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Dutch Roll

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Old 01-25-2003, 05:31 PM
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Steve Westphal
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Default Dutch Roll

A while back a friend told me he dropped the idea of building an XB-70 because of it's tendancy to dutch roll on take off. What is a dutch roll??? Thanks!
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Old 01-25-2003, 06:04 PM
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Ed_Moorman
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Default Dutch Roll

Dutch roll, in full scale planes, usually means a slight yaw-roll movement at cruise speed. It is very common on swept wing planes. The plane yaws slightly, picks up roll due to the yaw, then yaws back and rolls back. Inside the plane, it feels like you are rocking around.

The Boeing 707/USAF KC-135 Dutch rolled a lot until they installed a yaw damper on top the fin. If you look at the early 707s and KC-135s, you will notice a shorter fin. Later production planes had a yaw damper located in a vertically extended fin. The Air Force planes were retrofitted.

With yaw damper on, the plane was rock steady, but if the damper had problems, it could be all over the place. I reacall refeuling an F-4E during the Vietnam war on a 135 with a bad yaw damper. I was leading the flight and my roommate was on my wing. I hooked up to the tanker and was all over the sky trying to stay on the boom. All the time Mick was giving me this ration about how I couldn't fly. The F-4 was normally real stable on the boom, especially with a full load of bombs. After I topped off, he hooked up and had the same trouble. I radioed the tanker pilot and asked what was going on. "We had a yaw damper failure and are flying manually," was the answer. Made a big difference.

As for the B-70, you might have some yaw instability due to the closeness of the fin-rudder to the CG. Scaling a plane down normally results in the tail being too small for good RC flying. In addition, a delta configuration has most of the wing area located near the center so you get very little natural damping in roll. The deltas I have flown, except for the Diamond Dust, have tended to be roll sensitive around center. This could make it sporty on a test flight even if you are a really good pilot. Once you get it sorted out, exponential helps with the center sensitivity. Good centering servos also make it easier.
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Old 01-25-2003, 07:15 PM
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Default Thanks Ed

Thanks a lot Ed. That was an excellent and thorough explanation. I appreciate it!
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Old 01-25-2003, 07:43 PM
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Dsegal
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Default Dutch Roll

The cure for Dutch Roll in free flight models is to increase the vertical tail area.

Dave Segal
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Old 01-25-2003, 09:41 PM
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Default Dutch Roll

This is straight out of book #9 ,IF-89-620, Tech Order Manual dated 28 Aug 1956. Dutch Roll is when the air-craft creates a yawing motion coupled with a roll action causing the air-craft to fish-tail or skid in a roll. I had one of our pilots write this up, said it happened when he engaged auto-pilot. Don`t remember what I did to fix it, probably changed pilots!! >>big max 1935
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Old 01-26-2003, 09:03 PM
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Default We Salute You Ed

And thank you for your service to us and our country. I just missed the conflict, as it ended as I was graduating high school in 1974. It has and always will fascinate me though...
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Old 02-04-2003, 07:58 AM
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Grampaw
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Default Dutch Roll

We had Dutch Roll problems with two Ace All Star bipes. After a hand launch it looked like it was doing the Cha Cha Cha until it hit the ground. Too much damage to try again. Tried a ground takeoff with mine. Same results. Damage was minor and repaired on the spot, and tried again. I couldn't feed in TX corrections fast enough to gain control once it started. The design was sound and been flown with excellent results, and we built ours as designed. In desperation I set up for another takeoff. This time I just let it go on its own! It lifted off and started waggling and rolling like a drunk on ice skates! It kept climbing though, and in a few seconds settled down and flew beautifully. After that takeoffs were hysterical events for all present. I'd just stand there holding the transmitter at my side watching that fool thing climb out. No one ever asked to fly it, as my rule was " to fly it you gotta get it up there!" No takers. Some hi powered short coupled bipes are bad about that. Once read of a full size MiniPlane bipe whose builder extended the fuselage on his one foot to prevent Dutch Rolls. How about you Pitts guys? Any input on this?
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Old 02-05-2003, 02:08 AM
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Default Dutch Roll

I completed a Smith Miniplane in 1970 with a 125 hp Franklin Sport Four kit (engine came unassembled in a big crate of parts for $1400).

The only mod made from stock was extending the top of the fin and rudder ten inchs which increased area around 10%. This airplane never exhibited any dutch roll and was delightful to fly.

John
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Old 02-05-2003, 08:06 AM
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Default Dutch Roll

I believe you can also reduce dutch roll by increasing dihedral?
Can anyone confirm this, its been so long since I taught basic aeronautics.



cheers
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Old 02-05-2003, 01:53 PM
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Default Dutch Roll

I'm afraid more dihedral makes dutch roll worse.


Can vegetarians eat animal crackers.
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