SVSUSteve
Well-Known Member
If anyone needs something specific out of it, let me know. I have a copy.
Landing gear loads?
- Thread starter Tiger Tim
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Fly More
Active Member
On the subject of landing gear loads, hoping someone can help here:
I'm following the LSA ASTM for my scratch built project and am struggling with the gear loads. I'm a professional aerospace engineer, and I still can't figure it out.
What are they getting at with the variable "h" noted below? It says "drop height" - but for what? a test? and what is the little "m"? I can find no reference to it elsewhere in the doc. Can anyone give an example? Thanks.
I'm following the LSA ASTM for my scratch built project and am struggling with the gear loads. I'm a professional aerospace engineer, and I still can't figure it out.
What are they getting at with the variable "h" noted below? It says "drop height" - but for what? a test? and what is the little "m"? I can find no reference to it elsewhere in the doc. Can anyone give an example? Thanks.
See Federal Aviation Regulation Sec. 23.725 - Limit drop tests.
h is the height of the drop test used to demonstrate compliance. m is meter.
BJC
h is the height of the drop test used to demonstrate compliance. m is meter.
BJC
Maybe reading Jozef Pauliny's Master's Thesis will help:
Heliano
Well-Known Member
Fly more, the variable named m seems to be same as h. The ASTM text is a little confusing. The drop height is function of W/S, which in turn is directly proportional to the stall speed. The higher the stall speed, the higher the drop height. Drop height is related to the vertical speed on touchdown. The idea is: the higher the stall speed, the higher is the possible vertical speed on touchdown. Hope it helps.
I do not know the LSA standard... According to FAR Part 23 h = 3.6* SQRT(W/S) where W is max airplane weight in pounds and S is wing area in ft^2. H is in feet. I would expect the LSA rule to be similar, but can not be sure. As Heliano pointed out above it is intended to be set by stall speed of the airplane. The faster you are going when the wing lift sags away, the faster you will hit and the drop hieght is intended to reflect that.
Paz' book gets into all of this stuff in much more detail, derives useful equations for forces in the gear system, which in turn let's you design to the loads of your particular ship. Those of you hoping to do a Wittman leg will most likely need to do some more homework on tapered spring leg energy absorption and deflections. I worked up a model in Excel that lets any one design load up and deflect through iterations until it has absorbed the energy involved. We can talk about how to do this if anyone wants to talk in detail about it.
Bill
Paz' book gets into all of this stuff in much more detail, derives useful equations for forces in the gear system, which in turn let's you design to the loads of your particular ship. Those of you hoping to do a Wittman leg will most likely need to do some more homework on tapered spring leg energy absorption and deflections. I worked up a model in Excel that lets any one design load up and deflect through iterations until it has absorbed the energy involved. We can talk about how to do this if anyone wants to talk in detail about it.
Bill
henryk
Well-Known Member
If you want a Wittman type gear wouldn't you just figure out the loads and speeds and give those numbers to Grove or whoever is making the gear? They're the experts, after all.
Robbie Grove and Harmon Lang are experts at manufacturing high quality landing gear components of landing gear designs already in existence or of parts to your drawings. I know that the Grove website says they will design a gear leg to your needs, but Robbie Grove personally was clear that they do not design landing gear legs. Following the picks on Grove's website, they ask you to specify the gear leg dimensions. Harmon Lang has a similar set of instructions for specifying the gear leg to them. In neither case are the weights, wing loadings, drop hieghts, contact speeds, or other design parameters even mentioned. They both told me to supply a drawing and they would then quote or no-quote. Grove builds only 7075 legs and Lang builds only alloy steel round rod legs. Maybe that has changed, but I am skeptical that these small manufacturing houses have become full service landing gear design firms.
Billski
Last edited:
Fly More
Active Member
Thanks all - it looks like the LSA ASTM in this area is just a copy of the old part 23, which is much more descriptive and clear. I can figure it out from there.
Yes - planning to use Grove - but the landing gear is only as good as what it is bolted too!
Yes - planning to use Grove - but the landing gear is only as good as what it is bolted too!
Heliano
Well-Known Member
One last remark about the ASTM requirement: It assumes that ef x d is the same for rubber shock absorbers and spring shock absorbers (rubber rings and blade-type) but our humble experience does not exactly match that. Rubber shock absorbers seem to absorb more energy than a cantilever-type, metal blade legs. I would like to hear from any of you with related experience.
Fly More
Active Member
Pazmany gives steel spring efficiency of 0.5 and rubber of 0.6 - so he thinks you are right.
SVSUSteve
Well-Known Member
If anyone needs anything more out of the Pazmany landing gear book, let me know, I have the forum's reference copy. Sorry for the delay in replying...we have been swamped at work the past few months.
