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09-15-2012, 05:10 AM
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Join Date: May 2010
Location: Spring Hill Fl
Posts: 734
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Speed squared = kinetic energy
Bill
I believe your statement is about 90% correct.
However The kinetic energy required to stop an aircraft is a function of the weight (mass) of the aircraft multiplied by the square of its speed.
Land the aircraft faster than the design speed with loose bolts to many times by jumping on the breaks and getting tire skip things start to go wrong.
If information that I am getting from phone calls is truthful this is not from hard landings.
From talking with vans there demo aircraft has had its share of hard landings and no damage
Also we have heard that the 12 keeps flying and flying when they try to land.
This would be consistent with too fast of a landing and hard breaking.
( Pilot Error ) ( Learn your aircraft )
My view
Quote:
Originally Posted by Bill_H
OK, no plane is expected to fall 20 feet to the runway and not sustain damage. But ANY plane must be expected to be able to use maximum braking after touchdown and not damage itself in the process! I have trust in Van's engineering that they got that right and this problem has to do with something other than the use of maximum braking. I mean, that would be Structures 101.
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Last edited by joedallas : 09-15-2012 at 06:48 AM.
Reason: Added Note
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09-15-2012, 07:41 AM
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Join Date: Aug 2010
Location: Marshall TX (KASL)
Posts: 1,783
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Joe, I get your point. And I have been learning to approach really slow because I will eventually be operating off of a 1260 foot strip. But consider - The break sheds kinetic energy by gripping the spinning disk and heating it and the pads up. Is that rate of shedding HIGHER when the disk is initially turning faster? Or are the brakes actually less effective initially in that case? I dunno.
Also consider this. Say you have a nice smooth "landing" at 70 kts. The plane is not ready to land. There is not necessarily much weight on the wheels. The brakes are applied firmly. They could lock the wheels or one wheel. Skipping along in that condition might put a lot more backward force on one side compared to the other, although the amount of force on either side should be within the design limits (?). Actually doesn't a locked up wheel provide less braking action than a turning one? Also in such a case it seems to me that the plane would head for the edge of the runway real fast.
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09-15-2012, 07:51 AM
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Join Date: Oct 2010
Location: La Feria Texas
Posts: 3,822
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As come on now Bob, admit it, that seems to be aimed directly at me
Quote:
Originally Posted by NASA515
"I'm not shooting at anyone here but in my opinion, the best way to reduce the 'gross weight' that you fly at in the 12 is to go on a diet and exercise. There are few of us over the age of 50 who couldn't stand to loose some weight. I'll speak for myself and my wife who have in the last two years lost a bunch of weigt and now, with both of us in an airplane, we have a total combined weight of 325 pounds. All you have to do is to change the way that you eat - again, not shooting at anyone in particular. Shoot at me if you want, I'm a man."
Consider yourself "SHOT". BANG!
Bob Bogash
N737G
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09-15-2012, 12:47 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by joedallas
Bill
I believe your statement is about 90% correct.
However The kinetic energy required to stop an aircraft is a function of the weight (mass) of the aircraft multiplied by the square of its speed.
Land the aircraft faster than the design speed with loose bolts to many times by jumping on the breaks and getting tire skip things start to go wrong.
If information that I am getting from phone calls is truthful this is not from hard landings.
From talking with vans there demo aircraft has had its share of hard landings and no damage
Also we have heard that the 12 keeps flying and flying when they try to land.
This would be consistent with too fast of a landing and hard breaking.
( Pilot Error ) ( Learn your aircraft )
My view
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Joe, though not an official notification, this is pretty much my view also (as an explanation for the cracked C channels anyway).
As a (personal opinion) response to some of the comments that have been bouncing around the forum......
The ASTM's that the airplane was certificated under, have a huge # of tests that need to be complied with. Quite a few of them are related to the landing gear. Most of those are physical drop tests where a fuselage is loaded to a specific weight and dropped from specific heights in a variety of conditions meant to simulate hard touch downs at high alpha angle of attack, very flat (improper) touch downs (all three wheels at once), and even a test for nose wheel first (very improper touch down).
One of the gear tests is an aft pull load applied to the main gear legs. This is to simulate the potential load caused by heavy braking at gross weight, with the maximum friction coefficient that could ever be expected between the tires and the ground. An additional amount of load is designed into this test for a design safety factor just as is done for wing tests, etc.
I do not believe that the speed which brakes are applied, has a major effect on what load the gear leg feels. The load will peak at the moment that the tire is just about to begin to skid/slide. This load point will always be relatively constant regardless of speed. It is primarily, only effected by variables of brake pad/disk condition, tire condition and traction/grip rating, surface condition (pavement/grass, dry/wet, etc.), and to some degree, the airplanes weight. In theory, it could be argued that a higher load could be induced at a slightly lower speed, because the higher the speed, the more lift that is being produced by the wing (even once rolling on the wheels), thus reducing the amount of weight that is actually induced into the contact footprint between the tire and ground. Less weight on the tire will lower the possible load before the tire begins to skid/slide.
The RV-12 test fuselage passed all of the tests (BTW, technically a new fuselage could be used for each test, but in the RV-12's case, the same one was used for every test).
An RV-12, if built as designed, and flown properly, within the design gross weight, should perform the same way that the drop test fuselage did.
So, why have some airplanes gotten damaged? That is still not fully clear at this point. As of early this week, only 3 reports had come in to Van's. I believe more have come in during the week, but not as many as would be expected considering the # that Don has reported receiving information about. As Mike S. already mentioned, is the unwillingness for people to notify Van's an indicator of what caused their damage? We will probably never know.
Summery....
I think damage to the channel has been caused by excessively loose gear legs. At least one of the photos posted here by Don (as far as I know, it has never been sent to Van's by the airplanes owner) looks to have evidence of plastic deformation in the surface of the channel from the washer pounding into the surface (I,E, there appears to be a depression slightly bigger than the diam. of the washer, actually pounded into the surface of the channel).
Another owner here on the forum was recently petitioning for help in solving a nose wheel shimmy problem. From the description he gave, I posted that it didn't sound like nose wheel shimmy was a likely cause. A little while later he posted in this thread that he found the outboard bolts for his main gear legs loose. He has never stated so, but it is my guess that the loose bolts were the cause of what he thought was nose wheel shimmy.
Considering the tests that the design originally passed, and the operational experience that exists within the fleet, I don't feel that RV-12 owners need to be worried about going easy on their brakes. Particularly on grass runways (pavement should always induce the highest loads). Builders do need to be sure their legs are tight (there will likely be recommendations issued regarding this).
Finally... It is my opinion that the damage to side skins is a different issue, and not directly related to loose gear legs and/or damage to the channel. I believe it is pure overload from a very hard landing (possible even with some level of brake application at touchdown).
It is always possible that some owners have experienced a very hard landing, with loose gear legs, so that the overall problem would be amplified, but I think they are two individual issues, that in a few cases might at first look like the are linked, but are probably not. I don't think enough evidence has come into Van's to in any way substantiate they are directly related.
This issue is also being looked at, though it would be helpful if the owners that have experienced damage to side skins, would submit reports that included a download from their D-180.
Last edited by rvbuilder2002 : 09-15-2012 at 04:29 PM.
Reason: typo
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09-15-2012, 12:50 PM
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Join Date: Nov 2011
Location: Cheshire, UK
Posts: 10
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Have just checked our landing gear and found no cracks, wrinkled skin or any visible signs of anything amiss. We were however able to tighten all the outboard bolts between ? and ? turn.
For information we bought our 12 from the builder with 10 hours on the clock so have no knowledge of how tight the bolts were at construction. There are two of us flying the aircraft and we now have about 100 hours total operating from Manchester Barton (EGCB) in the UK which is not the smoothest of grass airfields! A rough estimate of the number of landings on grass would be in the order of 130.
Alan
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09-15-2012, 01:53 PM
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Join Date: Oct 2010
Location: La Feria Texas
Posts: 3,822
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For what it is worth, people that have communicated with me support your two different causes/effects theory. Of the three cracked channels, one of them had no skin wrinkles. Of the 9 reported skin wrinkles, two of those had cracked channels.
All but two of the wrinkled side skins deny any hard landings, two believe that a particular hard landing caused the damage, however the damage was not noted until quite some time after the event. I believe all the wrinkled skins reported loose bolts as well as the wrinkles. I have made no attempt to keep track of all the loose bolts reports.
Quote:
Originally Posted by rvbuilder2002
Joe, though not an official notification, this is pretty much my view also (as an explanation for the cracked C channels anyway).
As a (personal opinion) response to some of the comments that have been bouncing around the forum......
The ASTM's that the airplane was certificated under, have a huge # of tests that need to be complied with. Quite a few of them are related to the landing gear. Most of those are physical drop tests where a fuselage is loaded to a specific weight and dropped from specific heights in a variety of conditions meant to simulate hard touch downs at high alpha angle of attack, very flat (improper) touch downs (all three wheels at once, and even a test for nose wheel first (very improper touch down).
One of the gear tests is an aft pull load applied to the main gear legs. This is to simulate the potential load caused by heavy braking at gross weight, with the maximum friction coefficient that could ever be expected between the tires and the ground. An additional amount of load is designed into this test for a design safety factor just as is done for wing tests, etc.
I do not believe that the speed which brakes are applied, has a major effect on what load the gear leg feels. The load will peak at the moment that the tire is just about to begin to skid/slide. This load point will always be relatively constant regardless of speed. It is primarily, only effected by variables of brake pad/disk condition, tire condition and traction/grip rating, surface condition (pavement/grass, dry/wet, etc.), and to some degree, the airplanes weight. In theory, it could be argued that a higher load could be induced at a slightly lower speed, because the higher the speed, the more lift that is being produced by the wing (even once rolling on the wheels), thus reducing the amount of weight that is actually induced into the contact footprint between the tire and ground. Less weight on the tire will lower the possible load before the tire begins to skid/slide.
The RV-12 test fuselage passed all of the tests (BTW, technically a new fuselage could be used for each test, but in the rV-12's case. the same one was used for every test).
An RV-12, if built as designed, and flown properly, within the design gross weight, should perform the same way that the drop test fuselage did.
So, why have some airplanes gotten damaged? That is still not fully clear at this point. As of early this week, only 3 reports had come in to Van's. I believe more have come in during the week, but not as many as would be expected considering the # that Don has reported receiving information about. As Mike S. already mentioned, is the unwillingness for people to notify Van's and indicator of what caused their damage? We will probably never know.
Summery....
I think damage to the channel has been caused by excessively loose gear legs. At least one of the photos posted here by Don (as far as I know, it has never been sent to Van's by the airplanes owner) looks to have evidence of plastic deformation in the surface of the channel from the washer pounding into the surface (I,E, there appears to be a depression slightly bigger than the diam. of the washer, actually pounded into the surface of the channel).
Another owner here on the forum was recently petitioning for help in solving a nose wheel shimmy problem. From the description he gave, I posted that it didn't sound like nose wheel shimmy was a likely cause. A little while later he posted in this thread that he found the outboard bolts for his main gear legs loose. He has never stated so, but it is my guess that the loose bolts were the cause of what he thought was nose wheel shimmy.
Considering the tests that the design originally passed, and the operational experience that exists within the fleet, I don't reel that RV-12 owners need to be worried about going easy on their brakes. Particularly on grass runways (pavement should always induce the highest loads). Builders do need to be sure their legs are tight (there will likely be recommendations issued regarding this.
Finally... It is my opinion that the damage to side skins is a different issue, and not directly related to loose gear legs and/or damage to the channel. I believe it is pure overload from a very hard landing (possible even with some level of brake application).
It is always possible that some owners have experienced a very hard landing, with loose gear legs, so that the overall problem would be amplified, but I think they are two individual issues, that in a few cases might at first look like the are linked, but are probably not. I don't think enough evidence has come into Van's to in any way substantiate they are directly related.
This issue is also being looked at, though it would be helpful if the owners that have experienced damage to side skins, would submit reports that included a download from their D-180.
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09-15-2012, 02:46 PM
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Join Date: Oct 2011
Location: Manchester, UK
Posts: 9
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Bumpy Barton EGCB
Quote:
Originally Posted by p1lhs
Have just checked our landing gear and found no cracks, wrinkled skin or any visible signs of anything amiss. We were however able to tighten all the outboard bolts between ? and ? turn.
For information we bought our 12 from the builder with 10 hours on the clock so have no knowledge of how tight the bolts were at construction. There are two of us flying the aircraft and we now have about 100 hours total operating from Manchester Barton (EGCB) in the UK which is not the smoothest of grass airfields! A rough estimate of the number of landings on grass would be in the order of 130.
Alan
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As Alan says, Barton ... is not the smoothest of Grass Airfields.
http://www.youtube.com/watch?v=IERK-...feature=relmfu
John
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09-15-2012, 03:45 PM
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Join Date: Mar 2008
Location: Texas
Posts: 1,378
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drop tests where a fuselage is loaded to a specific weight
so besides the CG issue, what if the aircraft was loaded "over-gross?" would that fall in the category of hard-landing, side-skin damage?
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09-15-2012, 04:15 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by Peterk
drop tests where a fuselage is loaded to a specific weight
so besides the CG issue, what if the aircraft was loaded "over-gross?" would that fall in the category of hard-landing, side-skin damage?
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Definitely possible..... The structures are designed and rested with the expectation that the airplane will not ever weight more than 1320 lbs.
BTW, as an industry standard, when this type of testing is done, the fuselage is not loaded to equal the total gross weight value (1320 lbs). The reason for this is that when a wing stalls, the lift it is producing doesn't drop to zero. So an airplane that drops in fully stalled still does not hit on the gear with a force equal to it weight at the time.
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09-15-2012, 04:37 PM
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Join Date: Oct 2007
Location: Pensacola, FL & 2R4, Loveland, CO
Posts: 222
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Yust askin'
Does the astm testing include longitudinal impulse loads? Like locked brakes 'skipping / shuttering' or rough irregular surfaces that would cause the wheels to oscillate longitudinally. This type off load would be entirely different from what is tested by a 'drop test.'
FYI: Yust is southern for just.
-Dave
__________________
120500
Last edited by DaveLS : 09-15-2012 at 04:39 PM.
Reason: Yust clarification (iPad fingers)
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