---------------------------------------------------------- Lightning-List Digest Archive --- Total Messages Posted Fri 01/04/08: 6 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 02:47 AM - Re: Gear leg shimmy experiments (Malcolm Ferguson) 2. 04:13 PM - Re: Gear leg shimmy experiments (N1BZRich@AOL.COM) 3. 04:52 PM - Gear leg toe-in (Peter and Jan Disher) 4. 06:52 PM - Re: Gear leg shimmy experiments (Johnny Thompson) 5. 09:17 PM - Re: Gear leg toe-in (N1BZRich@aol.com) 6. 09:51 PM - Re: Gear leg shimmy experiments (N1BZRich@AOL.COM) ________________________________ Message 1 _____________________________________ Time: 02:47:27 AM PST US From: "Malcolm Ferguson" Subject: RE: Lightning-List: Gear leg shimmy experiments There was extensive discussion on another list that escapes me for the moment, concerning gear leg shimmy. It was a while ago but if my memory serves me right, the conclussion went along the following lines. The cure was slight toe out under all weight conditions, say half a degree or so. With any toe in the tyre will try to track inwards. As the leg slopes out the inward motion tries to stand the leg up. As the plane has inertia and a heavy weight it does not give, so ultimately the tyre is squashed down till it finally looses adhesion. As it flicks outwards it moves upwards slightly and travels past its neutral point then begins its cycle over again. It finds a natural harmonic at a speed peculiar to an individual aircraft and surface and does not happen on surfaces with reduced adhesion such as grass,dirt,stones etc. I have suffered from shimmy sporadically for a long while till I adjusted the toe in after following that discussion-relatively easy to do on my earlier design spring steel gear legs. As an aside I have been trying to decide what aircraft is to be my next project(if the drought ever breaks here in south OZ) and have narrowed it down to a Legacy FG or a Lightning with a Rotax 914turbo. In most cases the Lightning looses nothing to the Legacy except in sheer speed. The Lightning actually has a better useful cockpit load and half the fuel consumption. I reckon the 914 should be capable of cruising at VNE at FL150 on 23-24litre/hr which would put it only 40 kts less than the Legacy. One lovely advantage to the Legacy is its high glass transition temperature allowing sexy paint schemes with dark colours. Just dreaming about aviation is almost as much fun as flying. Buz, I eagerly await your next epistle. They are always enlightning. Malcolm Ferguson Early model Esqual 912s 340hrs ________________________________ Message 2 _____________________________________ Time: 04:13:46 PM PST US From: N1BZRich@AOL.COM Subject: Re: Lightning-List: Gear leg shimmy experiments Hi Malcolm, Good to hear from you again. I guess I did not realize you have a Rotax on your Esqual. How do you like it and how long to get your 340 hours. Do you only use auto fuel? What type and size prop are you using. Speed results? Too bad we are not closer, I think a race is in order. Also didn't know that you have the older style spring gear legs. That would make it easier to adjust toe in/out with shims. Interesting that the article you mention says that toe out is the key. Can you direct me to that discussion? What ever (in or out) I know that the "toe" will make a difference, but with an airplane as light as these there is a big percentage of weight change from it being at gross weight to it being fairly light. As a result, I think the toe would probably need to be different for different weights, tire pressure, etc., which we can't do. Therefore we need to know what works best for the range of aircraft weights. That may or may not be 1 degree toe in that the Esqual and Lightning kits call for. Actually the 1/2 degree of toe out that you mention makes more since in my mind (especially since these are not tail draggers). But I am convinced that to completely cure the shimmy, it will probably take a combination of the correct toe (in or out???) and some way to stiffen the legs to change the shimmy, harmonic vibration, or whatever we want to call it. That is why the RV guys use the broom stick method - it is a big help even if it is not the total real answer. Blue Skies, Buz Rich **************Start the year off right. Easy ways to stay in shape. http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489 ________________________________ Message 3 _____________________________________ Time: 04:52:53 PM PST US From: "Peter and Jan Disher" Subject: Lightning-List: Gear leg toe-in Hi Buz, In my manual for toe-in it states 2.25" over 57.5" for each side, 1 in 60 I though was app. 1 Deg. there for the manual would be over 2 Deg. would it not ? Pete Disher OZ ________________________________ Message 4 _____________________________________ Time: 06:52:08 PM PST US From: "Johnny Thompson" <14az@mysprocketmail.com> Subject: Re: Lightning-List: Gear leg shimmy experiments Hi All I am just days away from taxing my lighting so have no knowledge of this type of gear leg. I have been a pilot, test pilot and maintenance tech for about 39 years. The only vibration I have ever felt in the landing gear was caused by tires out of balance, bearings or bullets. Again I have never had this type of gear legs. I have installed the tires in the position recommended by the manufacture and have balanced the tires. This always helps and I would think these legs need all the help they can get. My questions: How many have or had a problem with leg shimmy? Has anyone spin balanced the tires and did you have a shimmy problem before or after the balance? What is the weight (heavy/light) of the aircraft when it is most prone to shimmy? Thanks Johnny Thompson ________________________________ Message 5 _____________________________________ Time: 09:17:58 PM PST US From: N1BZRich@aol.com Subject: Re: Lightning-List: Gear leg toe-in Pete, your math is right. When we recently adjusted Linda's I think I remember using 1" at 58" for 1 degree. I don't have the manual in front of me, but the numbers you quoted would give over 2 degrees. Some other important parts of setting the toe is to have some weight in the cockpit (not sure what amount the manual recommends) so that you have the gear legs spread to a normal position. Then make sure the centerline of the aircraft does not move while you do the measurements for toe. One technique (if you don't have the jig that they use at the factory) is to drop a plumb bob from the nose and tail then use a chalk line to connect the two spots. This line establishes the CL of the aircraft on the hangar floor. Then measure out perpendicular from that center line to the centerline of each main tire. That then becomes the start point for setting the 1" at 58". Of course, you probably already knew all of this. Buz **************Start the year off right. Easy ways to stay in shape. http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489 ________________________________ Message 6 _____________________________________ Time: 09:51:58 PM PST US From: N1BZRich@AOL.COM Subject: Re: Lightning-List: Gear leg shimmy experiments Hi Johnny, Certainly mounting aircraft quality tires on the rims in the correct way is important for balance. Some people don't seem to know what the paint spot on the tire is for. But you will have to wait for Nick to answer your question about how many Lightnings have had shimmy. I think he is still away from SYI and may even be in Arizona to make the initial flight on several Lightnings out that way. The shimmy situation has to do with the round rod gear leg (RVs I think initially identified the problem) and it varies from airplane to airplane, and seems to have something to do with toe, tire pressure, aircraft weight, speed, landing/taxi surface, and how you are holding your tongue. It does not occur on grass, and only happens at the slower speeds, say around 20 to 22 mph. And it does not happen on every landing (could be the tongue holding thing). On my airplane it happens on most every landing roll out (22 mph) and is worse when the airplane is light weight with higher tire pressure and on black top. Not quite as bad on one concrete runway I often land on. You will know it when it happens and if you have someone watching you they will see it as well. Don't confuse this with the possible nose gear shimmy that is caused by the castering nose wheel being too loose in the bracket that holds it on. Just tighten the 8 nuts/bolts a little so there is some "feel of tightness" when you have the weight off the nose and turn the nose wheel from side to side (the nose wheel is in the air for this test with someone pushing down on the tail of the airplane). As to spin balancing an aircraft tire, I have never done that for any of the 6 airplanes I have owned but it could not hurt. My experience in other airplanes that weigh about what a Lightning or Esqual does (my single seat Pitts) was that I never got any shimmy (no round rod gear leg), but tire pres sure certainly made a difference as to how hard it was to keep straight on landing roll out- lower pressure was easier. Of course the Pitts touched down much faster than a Lightning or Esqual. Bottom line, all the things we mentioned above will help or hinder the tendency to shimmy. The key seems to be get the toe right (not sure what right is at this point), make sure you have the tires mounted properly on the rims, start with no more than about 25 psi in the mains, stiffen the gear leg by some method as discussed before, and hold your tongue just right. Good luck on that first flight. Let us know how it goes. Blue Skies and check six, Buz My questions: How many have or had a problem with leg shimmy? Has anyone spin balanced the tires and did you have a shimmy problem before or after the balance? What is the weight (heavy/light) of the aircraft when it is most prone to shimmy? Thanks **************Start the year off right. 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