hopefully the stronger center section will not bend much, and the resulting elliptical dihedral, as measured at the wingtips can thus be less than a standard "V"-form dihedral. Holding the wing vertical to the steam, letting the steam flow past the upper and lower surfaces simultaneously, with the leading edge down, the trailing edge up and the wing level, as in a vertical dive, is the smartest way, by the way.
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Old Nov 10, 2014, 05:48 PM
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I grinned at the symptoms since it's exactly what we found with a buddy's RET model when at full throttle.

xlcrlee pretty much nailed what I'm thinking is the cause. It's likely that the airfoil used is flying at such a low angle of attack at high speed that it's operating in a range of angles where the lift doesn't alter much. Or perhaps it's in the range where it's being pushed to such a low angle of attack, and possibly even a negative angle, that the drag difference between the wings is changing how it responds to the rudder. Or you may have some other issue that is making the change in lift not occur in a linear manner with the angle of attack change.

I'll bet if you try to do a bigger size loop that the rudder will produce the proper roll in the loop. That's because the angle of attack is greater in the loop. That's what happened with my buddy's model.

And yeah, if you reduced the dihedral then you shot yourself in the foot I'm afraid.

In the end you may just need to practice greater throttle control and fly it a little slower so you use a little more up trim. You could also try xlcrlee's idea of adding a touch of nose weight along with some downthrust and re-trim the elevator as required. The added up trim due to the forward CG might be enough to restore proper behaviour.
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Old Nov 10, 2014, 06:23 PM
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For the record, I personally prefer to change CG by moving stuff, or sanding/cutting away stuff rather than adding weight .... if possible. Maybe the motor could be moved forward. "Too bad" it's diesel, else you could add a glo or other battery. Larger tank ....? Bigger wheels? Fwd-tilted L.G.?

Also .... just noticed that maybe it's plastic-film covered! Then adding dihedral/elliptical dihedral is a snap! Just use your covering tool to heat and bend.
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Old Nov 11, 2014, 04:44 AM
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Quote:
Originally Posted by xlcrlee View Post
B.t.w., to my eye, .... your new model has significantly LESS dihedral than the old design.
I just double checked and found that the dihedral of the new model is exactly 1 1/2" under each wing tip, which is as per the plan.
I definitely remember having built the first one with the 1 1/2" dihedral which is the reason for the "coolness" of this design.
The attached two pictures show my new one compared to Ted Strader's original from the Flying Models January 1961 article. No significant difference that I can see. Just themore powerful engine on my new one.
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Old Nov 11, 2014, 04:51 AM
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Quote:
Originally Posted by BMatthews View Post
I grinned at the symptoms since it's exactly what we found with a buddy's RET model when at full throttle...
It feels good that I'm not the only one. Thanks
Quote:
Or you may have some other issue that is making the change in lift not occur in a linear manner with the angle of attack change.
This sounds like the root of this behaviour.
Quote:
In the end you may just need to practice greater throttle control and fly it a little slower so you use a little more up trim.
I have no problem actually flying the model playing with the throttle, it is just that I was curious for an explanation. By the way, the little MVVS throttles very well, quite a nice little engine.
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Old Nov 11, 2014, 04:57 AM
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Quote:
Originally Posted by xlcrlee View Post
It couId just be an obstacle delusion, but if I were you I would double-check that! IF true, it's time to get out the old steam kettle and slowly, carefully bend in more dihedral, letting the wing decide were it wants to bend.
The wing won't bend at all, with the strong D-tube leading edge and the silk covering like the original and my old one. The only possibilities are either to saw the wing and reglue it with a bit of fiberglass reinforcement, or make a new one with the alternative 2 1/2" dihedral and see how it goes. Probably what I might do, it is a quick wing to build. In the mean time, next flying session I'll move the receiver battery forward by 4 inches, as your suggestion.
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Old Nov 11, 2014, 08:17 AM
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Quote:
Originally Posted by JMP_blackfoot View Post
The wing won't bend at all, with the strong D-tube leading edge and the silk covering like the original and my old one. The only possibilities are either to saw the wing and reglue it with a bit of fiberglass reinforcement, or make a new one with the alternative 2 1/2" dihedral and see how it goes. Probably what I might do, it is a quick wing to build. In the mean time, next flying session I'll move the receiver battery forward by 4 inches, as your suggestion.
Well, I first assumed doped silk! Please TRY using stream as originally described above. Even epoxy will move a bit that way!
[I have had to do stuff like this many times ... and one of my main "problems" has always been building too strong]

If you accidentally "ruin" any epoxy joint that way, you wont have to saw then!
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Old Nov 11, 2014, 08:21 AM
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Just now read above that the dihedral Is correct. So that's an easy thing NOT to change!

EDIT:

But wait, folks, there's more ....

[sorry, have a slow, tired brain in the moment, it starts to work now] Why I first thought the motor was pulling the nose down was because of the thrustline well above the center of lift [esp. with its relatively low dihedral]. But on the plans I now see the large downthrust, usually req. for relatively low, vs high, power.

I'll BET you can just reduce the downthrust!

And that should be easy ....

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Old Nov 11, 2014, 10:03 AM
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another odd duck
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Old Nov 11, 2014, 12:47 PM
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Here's an option that might help if you don't mind doing a bit of cutting. It would involve altering the wing tips as shown below. It's not a lot but the angled tip might just make enough difference to let you still roll despite the apparently marginal dihedral.

The tip piece being a heightened version of the airfoil's upper curvature so the upper covering runs fair to the upper surface as it comes off the wing.
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Old Nov 15, 2014, 05:12 AM
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Great suggestion!
I'll make some slip-on sleeves to fit over the wing tips with this type of upturned design as a test.
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Old Nov 17, 2014, 09:07 PM
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For me the mystery is how the original could have flown as well as you experienced, with no ailerons and so little dihedral and the low wing placement...

Quote:
Originally Posted by JMP_blackfoot View Post
I just double checked and found that the dihedral of the new model is exactly 1 1/2" under each wing tip, which is as per the plan.
I definitely remember having built the first one with the 1 1/2" dihedral which is the reason for the "coolness" of this design.
The attached two pictures show my new one compared to Ted Strader's original from the Flying Models January 1961 article. No significant difference that I can see. Just themore powerful engine on my new one.
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Old Nov 17, 2014, 09:14 PM
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dihedral is not like sweep

Quote:
Originally Posted by xlcrlee View Post
Since the dihedral-yaw effect of rudder-only depends on the angular difference of the AoA between the yawed wings, reducing that reduces the effect, in this case way too much.
I question this idea that the roll torque created by dihedral during a sideslip is highly dependent on the aircraft's overall angle-of-attack, i.e. the average angle-of-attack of both wings combined.

I'm at a loss to explain why the OP observed a better roll response to rudder inputs in low power, low-airspeed flight than in high power, high-airspeed flight but I think we should be careful about suggesting that this would be the behavior that we would normally expect from such a model.

My Gentle Lady glider rolls in response to rudder commands even when the net overall lift force is zero (e.g. during a "ballistic" arc) or negative (e.g. during sustained inverted flight). Not to mention in high-speed (low-angle-of-attack), positively loaded flight.

I think we are sending the op on the wrong track here.

If the roll torque were created by sweep, that would be entirely different, and the offered comments would make a lot of sense. But not when there is no sweep, only dihedral.

Still, the proposed fixes should work.

Maybe the critical difference between the new model and the older one is the vertical location of the CG-- if it is higher on the newer model, due to the heavier engine, there would be less "pendulum" stability or whatever you want to call it. With a higher CG the sideforce generated by the fuselage in a slip would contribute a stronger "upwind" roll torque, or a weaker "downwind" roll torque, than would be the case with the lower CG, and the "effective dihedral" of the model will be less. Etc....

Steve
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