Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

Abrupt Takeoff

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Old 08-18-2017, 01:37 PM
  #1
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Default Abrupt Takeoff

Iím trying to sort out a problem with takeoffs with a WM T-34 (tricycle gear). No matter how slowly I pull back on the elevator on the takeoff run, the plane leaps into the air and climbs out very steeply. Landings, on the other hand, are great because as soon as the mains are on the ground, the nose drops and the plane sticks like glue. The plane looks to be nose down when sitting on the gear. Also, using a level, the stab is definitely down in the front, about a couple of degrees. The balance is right as it flies well upright and inverted (with a bit of down.)

I surmise that, with the nose down stance, I need to put in too much up elevator to overcome the down force of the wing at that angle, but once it rotates, I have too much up. I think that if I raise the nose a bit (until the stab is level) the plane should be more controllable on takeoff. My landings may be a bit more interesting though!

What do you think?
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Old 08-18-2017, 02:00 PM
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Adjusting the nose gear would be easy to try and might solve the problem. Another thing check is to see where the main gear are relative to the CG. They should be just barely aft of the CG. If too far back, this will also require lots of up elevator to achieve rotation.
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Old 08-18-2017, 02:56 PM
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The engine is cowled in so somewhat of a hassle to take it apart to get to the nose gear. I did think of trying a larger wheel on the nose gear but have nothing the right size on hand. The mains are already worn down almost to the hubs (lots of flights!) so can't go smaller there. The mains are just aft of the CG. With an empty tank, the plane will rest on the mains and the tail.
What do you think about setting the stab parallel to the runway?
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Old 08-18-2017, 04:33 PM
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Jennifer Curtis
 
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Possible that the nosewheel is not
spinning freely. More likely the
nosewheel is too small. (this is
assuming you don't fly off a paved
runway.)

Jenny
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Old 08-19-2017, 02:34 AM
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It sounds like you need a longer nose strut.
I can't imagine it's that difficult to remove the spinner, prop and cowl to gain access to the strut.
There shouldn't be much difference between stab and wing (less than a degree or so), so select a nose strut that gives you 'a level stance or slightly nose down' on the bench.
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Old 08-19-2017, 07:47 AM
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I agree with the nosewheel being too short. If the wing is sitting at too much negative incidence it will make rotation difficult. The same thing will happen if a tail dragger is set up tail too high.
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Old 08-19-2017, 08:07 AM
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I put a drag on the nose wheel to help slow down the plane on landing. However, the abrupt takeoff issue was there long before that. We have a relatively-short paved runway and the drag helps keep the plane from rolling off the end. With the OS 46 and an 11 x 6 prop, the plane is up to flying speed in a few feet.

No question the nose strut needs to be extended (or a larger front tire). I took the front apart yesterday and extended the strut as far as it would go. It still is barely-perceptibly nose down but I'll try it like that and get a longer strut if needed. (I'll definitely need one when I replace the main wheels.) I did discover that one of the collars had slipped, letting the nose drop less than a 1/16 from the original setup.

A key question is whether the stab should be set parallel to the runway. We'll see how it goes on Monday.
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Old 08-20-2017, 05:05 PM
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Hi RC Fanatic, I have the same airplane as yours the World T-34 and have had reasonable success with it for a few years now as a warbird racer (RC Pro racing). I use a OS ,55AX with an Ultra Thrust pipe. It is indeed a great landing airplane and I think you will find it also is quite capable of those long slow rotation takeoffs too with no mods at all.. As noted it is necessary to have the stoke nose strut extended as far as it will go. I even use those tiny wheels since they are legal under the rules and I wanted them as small as possible.

Enjoy its a great airplane

John
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Old 08-20-2017, 06:27 PM
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Thanks, John. I gave up on racing after four years of being "tail end Charlie." Love the plane though. Have over four years on it, no idea how many flights. The canopy is quite darkened by the sun so can barely see the pilot (Barbie). It flies really well, so will see if this change improves the take-off. Maybe the eclipse tomorrow will help!

Have a Venus II on the bench so look forward to getting that in the air. I definitely prefer tail-draggers.
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Old 08-21-2017, 10:26 AM
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Well, extending the nose gear helped some, but I need to extend it more, which means a new nose gear as the current one is too short. Might need to pull one off another plane or get creative as the nearest hobby shop is 50 miles away!
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Old 08-21-2017, 11:51 AM
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Try reducing your elavator throw. There is a sweet spot where your plane is actually flying before you leave the ground. That's want you want to find,Once you do, you will be able to get airborne like a butterfly.Ha.
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Old 08-21-2017, 02:36 PM
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Good point, F4. I have a lot of elevator throw, a leftover from T-34 racing. It would probably help my landings as well. Maybe a dual rate with low for takeoff (and landing?) and high for beating up the sky. Smooth takeoffs with taildraggers are easy so I think I still need to get the nose higher to get rid of the snap. Extending the nose gear as much as I was able to got rid of banging the tail on takeoff.
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Old 08-22-2017, 03:15 PM
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I wouldn't worry about the angle of your tail on the runway. But the angle of you wing matters a lot. I set my nose gear planes up as close as I can to 1/2 degree of negative incidence on the runway. And I set the main gear so that the nose wheel has very little weight on it, just enough to not worry about it tipping back. In fact, the best landing plane I ever had was my Intruder pattern plane, which would rock back on the tail when there was no fuel in the tank.
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Old 08-22-2017, 04:04 PM
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The way to quickly check the balance of the T-34 is to see if it will sit tail down when the tank is empty. That's the first check when I get it assembled at the field. Was busy today getting a new front tire on my motorcycle but will head to the shop after dinner as I think I see a way to get the nose higher with the current nose strut.
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Old 08-23-2017, 09:20 AM
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The university heavy lift team designs they're aircraft to run with a plus 1 to 2 degrees on the main wing to aid in a smooth take off and landing. They lift upwards of 20 lbs of weight with a .65 engine. Here is a video of my heavy lift design limited to 1000 square inches and a OS .61 two stroke. No ailerons but spoilerons for roll control and + 1 degree when thrust line and elevator is at 0 degrees fully loaded with over 20 lbs of lead. Mine is the last plane. Large bounce on landing didn't flair soon enough LOL. Starts at 2:11


2011 University of Windsor maiden flight 14 ft span heavy lifter empty short take off a little to much elevator input for an empty configuration. the heavier it got the smoother the take off. Finished 2'd overall that year starts 1:40



Last edited by Propworn; 08-23-2017 at 09:36 AM.
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Old 08-23-2017, 10:41 AM
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Impressive planes. I like the kicked up tail. Really need some "Heavy Metal" music to go with the video, though.
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Old 08-23-2017, 02:08 PM
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Hmmm heavy lifters, here's mine eight pounds of sandbags and ten pounds of airplane. Not for any contest though, instead testing an extended and strengthened wing for a cross country airplane.
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Old 08-23-2017, 02:58 PM
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Hi RC Fanatic agine

Here is a picture of my old World T-34, have done of several specifically for bracket warbird racing and they make an excellent
Bronze or Silver airplane.

I believe when you were racing you may have been using excessive throws, its a very common mistake and the idea is when you reach the full vertical bank to also reach full up elevator. Throws beyond that will only serve to destabilize your grove and cause the all to common eratic course instead of the smooth steady elevation over the entire course.

If your main gear is in the stock location then if the tail drops without fuel your CG is too far back especially so for racing. I just had mine out this morning just for some hot laps and I double checked the CG and indeed it is actually at 25% even a little less. These T-34's are just about the best flying (for racing) ARF's out there. Double checked the elevator throw for flying the course and it is barely a quarter inch. If you like you could do a higher throw for landing with duel rates.
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Old 08-25-2017, 08:39 PM
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I don't like trike gears for my own models. But I understand from a lot of reading about them that an abrupt takeoff is often a sign that the mains are too far behind the CG location. This makes it harder to rotate off the nose wheel and when it does and the model finally does lift the excess up elevator needed for the rotation causes sharper than ideal climb angle.
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Old 08-26-2017, 06:54 AM
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A good rule of thumb for locating the main gear wheels is that it will sit on three wheels with an empty tank, but if you tip it back on its tail it will stay there. What you actually need is to be perfectly balanced on the mains when you are in a high fiare attitude, both for smooth takeoffs and smooth landings.
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Old 08-26-2017, 11:21 AM
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Nice Plane! I was wondering,what electrical source do you use for your power panel?
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Old 08-27-2017, 05:01 AM
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Phantom , my apologies for shortening your handle. My typing skills are poor: I abandoned the use of power panels long ago and instead go completely portable starter and glow driver as it is far more convenient and safer with no single or multiple umbilical cords. Another side effect of those things is the tendency of the fellows to keep increasing the glow current when working with a cranky engine which is exactly backwards of what you want when starting an engine. A white hot coil has the tendency to advance the ignition which as you know a retarded ignition is what is needed for engine starting.

What BMatthews and Jester posted about is dead on concerning gear placement for a tricycle gear, with BMatthews discussing placement too far to the rear and Jester talking about when it is too far forward i.e. closer to the CG. One thing to keep in mind and that is vital that one never change the CG when dealing with this type of problem. CG is always adjusted for in flight characteristics never for ground operations. That ground operations is always related to landing gear placement.

In the OP's case the choice of taildragger or tricycle or in mine above is that rules would not allow that kind of change since he was racing with the northern California groups and I with the folks mostly in Arizona and southern California (not the same organizations but very similar rules for bracket warbird racing). It is 'scale' racing and although the World T-34 is not all that accurate as a scale model but changing the gear from or to tricycle would not be allowed.

John
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Old 08-27-2017, 10:32 AM
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Quote:
Originally Posted by RC_Fanatic View Post
Well, extending the nose gear helped some, but I need to extend it more, which means a new nose gear as the current one is too short. Might need to pull one off another plane or get creative as the nearest hobby shop is 50 miles away!
On a couple of airplanes I smoothed the takeoffs by "lowering" the tail -- raking back the main gear, or spreading the main gear wider. I fly off grass, so it helps to have the plane stand with the wing several degrees positive relative to the "flat" ground.
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Old 08-31-2017, 08:28 AM
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I understand that this particular T34 is a pretty old model now from the comments earlier. Is it possible that the main gear legs have been bent back a little over the years and many dozens of landings? It may be as simple as re-forming the legs so the wheel axles sit a small amount forward of their present location. If this issue of not rotating well is fairly new on an older model that used to land and takeoff well then this may be the change that occurred.

What Jester said above or getting very close to what he said could be as simple as that small tweak of the main gear legs to move the wheels. Of course though if these are retractable gears and they work correctly then this is a non starter of an idea.
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Old 09-05-2017, 07:27 AM
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Hi!
Have you checked the C of G!? Too far forward in conjunction with a too far rearward placed main gear witll create those take offs you describe.
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