Max Landing Weight Determination
Thread Starter
Join Date: May 2005
Location: Australia
Posts: 49
Likes: 0
Received 0 Likes
on
0 Posts
Max Landing Weight Determination
Evening...
I'm interestd to find out how the maximum landing weight for light aircraft (eg: Cessna 182) is determined.
As a guess I would say the aircraft structure is required to withstand a certain rate of descent on touchdown, and then a factor is applied to that. But as I said it's a guess.
If anyone could give a brief explanation it would be much appreciated.
Thanks
I'm interestd to find out how the maximum landing weight for light aircraft (eg: Cessna 182) is determined.
As a guess I would say the aircraft structure is required to withstand a certain rate of descent on touchdown, and then a factor is applied to that. But as I said it's a guess.
If anyone could give a brief explanation it would be much appreciated.
Thanks
First thing, for virtually all light aircraft, it's identical to the MTOW / MAUW - although larger (or some military) have a separate and lower MLW, I can't say I've ever come across it for a light aircraft. That said, let's assume for the sake of argument that there is such a thing.
Second thing, generally MLW is not determined, it's a design parameter which is then proven and the structure designed to be able to take.
Having established that, there is a standard formula, repeated (with a few small variations) in most airworthiness codes, which says that the descent velocity of the aircraft may be considered to be 4.4 (W/S)^0.25 where W is MLW in lbf, S is wing area in ft² and the result is in feet per second.
Using that descent rate, plus knowledge of the necessary propeller clearances, plus an allowance for 2/3 of the weight to be acting as wing lift during the landing, the designer can work out the amount of deflection allowed (the more deflection the better since that reduces the forces acting on the undercarriage) and then moves onto a series of design cases which are a function of the determined maximum vertical reaction force (which is called Pz.max)
These design cases are fairly complicated, but simplistically are:-
- Maximum vertical reaction (Stall it in)
- Spin up (hard and fast normal landing)
- Spring back (bounce!)
- Sideforce (Crosswind / crabbed / didn't kick the drift off adequately)
- Braked roll
- One wheel.
So, using this determined value of Pz.max (which usually works out at about 3 times MLW) the designer must design a structure that will take all of these cases (although braked roll is usually a nonsense being generally much more benign than the spring back design case).
Then when that's all done we test it. There are two ways to experimentally test Pz.max, the favourite is to put a high gain accelerometer on the aeroplane and drop it (with the tyres landing on greased steel plates) from a predetermined height (based upon the rate of descent case I mentioned already). Drop tests are horrible and the best place to be when one is going on is usually in the next county. An alternative which is much nicer but only works if you've got no kind of fluid damper devices in it (whose characteristics are rate dependent) is to to a steady load .v. displacmement test with the aircraft sat on it's undercarriage (on greased metal plates again usually, although there are other means such as testing the various bits separately and adding the results up). The aircraft is then mounted up in a test frame and loads representative of all of the design cases applied and we see what happens (which filming or photographing everything for later analysis and proof).
Each of those tests is done to two loads, the first is "limit" which is the worst that should be seen in service - the undercarriage must take limit loads with no permanent deformation at-all. The second is "ultimate" which is a safety factor (typically but not always 50%) above limit, which the undercarriage must be able to take for at-least 3 seconds each time without catastrophic damage, although some lesser damage is probably inevitable.
Assuming that it then passed, this now completely wrecked test undercarriage is now thrown in a skip (dumpster to Americans) and a brand new one made and fitted to the aircraft for flight testing.
G
Second thing, generally MLW is not determined, it's a design parameter which is then proven and the structure designed to be able to take.
Having established that, there is a standard formula, repeated (with a few small variations) in most airworthiness codes, which says that the descent velocity of the aircraft may be considered to be 4.4 (W/S)^0.25 where W is MLW in lbf, S is wing area in ft² and the result is in feet per second.
Using that descent rate, plus knowledge of the necessary propeller clearances, plus an allowance for 2/3 of the weight to be acting as wing lift during the landing, the designer can work out the amount of deflection allowed (the more deflection the better since that reduces the forces acting on the undercarriage) and then moves onto a series of design cases which are a function of the determined maximum vertical reaction force (which is called Pz.max)
These design cases are fairly complicated, but simplistically are:-
- Maximum vertical reaction (Stall it in)
- Spin up (hard and fast normal landing)
- Spring back (bounce!)
- Sideforce (Crosswind / crabbed / didn't kick the drift off adequately)
- Braked roll
- One wheel.
So, using this determined value of Pz.max (which usually works out at about 3 times MLW) the designer must design a structure that will take all of these cases (although braked roll is usually a nonsense being generally much more benign than the spring back design case).
Then when that's all done we test it. There are two ways to experimentally test Pz.max, the favourite is to put a high gain accelerometer on the aeroplane and drop it (with the tyres landing on greased steel plates) from a predetermined height (based upon the rate of descent case I mentioned already). Drop tests are horrible and the best place to be when one is going on is usually in the next county. An alternative which is much nicer but only works if you've got no kind of fluid damper devices in it (whose characteristics are rate dependent) is to to a steady load .v. displacmement test with the aircraft sat on it's undercarriage (on greased metal plates again usually, although there are other means such as testing the various bits separately and adding the results up). The aircraft is then mounted up in a test frame and loads representative of all of the design cases applied and we see what happens (which filming or photographing everything for later analysis and proof).
Each of those tests is done to two loads, the first is "limit" which is the worst that should be seen in service - the undercarriage must take limit loads with no permanent deformation at-all. The second is "ultimate" which is a safety factor (typically but not always 50%) above limit, which the undercarriage must be able to take for at-least 3 seconds each time without catastrophic damage, although some lesser damage is probably inevitable.
Assuming that it then passed, this now completely wrecked test undercarriage is now thrown in a skip (dumpster to Americans) and a brand new one made and fitted to the aircraft for flight testing.
G
Thread Starter
Join Date: May 2005
Location: Australia
Posts: 49
Likes: 0
Received 0 Likes
on
0 Posts
Thanks a lot for the info Genghis! 
Very helpful and much appreciated.
It was actually the Cessna 182S which got me thinking. It has a MLW which is 150 lbs less than its MTOW.
Thanks again
Very helpful and much appreciated.
It was actually the Cessna 182S which got me thinking. It has a MLW which is 150 lbs less than its MTOW.
Thanks again
I've never done any cert work on Cessnas, nor flow a C182 but guessing wildly, they probably designed it to the limit of the safety factors and then discovered in test that the undercarriage wouldn't quite take the design MTOW and had to publish a slightly lower MLW - the alternative to be a very expensive redesign of the undercarriage.
These things happen.
G
These things happen.
G
