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Guest Douglas K

Turn Coordinator in C172

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Does anyone know what the limits of the turn rate in the turn coordinator in the C172 are? In theory books I found that it's 20deg/sec which is rather large (ie 1200deg/min). If this is linear (and since the needle is on a spring this is reasonable) my miniature plane rotates about 130 degrees max, which is a lot. The limit on the default gauge is about 40 degrees in comparison, so the miniature plane wings don't go over the balance ball cylinder, but (assuming a linear relationship) this corresponds to only 360 deg/min. Anyone knows how this works in real life?Cheers, Christian

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Should only be 180 degrees a minute. That is why putting the wings onto the dash on either side results in a "Two-minute turn"

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Sorry, I mustn't been very clear. I know the turn rate at the lines is 180 deg/min. I was wondering how much the maximum turn rate is that indicator will show (ie how much will the miniature plane move past the 180 deg/min lines)?Cheers, Christian

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If I understand you correctly Christian, you want to know the maximum angle of bank that the little airplane silhouette in the turn coordinator will display. This can vary by instrument (and keep in mind that I'm not a certificated instrument repairman); will be limited by internal stops in the instrument's dashpot. These stops are necessary to avoid stretching and possibly damaging the calibration spring.40 degrees is a very reasonable limit for the indicator, as I'm sure you're aware the turn coordinator is only meant to display rate information; rate of roll, and rate of turn once the roll rate is zero and the aircraft is established in the bank. I've never heard about the 20 degree per second limit (or is that the maximum rate?) for turn coordinators. That

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Thanks, Douglas.Yes, that was my question. I guess, you mean 40 deg tilt of the miniature plane. The limit I found in a book was max turn rate of 20 deg/sec which translates to 130 deg tilt of the indicator plane (which obviously is a lot more than 40 deg). The question remains, what does a 40 deg on the indicator translate to. Since a rate of 180 deg/min = 20 deg indicated, 40 deg indicated would be a rate of 360 deg/min, if the relationship is linear (given it's a spring, that's plausable). This doesn't fit with the 20deg/sec (which is 1200deg/min). The 20deg/sec limit stops the gimbal, so it's possible that there is a second limit on the spring?Cheers, Christian

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>>>>The question remains, what does a 40 deg on the indicator translate to.<<<< I'm going with the answer you've already worked out Christian (360), but the real answer is that it doesn't matter because the instrument is only certified to indicate a standard rate turn. The 20-degrees/sec limit you found is probably a theoretical limit on the gyro, you won't be making a procedure turn at that rate in a Skyhawk! And as I said before, internal stops will prevent such a rate from being displayed. To avoid confusion, and for those who are interested in the operation of the turn coordinator - note that the turn coordinator does not indicate the angle of bank with any precision. In a 172 performing a steady level turn at the standard rate (2 minutes) the wing of aircraft silhouette on the index mark will roughly correspond to a bank angle of 20 degrees. However, the instrument is only showing the rate of turn.The turn coordinator's gyro gimbal axis is tilted 30-35 degrees. This causes the instrument to precess when the aircraft rolls as well as when it yaws. This precession is responsible for driving the indicator in both the turn coordinator and the earlier turn and slip instrument. Unlike the earlier turn and slip indicator, which only senses yaw, the turn coordinator is not affected by adverse yaw at the beginning of the turn. Thus the turn coordinator will sense rate of roll as well as rate of turn.Try this experiment in MSFS, using the 172, establish the aircraft in a level turn at the standard rate. Now without changing the bank angle load a little G onto the aircraft with backpressure on the yoke to tighten the rate of turn. DO NOT increase the bank angle, check the bank angle before applying backpressure and maintain it as necessary. Now watch the aircraft silhouette indicate a higher rate of turn by banking further in the direction of turn.Try the same experiment with one of the newer aircraft that have instrument tooltips to indicate rate of turn, see how close you can come to 20 deg/sec.Douglas

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>I'm going with the answer you've already worked out Christian>(360), but the real answer is that it doesn't matter because>the instrument is only certified to indicate a standard rate>turn. ok. many thanks. I'll go with that.>The 20-degrees/sec limit you found is probably a>theoretical limit on the gyro, you won't be making a procedure>turn at that rate in a Skyhawk! I didn't say, I would :-lol. I was just interested in the theoretical possible max. I know that 20 deg/sec is quite a turn :).I'm not a real pilot, but do know a bit about how things work. I certainly wouldn't try to achieve such turn rates in reality. Having said that, I could imagine a situation of high turn (mainly roll) rates, and that's a spin (although I wouldn't rely on the TC in such a case). This could be tried with Rob Youngs C172 airfile, as the default C172 isn't capable of spinning. Anyway, I go with the 40 deg (360deg/min) angle. Anything higher is very unlikely anyway, I was just interested (I'm probably too much of a perfectionist).>Try this experiment in MSFS, using the 172, establish the>aircraft in a level turn at the standard rate. Now without>changing the bank angle load a little G onto the aircraft with>backpressure on the yoke to tighten the rate of turn. DO NOT>increase the bank angle, check the bank angle before applying>backpressure and maintain it as necessary. Now watch the>aircraft silhouette indicate a higher rate of turn by banking>further in the direction of turn.Another good way to show that the TC doesn't show bank angle is to fix a bank angle and vary the speed with the throttle. Note though that higher speed does not necessarily equate to higher turn rate. If my memory serves me well, just above stall speed for the selected bank angle will give best turn rate for that bank angle, and this makes sense if you think about it (faster car = bigger minimum turn radius). Ok, now I'm confusing things. Just above stall speed = tightest turn, but not highest turn rate. I think in most cases turn rate will increase with higher speed settings above stall speed, but then drop again at faster speeds, ie highest turn rate is when speed and turn radius are optimal taken together.Cheers, Christian

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>>>>I'm not a real pilot, but do know a bit about how things work. I certainly wouldn't try to achieve such turn rates in reality. Having said that, I could imagine a situation of high turn (mainly roll) rates, and that's a spin (although I wouldn't rely on the TC in such a case). This could be tried with Rob Youngs C172 airfile, as the default C172 isn't capable of spinning.<<<

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No, they go for about $365.00, just checked yesterday. :D

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>No, they go for about $365.00, just checked yesterday. :D>We're resurrecting a 2 1/2 year old thread here, but the $365.00 units are the cheapo Falcon types. The model I have is $879.00 & some run around a thousand. Mine has been replaced with an auto-pilot in the 3 1/8" hole, that includes an electronic turn coordinator and has a built in "ball". I need to sell it.L.Adamson

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While this is indeed an old thread, interesting read.Some misconceptions throughout it regarding the turn coordinator and some basic aerodynamic principles. ;)Rate of turn is based on:(PI()*velocity)/(RADIANS(180)*9.8*TAN(RADIANS(bank angle)))Radius of turn is based on:POWER(velocity,2)/(9.8*TAN(RADIANS(bank angle)))Rate of turn is proportional to velocity while radius is proportional to velocity squared.

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