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#31
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Mike wrote:
I've read the term ground loop in a lot of articles and accident reports but what exactly is a ground loop? --Mike A ground loop is when the airplanes skids around 360 degrees while continuing to move in the original direction on the ground. -- Darrell R. Schmidt B-58 Hustler History: http://members.cox.net/dschmidt1/ |
#32
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BTIZ wrote: and that's why tail dragger pilots have "happy feet".. never heard of a nose wheel aircraft "ground looping", the CG location relative to the main gear kinda help keep it straight.. My first solo in a pa28 came real close when I landed in wake turbulance. |
#33
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"David" wrote in message
... It seems to me that this explanation, though common, is oversimplified. What we are talking about is stability. When an aircraft (or a car for that matter) turns on the ground a sideways force is developed on the wheels. There is also a force generated by the sideways acceleration at the cg and, also, in the case of the aircraft, a side aerodynamic force. If the combined reaction of the first two is behind the cg than the yaw motion on the ground will be unstable and may or may not be able to be controlled by the pilot with the rudder. It is not that the main wheels are in front of the cg so much as that the side force developed by the tail wheel is very small (even smaller if it is a castoring tail wheel) and too much of the side ground force will be developed by the 'main' wheels. I'm not sure that's the case, David. The problem with a decelerating taildragger is that the inertial force at the CG associated with the deceleration occurs behind the retarding force of the mainwheels on the runway. This is unstable. If the two forces get out of line, the couple tends to increase. By contrast, having the retarding force on the main wheels behind the CG is stable. ^ | direction of motion in landing roll. ------------------------------------------------- Tricycle: ^ | * CG + wheels | v ------------------------------------------------- Conventional (tailwheel): + wheels | v ^ | * CG ------------------------------------------------- Look at the stability to a yaw. Of course the lateral forces you mention are usually available to control the situation, which is why the instability isn't manifested as a ground loop every time. If the aircraft starts to yaw it just gets more draggy, hence the situation just gets worse. In principle a tricycle gear aircraft should be susceptible to groundlooping on take-off, but you have the stabilising advantage of increased drag hampering acceleration as the aircraft starts to get out of shape. Julian Scarfe |
#34
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In article , Julian
Scarfe writes I'm not sure that's the case, David. The problem with a decelerating taildragger is that the inertial force at the CG associated with the deceleration occurs behind the retarding force of the mainwheels on the runway. This is unstable. If the two forces get out of line, the couple tends to increase. By contrast, having the retarding force on the main wheels behind the CG is stable. Thanks for the comments Julian. My first thought about your suggestions is that as long as the braking force is directly in the line of deceleration then there is not a problem but as soon as a yaw develops then side forces immediately come into play, if they didn't then the aircraft would not start to turn. It must be very similar to the oversteer/understeer problem of cars. In a car if you brake hard and the front wheels lock then the car tends to go straight on because the yaw stability is still provided by the rear tyres. If the rear wheels lock up then you will have great difficulty in stopping a spin. This corresponds to the lack of 'lift' developed by the tail wheel (particularly if it castors) meaning there is insufficient yaw stability. The couple that you mentioned that depends on the retarding force of the front wheels only produces an effect if a yaw develops. The correction of the yaw has to be provided by an inbuilt stability or by the rapid response of the pilot trying, with a big rudder, to introduce 'artificial' stability into the loop. It has some small similarities to the concept that a high wing aircraft is more stable because the cg is below the wing. It is more stable but not due to 'pendulum' stability but due to the way the air flows during a yaw. -- ----------------------------------------------------------- David Francis E-Mail reply to ----------------------------------------------------------- |
#35
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I'm in the process of a restorating a '58 Champion 7FC/EC that was
groundlooped in Reno, shortly after being converted to a raildragger. As my neighbor - from whom I purchased the project - said, they converted it just in time to groundloop it. It suffered a great deal of harm: broken wing spars, both main gear partially torn off, along with the fittings, prop curled back (one blade), a few bent ribs, scuffed and bent sheetmetal - cowl, etc. The lift struts made it through, as did the bulk of the airframe. I'm basically rebuilding it from the tires up - airframe has been faithfully converted to an EC by removing all vestiges of nosewheel paraphernalia, blasted and powdercoated, new interior, including floorboards. Even the seat frames got new powdercoating! I'm in the wiring stage, waiting for the radio trays to be repaired - not from crash damage, just abuse - then I'll finish the (brand new) panel wiring and plumbing. Covering isn't far off... Then it will look like an airplane, rather than a skeleton, in the garage. After that, it's on to tearing the engine down for inspection. Jon B. wrote in message om... "Mike" wrote in message ... I've read the term ground loop in a lot of articles and accident reports but what exactly is a ground loop? --Mike Mike, If you ever experience a ground loop, you will know it!!!!! A ground loop will make a bigger impression on you if you happen to be in the back seat of a tandem tail dragger that you own when the wild swing happens. I let some yahoo fly my 7ac Champ from the front seat because he was interested in buying it.....never again.....we ended up in the weeds at the side of the runway. Luckly there was no damage and he bought the airplane AND I was a LOT SMARTER. Ken Black |
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