Sizing the landing gear of a small airplane

[heitor]

I have a question about sizing a spring landing gear of a small airplane.

If I am reading the regulations correctly, FAR 23.723 requires drop tests for both takeoff and landing weights.

However, I can’t imagine why I should design the landing gear to withstand a MTOW landing. After reading the FAR 23.723, it seems that the landing gear (and the airframe structure) has to be designed to withstand load factors arising from a MTOW landing. Is it true?

Thank you for your comments.

 

[rb1957]

the MTOW test is at a reduced sink rate (6 fps ?, maybe 3) to represent landing immediately after t/o (hence MTOW) and the reduced sink rate reflects the pilot's extra care, knowing something's not right with the ship and that they're landing at MTOW and that this is a very infrequent occurrence.

 

[rb1957]

this isn't well spelt out in 23.473, but is better in 25.473 (says 10fps for landing wt and 6 fps for MTOW ... LIMIT)

not sure if there's an AC for 23.473

 

[moon161]

Engine out go around is one reason you might land at or near MTOW.

 

[berkshire]

I am not sure if it is spelled out in the far's but one drop test I have had to do in the past, was a simulation of a botched landing at MTOW i.e. high bounce to full stall on landing with no engine to catch the bounce.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[rb1957]

anything can happen, and eventually will ... but that does seem pretty severe ... the ol' "double failure" rule should come into play ... you've had one failure to cause you to land at max weight, and then you have to consider effing it up ? why not include encountering a micro-burst at the same time ? with under-inflated tires, and a CG too far aft (or fwd) ...

 

[btrueblood]

...carrying nitroglycerin as cargo...

 

[KENAT]

With a moose on the runway.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[MikeHalloran]

I've been involved with systems that were designed to the standard of only one failure allowed at a time, but I've always felt bad about it, because it was hardly ever not possible to design for two failures at once, i.e. it was an economic, not an engineering, limitation, and because In Real Life, I have personally experienced occasional double failures.

In this instance, I submit that a go-around at MTOW can induce enough stress on a pilot to make a high sink rate a reasonable expectation. I will however defer to the FAR's apparent assertion that every pilot will be as cool under pressure as Sully or Yeager, and/or the thing still has to be light enough to fly.






Mike Halloran
Pembroke Pines, FL, USA

 

[heitor]

Thank you for the comments.

I agree that the drop test at MTOW will be too severe. However, it seems to be required by the FAR 23.723.

By the way, do you agree with me that the FAR 23 is not clear when it deals with ground loads?

FAR 23.473(b) gives the definition of the design landing weight, which is a fraction of the design maximum weight. This requirement allows me to size the landing gear and the structure of my plane to withstand landing conditions at MLW (FAR 23.473(a)).

FAR 23.473(d) gives the descent speed at which the plane touches the ground in a landing. This speed is proportional to (W/S)^(1/4). Then, if the airplane weight is increased, the vertical speed will also increase. On the other hand, for big planes (FAR 25), we have to consider 6 ft/s at MTOW and 10 ft/s at MLW.

Now let's proceed to the drop tests. FAR 23.473(f) says:

"(f) If energy absorption tests are made to determine the limit load factor corresponding to the required limit descent velocities, these tests must be made under §23.723(a)."

FAR 23.723(a) says:

(a) It must be shown that the limit load factors selected for design in accordance with §23.473 for takeoff and landing weights, respectively, will not be exceeded. This must be shown by energy absorption tests ..."

So why I am allowed to take advantage of a lower weight to design the landing gear (23.473(a) and (b)) if I have to prove that the landing gear has to survive a MTOW landing (limit load) at a higher vertical speed?

This is confusing, isn't it?

Do you think I should be cautions and design a conservative landing gear (and airframe structure) considering the load factors evaluated at MTOW conditions (that is, MLW=MTOW)?

 

[MikeHalloran]

I suggest you need to make a table or a spreadsheet showing how the applicable regs evaluate in your particular instance, so you don't inadvertently miss a requirement.


Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

i'd talk to your local certification people. It is unreasonable (IMVHO) to apply 10fps sink rate with MTOW, in part 'cause this standard is higher than FAR25 (and FAR23 is generally a lower standard).

I think the obfuscation comes about because initially FAR23 (CAR3) probably didn't define sink rates, possibly expressed itself as the airplane has to survive loads expected.

10 fps at MLW and 6 fps at MTOW is what i'd propose in the certification plan (that the certification people accept as meeting the requirements).