[MrNomad]

Designing the front gear tests and other elements

Please, please avoid designing a test with too many variables (refer to my previous post re simple tests). The calibrated bump can be as simple as a strip of 1/2" plywood secured to the pavement that engages only the nose wheel. By varying weight, tire pressure, approach speed and approach angle w/ multiple runs at each configuration, you are going to have a lot of data to analyze. Once you accomplish all of the above, you can change the height of the bump to 1", etc. etc. That is a lot of testing. If possible, I would consider one camera at a fixed location on the ground ("global reference") and one camera on the trailer ("Local reference")LarryT

Larry: Please write up a series of tests you'd like us to conduct and email the WORD document to me. Chet has wood working skills and so he should be able to build any size/shape "road bump" we need. I have a website and can post the video and any observed measurements we're able to get in one location so everyone can view our progress.

I also have a laptop computer and assume that an inexpensive camera can connect to the laptop via a 20' RCA cable. The camera will be affixed to the trailer and the cable has to be long enough to connect to the laptop in the tow truck cab. I haven't done this so any suggestions for the camera are appreciated. If I break the laptop the wife will definitely kill me.

I will also need recording software for the laptop (Windows 7) and assistance with the software selection would be helpful too.

Overnite, I received an email concerning a Paypal account (I've never set one up) so hopefully another new friend will handle that detail should anyone wish to reduce our out of pocket expenses. We cannot destroy the gear leg we are using so taking the tests to destruction is not an option. However, if enough money comes in we will buy a replacement (latest design) gear leg & fork, tire, tube & wheel, and we'd be willing to submit the jig to more strenuous tests. If people have latest design parts they're willing to donate, that's good too.

Once we depart from my house, we wait until we are on the test road and relieve the lift the scissor jack affords. We can gradually decrease tire pressure and repeat the tests you suggest. Chet can drive ahead and keep resetting the road "bump" while I decrease air pressure for each subsequent test.

Thanks again for the help.

mrnomad57 "at" gmail.com

 

[Captain Avgas]

Sorry to disagree with you Bob, I do not think the the torsional moment contributes to a primary failure mode. Watching the taxiing videos, when the aircraft is moving there appears to be no noticeable torsional moment in the gear leg, that is no deflection, as the tire castors in response to the turning of the aircraft.

Hi Terry, I think you made some good points. It is true that there is no overt torsional moment evident in any of the current RV taxi videos (and for the reason you pointed out....the wheel is free castoring). But then again, the nose gear does not fail in any of the videos we have seen.

However if we had a video of an actual nose gear failure we might just see something completely different (and I expect we would).

The theory that says that there will be negligible torsional moment developed in the gear leg is predicated on the assumption that the wheel is completely free to rotate and that the yoke is completely free to castor. In an actual failure situation neither of these assumptions may be valid.

For instance loss of clearance between the ground and the wheel pant might cause the wheel to bind up. It might also prevent the yoke from castoring. The same might be true of loss of clearance between the ground and the yoke nut.

Virtually all of the photos I have seen of nose gear failures reveal that the nose gear is not only bent under....but it is noticeably twisted. This would suggest to me that torsional moment plays some significant role in the final yield sequence.

 

[Ron Lee]

Virtually all of the photos I have seen of nose gear failures reveal that the nose gear is not only bent under....but it is noticeably twisted. This would suggest to me that torsional moment plays some significant role in the final yield sequence.

Not necessarily. I am still of the opinion that the majority of the flips are due to encountering soft dirt. At some point the nose gear drops and the nut/bottom of the nose gear leg touches. Enough resistance and you have the pole vault initiated.

At this point, the plane may twist instead of going over "neatly." This may be what causes any torsional damage noted. Plane twists. Nose gear is somewhat staked....gear twists in response.

 

[MrNomad]

Put together a plan....

Hi Terry, I think you made some good points. It is true that there is no overt torsional moment evident in any of the current RV taxi videos (and for the reason you pointed out....the wheel is free castoring). But then again, the nose gear does not fail in any of the videos we have seen.

However if we had a video of an actual nose gear failure we might just see something completely different (and I expect we would).

The theory that says that there will be negligible torsional moment developed in the gear leg is predicated on the assumption that the wheel is completely free to rotate and that the yoke is completely free to castor. In an actual failure situation neither of these assumptions may be valid.

For instance loss of clearance between the ground and the wheel pant might cause the wheel to bind up. It might also prevent the yoke from castoring. The same might be true of loss of clearance between the ground and the yoke nut.

Virtually all of the photos I have seen of nose gear failures reveal that the nose gear is not only bent under....but it is noticeably twisted. This would suggest to me that torsional moment plays some significant role in the final yield sequence.

Great discussion! If you engineers & theorists can agree on a test sequence & write it up, Chet and I will try to make it happen. Personally, I like the idea of setting up a series of bumps, varying tire pressure, but remaining with a a straight ahead platform for the initial tests.

As evidenced by the scraping on the side of my fork, my front wheel definitely turned before the gear knuckled under. My tire and wheel were unscathed (no marks whatsoever) but the fork had deep scratch marks on one side and so did the gear leg as it bent under.

I still have the old fork.

 

[LarryT]

Wilco on a suggested test plan

Larry: Please write up a series of tests you'd like us to conduct and email the WORD document to me. Chet has wood working skills and so he should be able to build any size/shape "road bump" we need. I have a website and can post the video and any observed measurements we're able to get in one location so everyone can view our progress.

I also have a laptop computer and assume that an inexpensive camera can connect to the laptop via a 20' RCA cable. The camera will be affixed to the trailer and the cable has to be long enough to connect to the laptop in the tow truck cab. I haven't done this so any suggestions for the camera are appreciated. If I break the laptop the wife will definitely kill me.

I will also need recording software for the laptop (Windows 7) and assistance with the software selection would be helpful too.

Overnite, I received an email concerning a Paypal account (I've never set one up) so hopefully another new friend will handle that detail should anyone wish to reduce our out of pocket expenses. We cannot destroy the gear leg we are using so taking the tests to destruction is not an option. However, if enough money comes in we will buy a replacement (latest design) gear leg & fork, tire, tube & wheel, and we'd be willing to submit the jig to more strenuous tests. If people have latest design parts they're willing to donate, that's good too.

Once we depart from my house, we wait until we are on the test road and relieve the lift the scissor jack affords. We can gradually decrease tire pressure and repeat the tests you suggest. Chet can drive ahead and keep resetting the road "bump" while I decrease air pressure for each subsequent test.

Thanks again for the help.

mrnomad57 "at" gmail.com

It may take me a little while. I am preparing to be deposed 2x next week (two different cases). It is an "occupational hazard"

LarryT

 

[molson309]

I had the opportunity a few years ago to look at an RV-6A that someone attempted to land downwind - and too fast - on a grass runway. Floating down the runway they noticed the end coming up quickly, pushed the nose down to force it onto the runway, and flipped over. The nose gear leg was bent completely over and the nose gear pant had punctured the belly skin of the airplane. The torque on the gear leg was strong enough that the bolt fixing the leg to the engine mount had broken, allowing the leg to turn in the mount.

This got me to thinking: We've seen cases where the hole for this bolt is drilled overlarge, allowing for play and eventual elongation of the hole. Is it possible that in some of the gear leg fold-up incidents that this bolt may have been substantially weakened and the breakage of the bolt was the actual _cause_ of the incident? If it did break, the results of the gear leg rotating in the EM would probably cause a tuck under/flip over similar to what's been reported, and could happen at a lower speed than could happen otherwise.

Is it possible to measure the torque on the leg during these runs? Maybe there's more force there than we think.

Mark Olson N407V RV-7A N16XV F1-EVO Rocket

 

[pkill]

Barry,

Glad to see activity on the A Model Flip Problem, hate the thought of any RV flipping (especially mine).

NTSB study of the problem pointing to the ground to strut clearance as an issue?
http://www.ntsb.gov/publictn/2006/RV_Photos.pdf

Van?s solution increases the ground to strut clearance with a new fork design?.
http://vansaircraft.com/pdf/sb07-11-9.pdf

Wondering why the ground to strut clearance could not be increased even further, must be a reason (??), but seems like the more the better. Like to see some testing on the idea.

Check out the Tundra Nose Gear? note higher nut to ground clearance.
http://www.kitplanes.com/news/news/8389-1.phtml
Don?t know how many Nose Wheel Tundra models are in use so there might not be much operational history there. Luc Premontc at Tundra [email protected] could provide you with a Tundra Nose Gear set up for your tests, their website here www.dreamaircraft.com.

We have a Grove Nose Wheel on the 7A project here which reduced the rotational drag. Planning to incorporate anything else to reduce the potential of a flip also (skid plate in the fairing etc.). Hope you guys come up with something. Keep up the good work.

Paul

 

[flyboy1963]

Tundrs nose gear

Wow! looks like a gear we should pay attention to....it appears to be very similar to ours, with most of the improvements we want.
Look at the obvious benefit of the higher nut/fork clearance...it also shortens the overall length of the leg by about 6 - 8"....at least eyeballing it, that would decrease some of the tuck under tendency.
Kudos to those working on this project...will follow the thread and send a donation when possible! I have my old fork, but so must a lot of others closer to the test site!!??!

 

[plehrke]

Wondering why the ground to strut clearance could not be increased even further, must be a reason (??), but seems like the more the better. Like to see some testing on the idea.

Reason: higher fork requires larger nose wheel pant which creates more drag which makes you go slower. Would be nice to know how much drag and how much slower so you know the trade. Sounds like some experimenting for someone out there.

 

[Bevan]

nose wheel/pant drag

Increasing the size of the nose wheel and wheel pant must be less than 2 mph as that's what Van's website claims is the total difference between the current nose wheel over a taildragger.

An RV on it's backside sure doesn't look like it's going very fast.

Bevan

 

[Buggsy2]

My opinion is that unless you fully simulate the interaction that exists with the entire airplane (including the main gear) then you really don't have any way to correlate what ever your results are, to what would actually happen on an RV.

As a civil engineer whose career is in computer model simulations of water resources, I think the above is too pessimistic. In fact tests and simulation can exclude all sorts of details and features and still correctly simulate the desired features of the prototype (real-life object). The key, of course, is to know what's important and what is not. You learn that by gaining experience in the test itself. I encourage you to continue on course. Make some test drives, recording what you can, and share here. Testing almost always involves iterations, refinement, and backing out of dead-ends and trying something else. Testing is not a smooth, single, continuous path from idea to results.

I don't have high-speed cameras or other needed gadgets, but would seriously consider donating money to the project for specific items. PM if you want.