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Measurements.camber
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Neil Jepsen <jepsen@...>
This means you have -ve camber. Davids original notes recommended +ve
camber so does this mean camber is not all that important, or is -ve better than +ve? neil Jim Patillo wrote: David Gall,ADVERTISEMENT
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Jim Patillo <patillo@...>
Neil,
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Better ask David that question as I am not an eng gin eer. Jim Patillo N46JP Q200 ----- Original Message -----
From: "Neil Jepsen" <jepsen@...> To: <Q-LIST@...> Sent: Sunday, December 30, 2001 6:08 PM Subject: Re: [Q-LIST] Measurements.camber This means you have -ve camber. Davids original notes recommended +ve |
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David J. Gall
Neither am I!
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However, I still stand by my earlier notes that +ve camber is good. Several people have made the +ve camber change and reported an improvement in ground handling from this one change alone. On the other hand, some have not made the change all the way to positive camber, but have still reported better handling just reducing the negative camber. Maybe there's a "magic number" for negative camber -- go more negative and the handling gets really bad, but stay on the positive side of that number and things are OK. I had thought the magic number was zero, but Jim and Bob make a strong argument for -3 degrees. I just don't know. It's a lot like finding the aft CG limit. When you have positive stability, there's a whole range of CG locations that all offer positive stability, but when you go aft of the aft limit, there's a whole range of CG locations that are ALL unstabile. Maybe Bob Hoover can fly in this range on some airplanes on some test flights, but can Joe average pilot do that on all flights all the time? I'm not saying Jim and Bob are in this range, I just don't know where that range starts. I do think that range includes zero camber and small positive camber angles, so that's what I wrote. I entertained the possibility that some negative settings were okay when the first person to take my alignment advice, Michel Royer, advised that he had good results even though the brake mechanism made it impossible for him to get zero camber. Jim and Bob are reinforce that evidence. The bottom line: +ve is better than -ve. The question now seems to be, what is the -ve limit? David J. Gall -----Original Message-----
From: Jim Patillo [mailto:patillo@...] Sent: Sunday, December 30, 2001 10:00 PM To: Q-LIST@... Subject: Re: [Q-LIST] Measurements.camber Neil, Better ask David that question as I am not an eng gin eer. Jim Patillo N46JP Q200 ----- Original Message ----- From: "Neil Jepsen" <jepsen@...> To: <Q-LIST@...> Sent: Sunday, December 30, 2001 6:08 PM Subject: Re: [Q-LIST] Measurements.camber This means you have -ve camber. Davids original notes recommended +ve Yahoo! Groups Sponsor ADVERTISEMENT To unsubscribe from this group, send an email to: Q-LIST-unsubscribe@... Quickie Builders Association WEB site http://www.quickiebuilders.org Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. |
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Mike Perry <dmperry@...>
I read David Gall's two letters (copies below) on camber, wheel alignment
etc. David says, "I can't argue with success. . . . My head hurts just thinking about trying to understand it." In place of an aspirin I would like to suggest that: 1) The Jim-Bob six pack is more potent than camber. (or, Jim and Bob are wizard pilots) 2) Proper wheel alignment will still benefit ground handling. Mike Perry On 12/30/01 David Gall wrote two letters: Jim,At 11:44 PM 12/30/01 -0500, David Gall wrote:What does this suggest to you regarding your thinking on this matter?This suggests to me: Neither am I! ----- Original Message ----- |
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Bob Farnam <bfarnam@...>
The thing that has always bothered me is that all spring gear airplanes - C120,
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C140, C170, C180, C195, RV's, Christen Eagle, to name a few have considerable camber change under different conditions of load. You can include in that list bungee gear types like all of the ragwing Pipers. This has never, to my knowledge, been identified as causing a directional stability problem. My thinking is that camber, at least a few degrees either + or -, doesn't have much effect. I have felt all along that the primary problem was that the airplane as originally designed, lacked the full complement of steering tools present on other taildgraggers. Here's another point to think about. David G, in his original analysis, referenced a race car dynamics book which said that toe was about 5 times as effective in producing side thrust at the wheel as is camber, per degree. So we have airplanes which produce about -3 degrees of camber due to canard deflection which would make the left wheel, for example, produce right side thrust. Now if you divide 3 degrees by 5, you get 0.6 degree, which is exactly the toe out (left side thrust) set for the left wheel. Is is possible that Quickie understood this relaionship and that the 0.6 degree toe out was deliberately done to counteract the negative camber? Or is it just a coincidence? I don't know the answer to this, but the numbers do agree exactly. I'll say this. The negative camber does cause tire wear to be off center to the inside of the tire. I don't know whether that's good or bad. I let the tire wear on one side, then turn them around and wear the other side. I am getting about 175-200 hours per set of tires. I should go back and count the landings made in that time to get a better comparison with other airplanes. The Gall alignment should even out the tire wear, but maybe should include removing the toe out when it is done. Has anyone experience with a "zero-zero" setting for toe and camber? Bob F. N200QK "David J. Gall" wrote: Neither am I! |
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DorotheaKeats and ChrisWalterson <dkeats@...>
Neil Jepsen-------- I had built my little Quickie per plans and then
stuffed the 503 Rotax in the front end. I increased the gross to 600 lbs and this gave me quit a bit on - camber. I flew it about 30 hrs before I did the Gall mod and I think it was only my Dragonfly experience that kept me on the runway. After the Gall mod it was much better. Before, when all the wheels were on the runway it was not too bad , but if one wheel lifted before the other as in taking off , or landing in a crosswind it was a real handfull. Do an experiment. Grab a set of roller blades or ice skates. Go wide open and with both skates at 0 camber in a glide , lift one skate. You should still go straight ahead. Now do the same thing but give your skates some - camber and then lift one skate. You will turn away from the skate that is still on the ice. This is the same thing that happens on the airplane . If the right wheel is on the ground , you turn left if there is - camber. A little hard for your brain to digest when you are doing seventy down the runway with your ailerons and rudder saying you should be going right. Once the wheel is airborn, all is well. Small amounts of + or - camber are of no concern, but when you can visually see that the wheel is canted , be aware. My wheels are set to 0 camber and 0 toe out at gross and it all works for me. That's what I found.------------Canada Chris |
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Neil Jepsen <jepsen@...>
Hi Chris W
Thanks for your input. I've never seen a pair of ice skates in my life, let alone a frozen lake. I drove past a lake in our south island once, that sometimes freezes over, but thats the closest I've been. I saw a puddle frozen over once, but that was years ago! I take your point about steering when one wheel lifts. I haven't experienced that yet...the instability is with all 3 wheels down. I haven't made any progress since last week, as I've been aeway towing gliders for a week, and today is new yrs day, so I probably would get into trouble with "she who must be obeyed" if I went to the airport today. There was an interesting comment on the Dragonfly list yesterday regarding handling, and it alludes to the fact that maybe some ( a lot?) of my trouble may be the heavy footed pilot. However. someone said today that on a scale of 0-10, the Q2 was a 7 in the handling dept cf other tail draggers. I've flown TGs for over a 1000 hrs and many types, and on a scale of 0-10, my Q2 is 0 at present. Tomorrows another day. seasons greetings Chris neil DorotheaKeats and ChrisWalterson wrote: Neil Jepsen-------- I had built my little Quickie per plans andADVERTISEMENT
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david.cyr@...
I'll say this. The negative camber does cause tire wear to be off center to theinside of the tire. I don't know whether that's good or bad. I let the tire wear on one side, then turn them around and wear the other side. I am getting about 175-200 hours per set of tires. I should go back and count the landings made in that time to get a better comparison with other airplanes. The Gall alignment should even out the tire wear, but maybe should include removing the toe out when it is done. Has anyone experience with a "zero-zero" setting for toe and camber?> Bob Farnam, Yes, I have a zero-zero set up. I inadvertently "half corrected" my camber when I decided to remove the 2" toe-out that we had built into the Q2 at construction time. I say half corrected, because I sighted the axle holes dead center when the Q2 was on its mains with only its empty weight deflecting the canard. I believe I noted a significant improvement in stability as a result. After reading the David Gall article a few years later, I changed the camber to zero, still with zero toe-in/out. I recall seeing some additional improvement. I have (on occasion) rolled to a stop without moving the pedals, on a very smooth runway. BTW, I rely heavily on reverse aileron steering for extra help when needed... I don't have dual brakes, reflexor, belly-board or any other mod other than a vertical tail-wheel pivot. Dave Cyr |
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Bob Farnam <bfarnam@...>
David,
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It may be that "half-corrected" makes more sense since at the speeds most of the problems seem to occur, the wings are providing a substantial amount of lift and the canard is not seeing full gross weight to deflect it. Bob F. N200QK david.cyr@... wrote: I'll say this. The negative camber does cause tire wear to be off center to theinside of the tire. I don't know whether that's good or bad. I let the tire wear |
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David J. Gall
Bob,
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You're right on target. Landing gear directional stability is speed sensitive. All wheeled vehicles have a "critical speed" above which they are inherently directionally unstable. On Quickies, the camber becomes less negative/more positive as the canard flex diminishes due to the aerodynamic lift as speed increases. The canard flex at liftoff is approximately 3/8 of the static flex, so if you start the takeoff roll with negative five degrees of camber, you have about negative two degrees at liftoff. This is destabilizing. The critical speed is affected by the geometry, so as the camber changes, so does the critical speed. As the camber becomes less negative, the critical speed increases. The problem is that the critical speed does not always stay above the actual speed -- sometimes the airplane is operating above its critical speed. That's when ground loops happen. There are many factors that affect directional stability. On Quickies and most other taildraggers, the camber is variable with the load carried by the landing gear. That, in turn, varies with the gross weight and the aerodynamic lift. The aerodynamic lift is a function of airspeed, air density, and flap/elevator angle. The directional stability is a function of ground speed, so the relation between ground speed and airspeed can become critical -- wind becomes a factor. Runway crown affects camber angle since effective camber is the angle of the tire to the pavement, not just to vertical. Power setting also affects load on the landing gear. So with all these factors to consider, I think it's prudent to eliminate any that I can. I know that I can eliminate the destabilizing effect of negative camber if I build the plane to have zero camber at the worst-case, and to have positive camber (and a positive stabilizing effect) at all other conditions. This can be done by building in zero camber at gross weight, and adding a small positive amount for runway crown. But, if I build in a little toe-out, that becomes a constant known positive stabilizing "margin" that can cancel the need for the slight positive camber, so I figure zero camber for worst-case. (I also figure that I'll gain about three degrees of camber as I accelerate and judge that the tire wear will be acceptable since the loads on the tire will diminish as the camber increases. I can live with that, especially since it's way better than the wear conditions of the stock setup.) Sure, I can probably get away with some small negative camber at gross. During the takeoff roll, I'll gain about three degrees of camber, so (hopefully) my camber will become positive before I get near the critical speed (itself a function of camber, of course). It's that "hopefully" that you seem to be surviving on. So far, you have not encountered the conspiracy of all the other factors that will put you above the critical speed with negative camber. You have always had enough headwind/density altitude/flat runway/full-up elevator so that your camber was at the least negative and your ground speed was at the minimum possible for each moment of ground roll. But why chance it? No one has ever suffered from having too much directional stability on a taildragger! Put the camber at zero for the worst-case and you eliminate one powerful contributing factor to ground loops. Or, you can continue to play the "I think I've got enough camber" game until the day you try to land at Denver Centennial at gross weight with a quartering tailwind.... I'm not interested in pursuing an exercise in analysis to figure out what is the minimum camber I can get away with. Maybe "half-corrected" is enough for all forseeable operating conditions, maybe it's not. If yours is, indeed, "half-corrected," you may already have "enough" whatever that means. I KNOW that there are operating conditions that will exceed the capability of mere camber adjustments to prevent groundloops. I also know that putting the static camber at zero instead of at minus one or minus two or minus three moves the bleeding edge farther from my operating range and gives me a greater margin to accomodate crosswind gusts, forgetting to hold full-up elevator, and the occasional excursion over gross weight. It holds the added advantage of better tire wear, and it might just be sufficiently forgiving to let me use the stupid stock tailwheel and brakes that QAC gave us... although I DO plan to follow your excellent advice with the "six-pak." I'm not going to tell you that you need to change your camber, Bob. I don't know for a fact that what you have isn't "enough" to keep you out of the unstable range in all reasonable, forseeable operating conditions. You say that your tailwheel and other mods are all you need to make your plane satisfactory, and I believe you. David Cyr says he recalls seeing some additional improvement when he went from the "half-corrected" to the zero-zero setting, so maybe you will, too, if you choose to make the change. I'm just glad that you have made the effort to understand and appreciate the thinking that goes behind my conclusions. I could draw an analogy between this topic and the neutral point and static margin with respect to the CG location. And then we could discuss the fine points of a spirited aerobatic mount like a Pitts versus a workhorse like a Seneca or Caravan. if your plane is on the more-spirited side in directional stability and you have a good, positive control system (which you do!) then there's no harm in staying with it. I like a sports car better than a station wagon, and I simply recognize that the sports car may tempt me into a corner then give up its grip on the road. I want my Quickie to drive as predictably as a station wagon, that's all. That's what QAC promised.... ;-\ David J. Gall (Yeah, and 141 mph and 600 fpm climb on 22hp... and a bridge in Brooklyn!) -----Original Message-----
From: Bob Farnam [mailto:bfarnam@...] Sent: Wednesday, January 02, 2002 1:21 PM To: Q-LIST@... Subject: Re: [Q-LIST] Re: Measurements.camber David, It may be that "half-corrected" makes more sense since at the speeds most of the problems seem to occur, the wings are providing a substantial amount of lift and the canard is not seeing full gross weight to deflect it. Bob F. N200QK david.cyr@... wrote: to theI'll say this. The negative camber does cause tire wear to be off center inside of the tire. I don't know whether that's good or bad. I let thetire wear on one side, then turn them around and wear the other side. I am gettingabout 175-200 hours per set of tires. I should go back and count the landingsmade in that time to get a better comparison with other airplanes. The Gallalignment should even out the tire wear, but maybe should include removing the toeout when it is done. Has anyone experience with a "zero-zero" setting for toeand camber?>camber when I decided to remove the 2" toe-out that we had built into the Q2 atconstruction time. I say half corrected, because I sighted the axle holes dead centerwhen the Q2 was on its mains with only its empty weight deflecting the canard.I believe I noted a significant improvement in stability as a result. Afterzero, still with zero toe-in/out. I recall seeing some additional improvement.I have (on occasion) rolled to a stop without moving the pedals, on a verysmooth runway. BTW, I rely heavily on reverse aileron steering for extra helpwhen needed... I don't have dual brakes, reflexor, belly-board or any othermod other than a vertical tail-wheel pivot. |
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Peter Harris <peterjfharris@...>
David ,
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I have been out of the loop and missed some of the debate about camber . I am about to fit my original (salvaged ) wheel pants to the new canard. Original setup was plans built and I had no handling problems .I have a vertical tailwheel pivot but no other mods , no reflexor or belly flap . Close to take off speed the rudder is working anyhow. The plans built camber would be negative laden through to zero at lift off . Some negative camber would remain if the canard sagged with age .ie we are working with negative to zero camber in any case .So is there a problem with that ? Negative camber is widely used in motor racing for stability. What is the advantage in positive camber ? Won't it be more sensitive ? Peter H ----- Original Message -----
From: "David J. Gall" <David@...> To: <Q-LIST@...> Sent: Friday, January 04, 2002 3:53 AM Subject: RE: [Q-LIST] Re: Measurements.camber Bob, <, The canard flex at liftoff is approximately 3/8 of the static flex, so if you start the takeoff roll with negative five degrees of camber, you have about negative two degrees at liftoff. This is destabilizing.> David J. Gall To unsubscribe from this group, send an email to: Q-LIST-unsubscribe@... Quickie Builders Association WEB site http://www.quickiebuilders.org Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ |
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David J. Gall
Peter,
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Negative camber on the main landing gear of a conventional (taildragger) landing gear is directionally destabilizing. To say that "negative camber is widely used in motor racing for stability" is like saying that film is widely used in cameras for taking pictures. On the one hand, there is the "no, duh," response, and on the other hand there are the myriad technical questions gone begging to be addressed. Likewise for the questions of positive camber and sensitivity that you pose. The advantage of positive camber is that it guarantees that negative camber will not be the cause of any directional instability that you experience. That's the snake's answer. :-) The converse is that IF you have directional instability AND you have negative camber, removing the negative camber will go a long way toward removing the instability. This has been done by many Q-bird owners with the predicted beneficial result. It does NOT make the airplane more "sensitive;" if anything, it makes the airplane less sensitive. That is a good thing, because it reduces the airplane's directional response to bumps and crosswind gusts and such. If, by "sensitive," you are referring to control authority, then you have used the wrong word. However, I assure you that I have received not one report of an airplane that was so directionally "insensitive" to control input that it couldn't be made to swerve off the centerline at the pilot's command. :-) And finally, while I have the utmost respect for the Quickie's designer, Mr. Rutan never did "get it" about landing gear geometry in all his awkward and off-beat designs. His brother, Dick, even quoted him as being "astonished" that the airplane rolled straight on landing in an interview about the ill-fated Pond racer. Now, considering that source, could it be possible that a proper front-end alignment was never specified for the Q-birds? Sure, I know what the plans say to do, but what is the basis of that recommendation? I'm sure you've seen it before, but my basis is located at http://david.gall.com/files/Airplane/quickie1.txt. You say that your "original setup was plans built and I had no handling problems." Sounds like success to me. Did you test your ground directional stability at full gross takeoff weight in calm wind on a hot day? Did you do the "Jim-Bob six-pack" of brake and tailwheel mods? (I highly recommend them.) The only other thing I can recommend is that you are at an ideal place in the re-building process to do the camber change. Taking a few degrees off of the end of the canard before you glue the wheel pants on shouldn't present much of a problem. But, it's your call: you had a good setup before, why fix it if it ain't broke? Thanks for asking, David J. Gall -----Original Message-----
From: Peter Harris [mailto:peterjfharris@...] Sent: Tuesday, March 12, 2002 3:02 AM To: Q-LIST@... Subject: Re: [Q-LIST] Re: Measurements.camber David , I have been out of the loop and missed some of the debate about camber . I am about to fit my original (salvaged ) wheel pants to the new canard. Original setup was plans built and I had no handling problems .I have a vertical tailwheel pivot but no other mods , no reflexor or belly flap . Close to take off speed the rudder is working anyhow. The plans built camber would be negative laden through to zero at lift off . Some negative camber would remain if the canard sagged with age .ie we are working with negative to zero camber in any case .So is there a problem with that ? Negative camber is widely used in motor racing for stability. What is the advantage in positive camber ? Won't it be more sensitive ? Peter H ----- Original Message ----- From: "David J. Gall" <David@...> To: <Q-LIST@...> Sent: Friday, January 04, 2002 3:53 AM Subject: RE: [Q-LIST] Re: Measurements.camber Bob, <, The canard flex at liftoff is approximately 3/8 of the static flex, so if you start the takeoff roll with negative five degrees of camber, you have about negative two degrees at liftoff. This is destabilizing.> David J. Gall To unsubscribe from this group, send an email to: Q-LIST-unsubscribe@... Quickie Builders Association WEB site http://www.quickiebuilders.org Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ Yahoo! Groups Sponsor ADVERTISEMENT To unsubscribe from this group, send an email to: Q-LIST-unsubscribe@... Quickie Builders Association WEB site http://www.quickiebuilders.org Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. |
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Peter Harris <peterjfharris@...>
David ,
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Thanks for your detailed ideas re the advantage of + camber to stabilise the Q. David you have devoted a lot of time in analysis of the reports of handling problems which you attribute primarily to plans built negative camber .You have stated that your Q is not yet airbourne . Have you considered that some of these incidents may result from pilot error ? There is a learning curve and first encounter is a bit daunting . Although my Q sometimes "feels" unstable it has never ground looped . .There have been times when I have consciously attempted to steer it and then my slow clumsy reactions cause havoc by over correction, but even then it wants to go straight (tyres squealing). For me the best technique is to watch the end of the runway and let your brain stem intuitively respond with many tiny unconscious adjustments .Even though at times it "feels" slightly momentarily unstable I have gained the confidence to trust it and let it "sort itself out" . It wants to go straight .Any divergence is self corrected . I hold hard aft stick during the ground roll and get immediate response from tailwheel/rudder inputs. It is never boring , always a buzz . I have a pneumatic tail wheel with a vertical pivot . Control is easier if this tyre is a bit soft . Tailwheel tyre wear is fairly heavy . It must be doing a lot of work . I have been using a 10 yd. wide gravel strip at Noosa in Queensland .Main gear tyres needed replacement after 250 hrs. No T tail , no reflexor , no dual brakes .I think a reflexor would be an advantage to overall performance . I have thought about your idea and respect it but I want to recover the handling and everything else just like I had it before and I have decided to stay with the plans in this case.Like "if it aint broke" . Well the canard was broke but not due to negative camber .The fence was 3" too high . Regards , Peter H ----- Original Message -----
From: "David J. Gall" <David@...> To: <Q-LIST@...> Sent: Wednesday, March 13, 2002 3:01 PM Subject: RE: [Q-LIST] Re: Measurements.camber Peter, <Negative camber on the main landing gear of a conventional (taildragger) landing gear is directionally destabilizing.> To unsubscribe from this group, send an email to: Q-LIST-unsubscribe@... Quickie Builders Association WEB site http://www.quickiebuilders.org Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. To unsubscribe from this group, send an email to: Q-LIST-unsubscribe@... Quickie Builders Association WEB site http://www.quickiebuilders.org Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ |
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John Cartledge <urecomps@...>
Peter,
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Sounds like you have a great tailwheel. Can you please advise its details and source. IMHO a soft narrow tailwheel is a very big contributer to directional stability. Regards John Cartledge VH LOQ Q200. ----- Original Message -----
From: "Peter Harris" <peterjfharris@...> To: <Q-LIST@...> Sent: Wednesday, March 13, 2002 6:29 PM Subject: Re: [Q-LIST] Re: Measurements.camber David ,the Q.handling problems which you attribute primarily to plans built negative camber .Youerror ?then it wants to go straight (tyres squealing). For me the best technique is towith many tiny unconscious adjustments .Even though at times it "feels"slightly momentarily unstable I have gained the confidence to trust it and let it |
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David J. Gall
Peter,
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Have you ever operated your plane from a paved strip? Your description is of a classically unstable machine. Unfortunately, that accounts for 99% of all taildraggers ever built, so the status-quo is acceptance. However, even the level of instability can be "good" or "bad" as evidenced by your comment about slow, clumsy reactions. In any static instability, there is a time constant of divergence that can be measured. The degree of instability can be classified according to this time constant. You accuse yourself of being too slow when it is, in fact, that your plane is too fast for you - it has too short of a time constant. Even if you never make your plane directionally stable, you can reduce the level of instability and, thus, increase the time constant, hopefully to something larger than your slowest reaction time. Regarding pilot error: There comes a point in any investigation of a recurring phenomenon whereat one must consider that not all of the instances are attributable to the pilot. That is the point where I BEGAN. As taildragger pilots, we choose to drive what amounts to little more than a shopping cart, BACKWARDS. At high speed. With a free-castering tailwheel, this is a classic directionally unstable situation. HOWEVER, with the pilot's feet on the rudder pedals, it is an entirely different situation. The pilot's feet render the tailwheel somewhere between free and fixed, or even "anti-free" (active control). Looking at the situation with the tailwheel LOCKED, we can take the pilot's inputs out of the equation. Unfortunately, there are no operator reports of Q-birds using a locking tailwheel. However, there are lots of operator reports where the pilot reported applying a corrective rudder input (active control) to cancel a swerve, and the result was an amplification of the swerve into a loss of control. That's as good as a locked tailwheel report of directional instability. Ergo, the device is directionally unstable. What are the factors affecting directional stability of vehicles? They are remarkably analogous to the factors affecting the familiar pitch stability of airplanes. There is some point called the neutral point, and if the CG is ahead of that point, the vehicle is directionally stable (statically, for the purists). Just like the pitch stability neutral point is a function of the slopes of the lift curves of the front and rear wings, so the directional stability neutral point is a function of the tire performance curves of the front and rear tires. Unfortunately, for Q-birds, the tire performance curve of the rear tire is dynamically modified by the pilot's inputs, and that of the front tires is dynamically modified by the compliance of the suspension - read: camber change with load. Putting on a pneumatic rear tire is one way to modify the static stability. Running that tire in an under-inflated state can further improve stability, but only marginally. Replacing the hard rubber tire with a pneumatic one is analogous to putting a bigger stabilizer on the rear of the plane. Putting more downforce on the tailwheel is analogous to using a higher-aspect ratio tail and is another method of improving the stability (reflexor; T-tail). These have inherent disadvantages and are, in my opinion, not necessary. (Users report otherwise.) So, these approaches all concentrate on the tailwheel. However, there are two things to consider about tailwheel changes: 1) they are completely at the mercy of the pilot since he can vary the amount of "fixity" of the tailwheel, and 2) they only address ONE-HALF of the available options to change the neutral point. The other half of the options are in the main wheels. Whatever was done to the tailwheel to make it more effective, the opposite is needed at the main wheels to make them LESS effective. By analogy, we want to reduce the slope of the "lift" curve of the main wheels. Increase the tire pressure? Reduce the load they carry? Change to solid rubber tires? These all seem to be silly avenues to pursue, but there is one thing that can be done: change the alignment! The camber acts as an amplifier of sorts. Whichever way the tire is cambered - leans with respect to the road surface - the tire's lateral force response is enhanced when it is steered in that direction, and decreased when it is steered opposite that direction. For the Q-birds, structural sag makes the camber "inward." Because there are two front tires, the small inward force due to camber cancels and the plane rolls straight ahead. But, when turning, centrifugal force transfers the weight to the outboard tire, so the camber effect of that tire predominates and the net effect is of an enhanced response. By putting the static camber to an "outward" stance, the weight transfer in turns then decreases the overall response, making for the equivalent of a reduced slope of the lateral force curve: PERFECT, just what we wanted. And, it does not rely on the pilot! Note that this is a discussion of the stability of the machine. Therefore, I have ignored controllability. Thus, changing the steering gear ratio or adding another directional control device (larger rudder or differential brakes) is NOT an option for modifying the stability. There are, certainly, questions of controllability. If we increase stability, won't we sacrifice controllability? Yes. Does it matter? No. Your trusty steed will still corner like a Porsche, only now it'll do it like it's on rails. Well, maybe. That all depends on the numerous little details of the control system - brakes, tailwheel pivot, steering gear ratio, and rudder size. I have deliberately chosen not to delve into those issues. I have also not addressed the placement of the wheels with respect to the CG, and I won't today. Well, I've blabbed enough for one day. Back to the hunt. David J. Gall "There's them that has, and them that's gonna." P.S. Did I mention that directional stability decreases with increasing speed? -----Original Message-----
From: Peter Harris [mailto:peterjfharris@...] Sent: Wednesday, March 13, 2002 2:30 AM To: Q-LIST@... Subject: Re: [Q-LIST] Re: Measurements.camber David , Thanks for your detailed ideas re the advantage of + camber to stabilise the Q. David you have devoted a lot of time in analysis of the reports of handling problems which you attribute primarily to plans built negative camber .You have stated that your Q is not yet airbourne . Have you considered that some of these incidents may result from pilot error ? There is a learning curve and first encounter is a bit daunting . Although my Q sometimes "feels" unstable it has never ground looped . .There have been times when I have consciously attempted to steer it and then my slow clumsy reactions cause havoc by over correction, but even then it wants to go straight (tyres squealing). For me the best technique is to watch the end of the runway and let your brain stem intuitively respond with many tiny unconscious adjustments .Even though at times it "feels" slightly momentarily unstable I have gained the confidence to trust it and let it "sort itself out" . It wants to go straight .Any divergence is self corrected . I hold hard aft stick during the ground roll and get immediate response from tailwheel/rudder inputs. It is never boring , always a buzz . I have a pneumatic tail wheel with a vertical pivot . Control is easier if this tyre is a bit soft . Tailwheel tyre wear is fairly heavy . It must be doing a lot of work . I have been using a 10 yd. wide gravel strip at Noosa in Queensland .Main gear tyres needed replacement after 250 hrs. No T tail , no reflexor , no dual brakes .I think a reflexor would be an advantage to overall performance . I have thought about your idea and respect it but I want to recover the handling and everything else just like I had it before and I have decided to stay with the plans in this case.Like "if it aint broke" . Well the canard was broke but not due to negative camber .The fence was 3" too high . Regards , Peter H ----- Original Message ----- From: "David J. Gall" <David@...> To: <Q-LIST@...> Sent: Wednesday, March 13, 2002 3:01 PM Subject: RE: [Q-LIST] Re: Measurements.camber Peter, <Negative camber on the main landing gear of a conventional (taildragger) landing gear is directionally destabilizing.> |
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David J. Gall
Agreed! Please share the geometry and other details....
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David J. Gall -----Original Message-----
From: John Cartledge [mailto:urecomps@...] Sent: Wednesday, March 13, 2002 3:57 AM To: Q-LIST@... Subject: Re: [Q-LIST] Re: Measurements.camber Peter, Sounds like you have a great tailwheel. Can you please advise its details and source. IMHO a soft narrow tailwheel is a very big contributer to directional stability. Regards John Cartledge VH LOQ Q200. |
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Peter Harris <peterjfharris@...>
John ,
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The kit tailwheel assembly was modified by Alan Mollenhauer according to info available at the time . The pivoting axle is inclined from vertical ,and there appears to be a variation in camber as the wheel turns right or left . I will get a photo , but I think this mod was widely used . Regards , Peter H ----- Original Message -----
From: "John Cartledge" <urecomps@...> To: <Q-LIST@...> Sent: Wednesday, March 13, 2002 6:57 PM Subject: Re: [Q-LIST] Re: Measurements.camber Peter, Sounds like you have a great tailwheel. Can you please advise its details and source. IMHO a soft narrow tailwheel is a very big contributer to directional stability. Regards John Cartledge VH LOQ Q200. ----- Original Message ----- From: "Peter Harris" <peterjfharris@...> To: <Q-LIST@...> Sent: Wednesday, March 13, 2002 6:29 PM Subject: Re: [Q-LIST] Re: Measurements.camber David ,the Q.handling problems which you attribute primarily to plans built negative camber .Youerror ?then it wants to go straight (tyres squealing). For me the best technique is towith many tiny unconscious adjustments .Even though at times it "feels"slightly momentarily unstable I have gained the confidence to trust it and let it To unsubscribe from this group, send an email to: Q-LIST-unsubscribe@... Quickie Builders Association WEB site http://www.quickiebuilders.org Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ |
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Neil Jepsen <jepsen@...>
Peter
I am interested in two of your comments. Firstly, you say you look at the end of the runway. I can't see the end of the runway. I can only see out at about a 20-30deg angle. ( i'm currently at the "frighten yourself silly taxying stage") I have concluded that if I coyuld see better, taxying in a straight line would be easier. Secondly, what size is and where did you get the pneumatic tail wheel from. My tauil wheel is the original and I am of the opinion that it doesn't get a very good grip on the ground, especially if the trmac is a little rough. neil NZ Peter Harris wrote: David ,ADVERTISEMENT
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Neil,
If you're only taxiing your Q you should reflex the ailerons all the way up to kill the lift on the main wing. Approach the throttle slowly and pull out the throttle slowly as the yaw changes alot if you do the throttle changes abruptly. However if you get in trouble a quick pull on the throttle is in order to slow it down. The hardest part to handle is taxi. If it feels controllable you're ready. The soft solid rubber tail wheel is much better than the original. Speaking from experience. On 1st flight don't leave the aileron reflexed all the way up as it will make the pitch stability almost impossible to handle. I reflex mine to about 1/2" up to the trailing edge of the main wing. Your mileage may vary. Bruce Crain On Thu, 14 Mar 2002 13:04:47 +1300 Neil Jepsen <jepsen@...> writes: Peter________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. |
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Peter Harris <peterjfharris@...>
Neil ,
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I will get that tyre detail for you .It is readily available from your discount tyre franchise and used on trolleys etc . Do not get a grey tyre . They are meant for indoor trolleys and the bead is too loose . It should be possible for you to "just" see the end of the runway . May need to use a cushion . I am less than 180cm . My head is just about touching the canopy .I can see the runway end over the spinner. You may be interested in the tail wheel mod refer my reply to John . I am sure you will find a very big improvement by going to a pneumatic tyre set up . Peter H ----- Original Message -----
From: "Neil Jepsen" <jepsen@...> To: <Q-LIST@...> Sent: Thursday, March 14, 2002 10:04 AM Subject: Re: [Q-LIST] Re: Measurements.camber Peter I am interested in two of your comments. Firstly, you say you look at the end of the runway. I can't see the end of the runway. I can only see out at about a 20-30deg angle. ( i'm currently at the "frighten yourself silly taxying stage") I have concluded that if I coyuld see better, taxying in a straight line would be easier. Secondly, what size is and where did you get the pneumatic tail wheel from. My tauil wheel is the original and I am of the opinion that it doesn't get a very good grip on the ground, especially if the trmac is a little rough. neil NZ Peter Harris wrote: David ,ADVERTISEMENT
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