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Model Forum / Radio Controlled / Air Models / September 2007Can't Taxi Straight
Speaking of landing gear, I was going to take off from a road yesterday with my Commander 2. The problem was that the plane kept making a hard right turn under power, even though that brought it slightly 'uphill' and over the crest of the road. The blasted thing went farther sideways than forward. The landing gear is really cheap and loose, so I'm not sure how much adjusting I can do. Maybe change the direction that the wheels generally point...? > Speaking of landing gear, I was going to take off from a road > yesterday with my Commander 2. The problem was that the plane kept [quoted text clipped - 3 lines] > loose, so I'm not sure how much adjusting I can do. Maybe change the > direction that the wheels generally point...? "Toe in" seems to be the generic fix. > Speaking of landing gear, I was going to take off from a road >yesterday with my Commander 2. The problem was that the plane kept [quoted text clipped - 3 lines] >loose, so I'm not sure how much adjusting I can do. Maybe change the >direction that the wheels generally point...? A "hard right" would seem to indicate that perhaps the right wheel is not turning freely. Check it with a little weight on the wheel... if it is free, a bit of toe-in (or out) would likely cure the problem. Cheers, jc Hand launch it. >> Speaking of landing gear, I was going to take off from a road >>yesterday with my Commander 2. The problem was that the plane kept [quoted text clipped - 10 lines] > Cheers, > jc Then how will he learn to taxi and take off? Kinda like saying to somebody whose engine wont start to just push the car, isnt it? Just my opinion...  SignatureDan AMA605992 KE6ERB http://www.fubar1.net "I've heard the screams of the vegetables..." Take out the "trash" to reply > Hand launch it. >> [quoted text clipped - 12 lines] >> Cheers, >> jc > Then how will he learn to taxi and take off? Kinda like saying to somebody > whose engine wont start to just push the car, isnt it? > Just my opinion... He should hold the plane in his hand while waving it around in the air making motor noises. ;-) | Then how will he learn to taxi and take off? Kinda like saying to somebody | whose engine wont start to just push the car, isnt it? ... except that this sort of plane is meant to be hand launched. The landing gear is there and works, but it's sort of an afterthought. But you can be a great pilot and have never flown a plane with a landing gear, let alone having taxied around and rose off the ground. In any event, if the plane flies straight (this wasn't mentioned, but I'll assume it) but taxies crooked, that suggests that the wheels are crooked or one is dragging. Or perhaps there's a crosswind? Toe-in can help make it more stable on the ground, but if it always veers to the same side, that suggests that the problem is not the sort of problem you correct with toe-in -- it's something more fundamental, something is assymetrical. SignatureDoug McLaren, dougmc@frenzied.us You could get a new lease on life -- if only you didn't need the first and last month in advance. Hmmmm ... I had the same results in my initial ground tests with a very similar aircraft. After doing a LOT of reading here ... I believe that my ( and perhaps your) solution might lie in the fact that I was trying to take off under FULL throttle. Some suggestions on this forum, and on other groups, have indicated that these little planes just don't need full throttle. Using full power can cause them to cork-screw, either in the air or on the ground. So, I'd just try taxiing slower and then very gradually increasing speed until it either takes off or goes back into the unwanted ground turn. Take any and all of this post with a very large grain of salt since I am a rank newbie with virtually no hours under my belt. The radio that came with my beginner plane is giving me trouble ( it was a used plane and radio set ). Until I either replace the speed rheostat or find some other solution, I am grounded with that plane. regards, Rich > Hmmmm ... I had the same results in my initial ground tests with a very > similar aircraft. [quoted text clipped - 15 lines] > > regards, Rich That's easy to try, and I will, and report back. Thanks for thinking about it. >> Speaking of landing gear, I was going to take off from a road >>yesterday with my Commander 2. The problem was that the plane kept [quoted text clipped - 9 lines] > Cheers, > jc Thanks for all the responses. Funny thing: reversing the landing gear, which puts the wheels on opposite sides, has no effect on the problem... >Commander 2. Is this the one?: http://www.rchobbies.org/firebird-commander2.htm >The problem was that the plane kept >making a hard right turn under power If that's how the design is built, it is going to be practically impossible to take off. The main wheels are WAY too far forward. The wheels need to be placed slightly in front of the CG. Too far forward, and it will not go straight; too far back, and it will nose over. Since the propeller is so high up, the tendency to nose over will be stronger, so finding the sweet spot will be even more difficult than with a more conventional tail dragger. SignatureRoRo I agree with Robert. Here is an intuitive explanation of the problem. The prop is where the thrust is effected, and in this plane it is about the same position (longitudinally) as the landing gear. Imagine trying to push a beam with free wheels at its ends using your finger. If your finger is perfectly centered, it will go about straight. If the finger is slightly off center, the wheels will turn very quickly and the beam will rotate. On the other hand, if you pull (or drag) the beam and wheels using a long, rigid stick instead (equivalent of having the landing gear way behind the propeller), it is much easier to control. I had the same problem in an airplane. Past some threshold speed, the airplane would turn erratically when trying to taxi. My solution was to move the motor + propeller forward, thus increasing the distance between propeller and landing gear. This solved the taxi control and center of gravity problems at the same time. But you can't do the same with the Commander 2 easily, because it is a pusher. Then I would seriously consider the option of hand lanuching it. Happy flying! Angel >>Commander 2. > [quoted text clipped - 13 lines] > will be stronger, so finding the sweet spot will be even more > difficult than with a more conventional tail dragger. > I agree with Robert. > [quoted text clipped - 8 lines] > stick instead (equivalent of having the landing gear way behind the > propeller), it is much easier to control. Except that isn't how it works. There are many real aircraft using pusher arrangements and they're no harder to fly than tractor setups. An engine is mounted rigidly to a fuselage and points in whatever direction the fuselage points, so the thrust is always along the fuselage centerline whether the airplane is pushed or pulled. Your finger isn't rigidly mounted to the fuselage and its thrust therefore isn't kept in alignment with it. The OP just need to use the rudder to keep the airplane straight in the takeoff roll. It's all part of learning to fly. Dan >> I agree with Robert. >> [quoted text clipped - 20 lines] > > Dan You completely ignore moment arms - and clearly a tractor has a much longer moment arm than any pusher. That means the tractor is intrinsically more stable under thrust than the pusher - and that difference could indeed aggravate an inexperienced driver... /daytripper >>> I agree with Robert. >>> [quoted text clipped - 25 lines] >stable under thrust than the pusher - and that difference could indeed >aggravate an inexperienced driver... Here's a corollary: it is much easier for an inexperienced driver to negotiate snow-covered roadways with a front wheel drive vehicle than a rear wheel drive... /daytripper > >>> I agree with Robert. > [quoted text clipped - 31 lines] > > /daytripper And how does that equate to a tractor being more stable than a pusher? Does the propeller have something to do with sliding around sideways when it slips? Dan > ... And how does that equate to a tractor being more stable >than a pusher? Does the propeller have something to do with sliding >around sideways when it slips? I don't have a pusher aircraft like the Commander--or a pusher of any kind. I do wonder whether the gyroscopic precession forces might act differently with the propeller behind the CG instead of in front of it. Most tractor taildraggers of my acquaintance lurch a bit to the left as power is brought up during the takeoff roll. It is practically second nature to me now to lean on the right rudder as I bring the throttle up. Cubs are particularly tricky with their narrow gear. Something like that may be going on here, but in reverse. Marty SignatureBig-8 newsgroups: humanities.*, misc.*, news.*, rec.*, sci.*, soc.*, talk.* See http://www.big-8.org for info on how to add or remove newsgroups. > >> I agree with Robert. > [quoted text clipped - 27 lines] > > /daytripper You'd better expand on moment arms and sources of thrust. I'm trying to figure out what a pusher is less stable than a tractor. In my 34 years of full-scale flying I have never heard of this. Dan >>>>I agree with Robert. >> [quoted text clipped - 33 lines] > > Dan I'm a complete newbie here, but I'm guessing he means that thrust from the rear of a plane will produce more movement from incorrect or unintended inputs than thrust generated at the front. I should also note, though, that the Commander and other Hobbyzone planes are 'mid-engine pushers'. That is to say thrust isn't generated at the rear, but near the CG. This is probably the case with most pushers (except maybe for some thrust vector planes like my Vulcan gliders), so the difference probably isn't that big... >>>>> I agree with Robert. >>> [quoted text clipped - 48 lines] > maybe for some thrust vector planes like my Vulcan gliders), so the > difference probably isn't that big... He is actually talking bollocks, because unlike e.g. someone pushing a wheel barrow, the thrust is always perfectly aligned along the aircraft axis, therefore the position of where the thrust is aligned cannot affect the lateral stability. > ... The OP just need to use the rudder to keep the airplane >straight in the takeoff roll. It's all part of learning to fly. I have a feeling that the plane has ruddervators rather than rudders. I may be wrong. But if so, they may give less control on the ground than a rudder mounted on the vertical fin. Marty SignatureBig-8 newsgroups: humanities.*, misc.*, news.*, rec.*, sci.*, soc.*, talk.* See http://www.big-8.org for info on how to add or remove newsgroups. >>... The OP just need to use the rudder to keep the airplane >>straight in the takeoff roll. It's all part of learning to fly. [quoted text clipped - 8 lines] > > Marty True on both counts. They seem to be useless at low taxi-ing speeds. >> I have a feeling that the plane has ruddervators rather than >> rudders. >> But if so, they may give less control on the ground than a >> rudder mounted on the vertical fin. > True on both counts. They seem to be useless at low taxi-ing speeds. Standard aircraft get prop wash over the rudders. Another factor in your case may be that the ruddervators are not solidly in that airstream because of the location of the propellor. Marty SignatureBig-8 newsgroups: humanities.*, misc.*, news.*, rec.*, sci.*, soc.*, talk.* See http://www.big-8.org for info on how to add or remove newsgroups. Dan, What you say is completely true and I agree, but the argument may fail in the assumption on rigidity. If the landing gear is not rigid, which I think is the case, any disturbance will make the pusher less stable than a tractor. It is really difficult to figure out why it is not being able to taxi straight because many variables are involved, but intuitively speaking, if you are dragging something (even if it does not have wheels), it is more stable than if you are pushing it. If rigid and symmetric, it should not matter at all (we agree in that), but small deviations from perfect rigidity or symmetry could effectively produce a big difference in stability. My last handwaving argument. If you are pushing and the airplane does not move straight, e.g., because the thrust is off axis or the landing gear is not symmetric, the thrust will produce a moment that will help the airplane turning further, making this inherently unstable. This is different from the tractor airplane. Best regards, Angel >> I agree with Robert. >> [quoted text clipped - 24 lines] > > Dan > Dan, > [quoted text clipped - 16 lines] > turning further, making this inherently unstable. This is different from the > tractor airplane. If the thrust is not straight (P-factor) or if the landing gear drags on one side, the tractor will deviate as much as a pusher. All jets are pushers, especially the MD series or most business jets, and they have no more trouble maintaining alignment than tractors. Anytime the thrust and drag aren't in line, there'll be deviation. Rudders were invented to control that. Propeller thrust isn't vectored, like the wheel thrust on a front-wheel-drive car. It's rigidly aligned with the fuselage, and it wouldn't matter if the prop was clear at the end of the tail (like the Taylor Aerocar, Spratt Controlwing, and several other) at the aft end of the cabin (like the Seabee or Lake Amphibian or a million ultralights), atop the cockpit (like the Teal amphib or Volmer Sportsman) in front (like most lightlanes) or WAY out front (like the turboprop singles like the Turbo Thrush or Turbo Air Tractor or Pilatus PC-6). Google any of these. None of these airplanes have zigzag habits. Point the airplane and the propeller drives it in that direction. Same as a ship, which is also a pusher. Dan > I agree with Robert. > [quoted text clipped - 39 lines] >>will be stronger, so finding the sweet spot will be even more >>difficult than with a more conventional tail dragger. What about bending the landing gear to relocate it slightly rearward...? If you bend the landing gear backwards, behind the CG, the nose willl fall... >> I agree with Robert. >> [quoted text clipped - 42 lines] > What about bending the landing gear to relocate it slightly > rearward...? >>Commander 2. > [quoted text clipped - 13 lines] > will be stronger, so finding the sweet spot will be even more > difficult than with a more conventional tail dragger. I've seen videos of stock Commanders taking off ok, and read reviews in which they did so. I'm nonplussed about why mine is so squirrely. >>>Commander 2. >> [quoted text clipped - 16 lines] > I've seen videos of stock Commanders taking off ok, and read reviews >in which they did so. I'm nonplussed about why mine is so squirrely. Ever consider the possibility of it being the pilot ? Ken > The main wheels are WAY too far forward. I think when the mains are way forward, it is actually easier to take off. When I get a squirreley plane, I just give it more throttle right from the start so the rudder takes over the directional control right away, rather than gradually building up ground speed. Also a little toe-in helps. Signaturem-m http://www.mhmyers.com/rc.tn.html <--Lots of RC photos In article <nospam.m-m-CF8210.10021427092007@cpe-76-190-186-198.neo.res.rr.com>, > > The main wheels are WAY too far forward. > [quoted text clipped - 6 lines] > > Also a little toe-in helps. Oops. I just took a look at the plane: http://www.rchobbies.org/firebird-commander2.htm All bets are off for ground takeoff on THAT thing. :) I would hand-launch it. Signaturem-m |
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