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takeoff control

Old 05-23-2015, 07:47 AM
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acdii
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Default takeoff control

I have a T-Clips 70 that is squirelly as all get up in take off. If I give just enough throttle to raise the tail, and just let it go without any rudder it will go around in a left circle. Now that I know is normal do to the prop wash on the rudder, and I also know that to correct you input some right rudder. Yep that works well on all my planes, except this one. I have dialed in Expo, reduced rates, etc. but no matter what I try, as soon as I try to correct it over corrects and now the plane veers right. It is all over the place on take off, and one other thing to note, when it leaves the ground it does so in a left bank. Once it has enough speed it flies perfectly fine, level, and performs very well. Landings are also non-eventful.

I am almost to the point of putting a heading hold gyro in. It was fun to watch it go around in a circle though. Yes both wheels turn freely too, and tail down handling is also very good, its just when the tail comes up that it becomes a handful. Just wondering if thrust angle is off.
Old 05-23-2015, 10:28 AM
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scale only 4 me
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Don't know what to say but keep practicing.

If it's going hard left on take off, you're holding or pulling too much elevator on take off, going to snap stall that baby if you're not careful

Good luck
Old 05-23-2015, 05:57 PM
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acdii
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Nope, not me. My take offs in all my other planes is nice and smooth. On this one I barely touch the right stick for it to leave the ground. What I notice is the faster I can get it going with the wheels down, the less likely it will bank left, so its speed related, not sure what it is though.

I probably should have worded the title better as I dont need take off assistance, I can take off and land just fine, this is an issue with the plane itself being overly sensitive at low take off speeds and wonder if its thrust related. I remember having thrust issues with my 4*120, but not what the issue was that got corrected.
Old 05-23-2015, 08:08 PM
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Start with the model pointed left of straight down the runway.

I was just watching a WWII documentary of P-40's in Africa and the pilots stood on the rudders on take off. Pumping to full deflection repeatedy as necessary.


My Cubs like a lot of right rudder on takeoff. Some models do.
Old 05-23-2015, 09:07 PM
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52larry52
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acdii, Have you looked at the "alignment" of the main gear wheels? Both wheels should point forward at the same angle with just a very slight toe in. At low speed before the rudder is effective the tire angle is going to dictate where the plane is headed. Also, as you apply throttle starting on the takeoff roll do you slam the throttle quickly to full throttle, or feed it in very gradually as you pick up speed? Are you flying off grass or a paved runway? I normally fly off grass but when I do fly off pavement I find my Cubs and other Cub-like taildraggers require extra smoothness on the controls (both rudder and throttle) or I will start the zig-zag tail chasing heading for a ground loop. Once you start that zig-zaging it's best to abort the takeoff, line back up and try again. Taildraggers on pavement are much less forgiving to any combination of misaligned wheels and or over sensitive control . Once the tail is up, all steering control is from the air on rudder surface and the main gear wheel alignment if you are feeding in the power gradually. Just some random thoughts on your problem. Also check that the tail wheel is parallel to the rudder.
Old 05-23-2015, 09:31 PM
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It's never going to be as easy as a nosewheel, but maybe a better understanding of the physics of a ground loop will help you overcome the problem.

First, the fact that the airplane goes left at low speed is no coincidence. What you are seeing is not really so much motor torque as it is gyroscopic precession. Precession sounds like a mouthful, but what it really means is that the spinning prop is a giant gyroscope. When the tail comes up, the prop feels like something is pushing forward at the top of the disc. But precession means that the gyroscope reacts as if it were being acted upon 90 degrees later in the rotation path. In other words, you push at the top of the disc, but the disc reacts as if you had pushed on the right side of the disc (right as seen from the pilot's seat). So the rapid rise of the tail becomes a strong push on the right side of the prop disc, making the airplane go strongly to the left. Since the model is very slow when the tail comes up, the rudder effectiveness isn't as strong as the gyroscope.

The thing to take away from this gyro business is the understanding that you need a major quick rudder input while the tail is coming up. If you wait to correct when you see the nose heading left, the harm has already been done. Learn to poke in a healthy dose of right rudder at the moment that the tail begins to rise. When I say, "poke", what this means is that you don't simply put in some right rudder and hold it. You need to put in far more rudder that you might think, and then get right back off it even before you see the plane respond. Just one big, quick poke, let go, and see how it's going. If the plane is still swinging to the left, poke the right rudder again. But don't push and hold the rudder. You will get out of phase with the action and wind up looping to the right. Notice what Charlie P. said about the P-40: "Pumping to full deflection repeatedly as necessary."

Another thing that will work in your favor is to change from a plastic propellor to a wooden. Remember, the prop is a spinning gyroscope. If you make the gyroscope lighter (wood prop instead of plastic), the precession torque will be reduced. This is very apparent on full scale airplanes if you fly the same airplane with a wood prop and then a metal prop. The wood prop airplane will seem like a ***** cat after flying the metal prop version.

Another trick is to slow down the rate at which the tail rises. If you apply and hold just a tiny bit of up-elevator before advancing the throttle, the tail will come up a little later and won't come up as quickly. The slower tail rise will reduce precession torque. I usually sort of lock my thumb on the elevator stick with 5% to 10% up elevator and hold it there until I break ground. Unless your model is overly sensitive to elevator, there won't be any danger of stalling with that small elevator input.

You mentioned using a heading hold gyro as a last resort. That's not a bad idea, although it's not necessary to use heading hold mode. I've experimented with a gyro in simple rate mode and found that it tames the model significantly. Better yet, get one of the 3-axis gyros such as the Eagle A3. Having rate damping on the elevator as well as rudder slows down the rate at which the tail comes up. Remember precession ? Slowing down the rate of rise of the tail reduces precession and makes it that much easier for the yaw channel of the gyro to control the left pull. Putting a gyro on yaw or pitch alone will make your job easier. Putting a gyro on both axes makes takeoffs duck soup.

Dick

Last edited by otrcman; 05-23-2015 at 09:35 PM.
Old 05-24-2015, 02:19 AM
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RBACONS
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With the receiver and transmitter turned out, but without touching the controls, if you give it a push on a flat, paved surface does it roll in a straight line?
Old 05-24-2015, 02:51 AM
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All T-Clip exibit this problem.

I solved it by slowly increasing the throttle on take off and letting the model rise without elevator input.

A friend solved it by adding a gyro, as you are considering.
Old 05-24-2015, 05:26 AM
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acdii
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This is what I was looking for. If its inherent to the plane. I will give the LG a review just in case its out of whack, though it is a fixed aluminum gear, but it could still have a twist. I usually throttle up slowly too, and let the tail come upon its own. I learned quickly not to give any elevator on this plane as it will leave the ground before it is ready and I have cartwheeled it twice due to this. I will try the rudder bumping next time and see if that helps. On my other planes, I just give a touch of right rudder as the plane picks up speed just at the point the rudder leaves the ground, and on this one, when I do that, it veers right, but not right away, so bumping sounds like the trick to use. Maybe its because of the short fuse that I have difficulty with it as all my other planes are long in the fuse.


Now if you had seen my maiden with my new Cub, you would have freaked! It left the ground in 10 feet and went straight up. Took me a couple minutes to get it flying level. I WAY over powered that plane with a Satio 100. Hopefully my next flight wont be so dramatic. I have to put it on the bench and make some adjustments to the elevator and ailerons, I have 56 down and 47 right on the JR for it to fly level. Also need a larger prop, I don't want to tear the wings off. Anyone 3D a cub yet?
Old 05-24-2015, 07:03 AM
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otrcman is absolutely correct.

Back in the bad old days, I've owned and extensively flown two different Cessna 170B models. Same year yet each was quite different in its take off characteristics. The second was more sensitive to rudder and required much different rudder application technique - a finesse which precluded the almost mindless, inattentive handling which worked with the other.

Every plane, even of the same model, has its own character and variations in behavior. On my second 170B, the gyroscopic effect of the prop was most dramatic. The amount of right rudder required was dependent on aircraft speed and engine power as well rate at which the tail came up to control direction on take off. You learned just how much for the conditions so that you - and this is critical, just as he says - applied the rudder in advance of starting the take off roll or lifting the tail, sometimes with variation as the tail rose and then let it off, holding just a tad as you continued the take off roll with the tail high. It was like a fine dance on those rudder pedals, going to full right for a quick take off and/or rise of tail - but just for a second or two, back to a much smaller deflection. In gusty conditions, you would be ready for more rudder. It became an automatic, learned action.

It's critical to realize that it was/is necessary to anticipate the aircraft behavior and act in advance.


Practice, practice, practice.
Old 05-24-2015, 07:19 AM
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flycatch
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Larry, your right but I believe the poster is just trolling.
Originally Posted by 52larry52 View Post
acdii, Have you looked at the "alignment" of the main gear wheels? Both wheels should point forward at the same angle with just a very slight toe in. At low speed before the rudder is effective the tire angle is going to dictate where the plane is headed. Also, as you apply throttle starting on the takeoff roll do you slam the throttle quickly to full throttle, or feed it in very gradually as you pick up speed? Are you flying off grass or a paved runway? I normally fly off grass but when I do fly off pavement I find my Cubs and other Cub-like taildraggers require extra smoothness on the controls (both rudder and throttle) or I will start the zig-zag tail chasing heading for a ground loop. Once you start that zig-zaging it's best to abort the takeoff, line back up and try again. Taildraggers on pavement are much less forgiving to any combination of misaligned wheels and or over sensitive control . Once the tail is up, all steering control is from the air on rudder surface and the main gear wheel alignment if you are feeding in the power gradually. Just some random thoughts on your problem. Also check that the tail wheel is parallel to the rudder.
Old 05-24-2015, 08:50 AM
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as far as the left bank after plane leaves the ground sounds like you are releasing the right rudder too soon. I like what one poster said if you give it a push on flat surface motor off radio on and it doesn't roll straight check the landing gear and neutral tail wheel alignment
Old 05-24-2015, 11:07 AM
  #13  
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Relative to over-controlling when the tail wheel lifts, if your plane rolls straight with the push test, then you may have a built-in imbalance between rudder authority at a given stick position and the tail wheel authority at the same stick position. This can be a frequent problem when the tail wheel is driven by a non-adjustable direct connection to the rudder rather than driven by the servo or an adjustable connection. This may explain why the problem lessens the faster you go with the wheels down. Typically a tail wheel in contact with the ground exerts a more positive yaw control than the rudder in the air at low speeds. If you go fast enough before the tail wheel lifts, then the rudder authority has caught up with or exceeded the tail wheel authority, hence there is no noticeable change in yaw when the tail wheel lifts.

If you can make your tail wheel throw adjustable, tame it down so that it is less responsive and more in line with the rudder authority at slow speed.
Old 05-24-2015, 11:13 AM
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I had the same problem with an old pattern plane , an Airtrax 46. This was ground looping as soon as it got light on the tail, rudder would not be effective at all.
Now I take off with full elevator up to keep pressure on the tail wheel. This works out great, just remember to ease off the elevator as soon as it leaves the ground.
Old 05-24-2015, 03:49 PM
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Motor torque. Happens with all over powered tail draggers. Look at some old videos of Corsairs taking off of carriers - they have that rudder cranked full right on take-off. It's why the Navy stopped doing it in favor of putting them on land based forward air strips. It shows up as left force on takeoff roll and left bank after takeoff.
Old 05-24-2015, 03:56 PM
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1. So did you ever check the thrust line?
That was my first thought is you might have a slight left not right thrust. I would use about 1/8 to 1/4 inch less distance on the starboard side prop tip when horizontally placed. You can measure with string from the rudder post to each side of the prop but keep the tension the same when you do or you will stretch the string and get a false reading. Starboard side must be shorter when sitting on the wheels.

2. consider adding 1/4 to 1/2 oz lead to the starboard wingtip on the under wing side, not visible. This tamed many of my warbirds and I do consistently on most now as just another step and epoxy it inside the wingtip.
Old 05-24-2015, 08:44 PM
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52larry52
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flycatch, No, acdii is not a troll, he's legit. If you go over to the Cub brotherhood thread in the kit builders forum you can see the neat biplane J-3 Cub he is building. I have been recently sucked in by a troll on another thread, but not this time. Acadii is real. Cheers
Old 05-24-2015, 09:21 PM
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My Cub was a monster on the ground when I first got it. I would get started rolling, then it would go left as Cubs will do. When the tailwheel lifted, it would go hard right and then I'd chase it until it was flying. It turned out the main gear wheels were crooked with some tow out which had made me also make the tail wheel crooked to compensate. I diagnosed the problem by getting onto some flat concrete and rolling the plane with the tailwheel off the ground. I got behind the plane and pushed the rudder toward the spinner, not steering the plane to go straight but rather just applying pressure directly forward. The plane pulled left when I did that, so I took it home and used 3 sticks to get the gear set up right. All were about 3 feet long. The first was taped to the bottom of the fuselage as perfectly straight with the centerline as I could get it. Sticks 2 and 3 were taped to the wheels. With the 3 foot length, I could get very precise measurements of the angle of the axles relative to the fuselage centerline. All wheels wobble, so I measured each wheel twice to get a minimum and maximum distance. I tweaked the main gear until the center of the min/max measurement was the same, then I turned the wheels a half turn to compare the distance to the centerline facing backwards so I could see how much tow I had. I set it with none because I wanted to start there and make changes if needed. When I rolled the plane on the concrete again with the tailwheel off the ground, it tracked straight. When I put the tailwheel down, I guess it would have made about a 40 foot circle. So I tweaked the tailwheel to make the plane roll straight (and I mean really straight as in slow taxiing about 200 feet down the runway with no more rudder correction in one direction than the other). My next takeoff was a thing of beauty; just a little left pull when I advanced the throttle which I expected, the tail came up, and the plane started flying with a 30 degree or so left turn. Those left yaw tendencies became very consistent with no other changes, so after a few practice takeoffs I can run the plane straight down the runway for a scalelike takeoff.

My Cub takeoff routine now, which should work just as well with a T Clips, is to advance the throttle to about 1/3 over 2 seconds or so with about 30% right rudder applied as soon as the wheels start rolling. At this point, I'm looking to build enough airspeed for the rudder to become effective. I let the plane accelerate for a second or two, then advance the throttle to around 60%. I release most of the rudder when the tail comes up on its own, then go to full throttle. I then pull back on the elevator to take off while giving it a bump of 50% rudder only during the rotation. The plane is a H9 .40 size Cub with a Saito .72, so there's a whole lot of torque and P factor to contend with on this plane. If your plane won't respond well to a similar takeoff technique, I would suspect the landing gear before anything else.
Old 05-25-2015, 03:19 AM
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Originally Posted by acdii View Post
This is what I was looking for. If its inherent to the plane. I will give the LG a review just in case its out of whack, though it is a fixed aluminum gear, but it could still have a twist. I usually throttle up slowly too, and let the tail come upon its own. I learned quickly not to give any elevator on this plane as it will leave the ground before it is ready and I have cartwheeled it twice due to this. I will try the rudder bumping next time and see if that helps. On my other planes, I just give a touch of right rudder as the plane picks up speed just at the point the rudder leaves the ground, and on this one, when I do that, it veers right, but not right away, so bumping sounds like the trick to use. Maybe its because of the short fuse that I have difficulty with it as all my other planes are long in the fuse.


Be careful with bumping the rudder. I had better luck by "leaning" on it.

The T-Clips has the least forgiving ground handling in any model that I have encountered. It does not react well to sudden control inputs, when on the ground.
Old 05-25-2015, 04:12 AM
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I've never flown one, but with that gear so far forward of CG, I could imagine it's very squirrelly much like a Cub but worse

Most common mistake I see is over controlling, what you want to do is put small rudder inputs and be patient for the plane to come around
Old 05-25-2015, 04:16 AM
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Originally Posted by scale only 4 me View Post
I've never flown one, but with that gear so far forward of CG, I could imagine it's very squirrelly much like a Cub but worse

Most common mistake I see is over controlling, what you want to do is put small rudder inputs and be patient for the plane to come around
Bingo !

That's what I've been trying to say.

Small, smooth, control inputs, including throttle, are what is required to successfully fly this model.
Old 05-25-2015, 06:10 PM
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Acdii isn't a troller, nuff said. My full wing cub did the same thing so the first thing I did was as suggested and with the radio on and my hands off the sticks I just rolled it down a small hill. It went sharp left. I had the LG mounted a bit off. After I got that straightened out the plane was still a big drifter but after a bit of time I got used to it. It was never as bad as my Fokker D-VII though. That plane rolled straight but on take off it wanted to go sharp left every time. Once I knew what it was going to do then as scale said, practice!!
Old 05-26-2015, 07:42 AM
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The problem with over controlling a taildragger is often
caused by the tailgear being angled back too much
in combination with looseness in the rudder pushrod.

When you give it a little right rudder (even the smallest
amount) the weight of the tail of the plane shifts to one
side, pushing against the tailwheel. If the tail gear was
pointed straight down this side pressure would do nothing.
(note that I said straight down as in towards the ground,
not in line with the rudder hinges because the rudder is tilted
back when the tailwheel in on the ground.)

So when the tail gear is angled back, (probably to make it
springier) the side pressure makes it want to turn even harder.
If there is any mushiness in the pushrod, like you get with
goldenrods, or sloppiness in the linkage, it makes it even
easier to get the unwanted extra turning force.

Once I figured this out, I started building all my planes with
the tail gear going straight down, and I never had ground
loops occur again. Planes under about 5 to 6 pounds
don't really need springiness in the tail wheel when flown
off grass. Larger planes and those flown off pavement
can have an actual spring landing gear installed (with
the pivot vertical).

The tailwheel on the t-clips 70 can be bent straight down.
It may raise the tail a bit in take-off and landing attitude,
but it should not cause any difficulty.

Jenny
Old 05-26-2015, 10:27 AM
  #24  
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And raising the tail wheel actually makes takeoffs easier. The tail on my Cub sits higher than scale fidelity should allow, but with less angle relative to the wind and less rotation required to get horizontal I don't have as much P factor to deal with.
Old 05-27-2015, 03:26 AM
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