far23 propeller clearance, and drop tests (landing gear) 1

[superenginee]

Hello,

I have some questions about the regulations for light aircrafts.

Limit drop tests
As stated at JAR 23.726, limit drop test should able to devolop 1.5 times the limit load factor.
Should this test be applied on each individual Landing Gear? For example if the worst case for a configuration with tricycle (nose gear) is the tail down landing, should the energy absorbed by the nose gear during the nose down be ignored? So regardless of the rest gears, should each of the gears develop 1.5*limit load factor induvidually?


Propeller clearance
At Jar 23.925 propeller clearance the regulation states that the propeller must have positive clearance from the ground with a deflection correspoinding to 1.5g.
Does this mean that the propeller should have positive clearance when the corresponding ultimate load factor has been reached (1.5* limit load factor, limit drop test),
Or is this factor 1.5*g refered to a normal landing case.
As 1.5* (the limit load factor) is probably not a real life situation and just a safety factor, then the (1.5*g) might mean it is just a normal landing case and the regulation does not include propeller damage prevention in such an adverse situation.

quick link for the reg

https://www.fzt.haw-hamburg.de/pers/Scholz/vorschriften/JAR-23.html

Do you know any book, or site that has a better explanation and examples of Jar23/cs23?

Thank you in advance

 

[Sparweb]

Your references to JAR's indicates that you may not be trying to access the current regulations published by EASA, if you are based in Europe. The regulations and the certification standards are published on the EASA website.

While you are there, you may be surprised to discover that even the CS regulations for part 23 aircraft are changing. I think EASA is implementing a change to specifications published by ASTM. The FAA already did that a few years ago. The rest of the world will probably follow suit. If you are working on a new design, you had better get up to date on the status of the requirements.

For CS 23.726:
The drop test needs to be 1.5 * limit for whatever load is the critical load on each landing gear leg. The regulation's own text tells you that you must explore all conditions, and identify the critical loading case for test. If the test is done on a single landing gear, then the critical (worst-case) load on that particular wheel must be applied. If the test is done on a rig simulating the airframe with all of the landing gear legs and wheels on it, then it has to be dropped from the critical attitude, at the critical weight, from the necessary height. Furthermore the airframe used in the test must be sufficiently similar to the actual plane to apply the same loads, moments and elastic/plastic responses to the impact. The propellor clearance is not considered in the drop test.

The regulation 23.726 probably looks different in the ASTM standard. Showing compliance in the same way as the above may be adequate for the ASTM requirement, but it may not - it's too new and I don't know the details yet.

For CS 23.925
The drop test is not considered relevant to the propellor clearance requirement. Landing load factor of 1.5g is abnormally high, even for a student pilot, but nowhere close to the limit load. I assume you are asking this way because your landing gear struts are non-retractable fixed gear legs?

The regulation 23.925 probably looks different in the ASTM standard. Showing compliance in the same way as the above may be adequate for the ASTM requirement, but it may not - it's too new and I don't know the details yet.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[superenginee]

Sparweb, thank you for your fast and comprehensive answer.

In fact i am using Stanag4671 for a UAV which is almost the same with CS-23, at least at the sections i am refering to.

I am using non retractable leaf springs (nose tricycle LG configuration, aft mounted propeller). The limit load factor is about 4.5(3m/s sink speed). The leaf spring and the tire is designed so as not to yield on developed loads of the ultimate load factor(1.5*4.4).

My question is about the propeller clearance. According to Cs23.925(and stanag) there must be positive clearance at a deflection developed with 1.5g. As you can see 1.5g is significantly lower than 4.5g(limit load factor). I do understand that 6.75 L.F. is an extreme case, but is there any case that the regulation is about the ultimate load factor (1.5*4.5=6.75) and the propeller should have positive clearance at this ultimate load factor?

Best regards

 

[Sparweb]

In principle, ultimate loads must be taken without failure that endangers the occupants of an aircraft, so smashing the propeller is secondary. For some aircraft, the debris can hit the fuselage and if it penetrates could injure people or ignite fuel, for other aircraft, it's not a hazard to the people at all. With an aft propeller the hazard is lower.
And for a UAV that hazard isn't relevant.

Also, in principle, limit loads apply to events that must not cause damage to the aircraft, and certainly not lead to injury of occupants. Again, in a UAV, the hazard to people is not relevant, so we'd only consider limit to be the load that the aircraft must support without deformation, damage, or difficulty to control.

You can get into trouble using regulations for one type of machine for a different kind. As I've pointed out already, the CS and FAR are built on occupant safety and those requirements saturate the reg's everywhere and in subtle ways. Using for a UAV you could be working hard for a requirement that doesn't matter at all.

If you want to continue using CS/FAR regulations, you might want to note that the FAR 23.925 has an additional requirement for aft-mounted propellers

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[superenginee]

So, in general the ultimate loads aim to keep the passengers safe, and consequently the 1.5g refering to the positive propeller clearance refers to normal landing conditions. Though with extreme landing conditions (limit load factor,3m/s sink speed) the propeller is being damaged.

As the regulation for UAVs is stanag, and it is probably a "copy" of the cs/far regulation, do you know any official organisation or source, i can refer to and ask some more questions?

 

[Sparweb]

I always thought the STANAG requirements were taken from a military context. Doesn't NATO publish this?
Sorry I work on piloted civilian aircraft and rotorcraft only. I don't know much at all about the military stuff.

I had to google it.
FAA regulates it:

https://www.faa.gov/regulations_pol...fm/go/document.information/documentID/1031867

China will regulate it:

https://uavcoach.com/caac-uav-airworthiness-certification/

This only applies if the drone is used for a commercial purpose. If you're just taking pictures of your own backyard, airworthiness is not required. But in all cases you probably need a drone operator's certificate.

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[superenginee]

I see..
In reference to the limit drop test and tyre loading: Should the tyres be bottomed by the time the load factor becomes ultimate?
In particular according to the regulation should the tyres be designed so as to be bottomed at and after the ultimate load (1.5*limit L.F.)?
Is it acceptable to be bottomed while the landing gear is loaded with limit load factor?

 

[Sparweb]

Again, you are pushing the limits of my knowledge.
But I don't need to remember everything:
Advisory Circulars
Notably:

https://rgl.faa.gov/Regulatory_and_...6e937e68f86256e51006929d4/$FILE/AC 23-15A.pdf

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[superenginee]

Thanks a lot for help!

 

[rb1957]

ok, let's understand what you're trying to do. is this a military program, or civil ?

either way, talk to the people approving your design for the best guidance.

FAA's ACs are a good starting point to understand these questions. Sure you can use JAA material for reference but there should be more current interpretations around.

You can use Part 25 guidance for Part 23 designs.

"limit drop test should able to devolop 1.5 times the limit load factor" probably should be "drop test should able to devolop 1.5 times the limit load factor". This is in FAA FAR23.726a, they want an ultimate load test. A problem you find in gear tests is that sometimes you cannot create a "true" ultimate load test, like with a lever gear … these will usually have ultimate loads applied to a gear with limit deflection.

FAR23.925a says (in effect) that leaf sprung gear are more dynamic have conventional gear so demonstrate the clearance at 1.5g rather than 1g. The idea is this is during taxi and maintaining the 7" clearance in taxi "should " give you clearance in landings. Leaf springs create their own design problems. For example we'd typically show clearance with the tires flat and the shocks fully compressed … but you can't do this. I would think showing clearance with a limit landing load would be appropriate. An important question is "is this a nose engine ?" (in which case the consequences of prop failure are less than if it's a twin, with prop planes near the cabin.

I wonder what they did for the Cessna 337 ? or many other Cessna's (and other same planes with leaf gears.

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

SE... a bit of information that would be VERY useful to this Thread... that I am unclear about...

What is the weight/size category for Your UAS?

Small/light UAS will have far different operating specs/standards than a military heavy UAS such as Global Hawk.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[Sparweb]

I wonder what they did for the Cessna 337 ? or many other Cessna's (and other same planes with leaf gears.

You drop one!

Superenginee: The above video is NOT the certification drop test. I'm posting it for fun and I hope you enjoy it on the general subject of drop tests.
But be advised that the plane is dropped from a higher elevation than required for certification, and the sand used in tests 2 & 3 is not called for in certification either.
This test seems to have more to do with human factors and setting off ELT's.
Here's the NASA Report generated by these tests.
This may give you an idea what a drop-test method might look like - though quite exaggerated, for an extreme test condition.


No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[superenginee]

Very intresting video!

By the way the uav is mid-short range, about 350 kg, so its considered pretty light.

 

[WKTaylor]

SE... 350-Kg is definitely on the light side of what I was imagining.

One element that has always made me curious and fascinated in drop tests... need for tire-spin-up in opposite to normal landing roll to simulate the instantaneous tire spin-up 'kick' on touchdown.

I have witnessed close-up real-world/operational break and tire failures... made messes... and a few dents... but LDGs were replaced with only minor damage... with exception of one F-16C. Tire failure can be particularly damaging/dangerous at high speeds

Very detailed description of RV-14 Design combined with Drop Testing... with tubular 'spring' gear struts.

https://www.youtube.com/watch?v=KbFMogBNUa0

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]