Gross Weight

They say there isnt such thing as a dumb question, just inquisitive idiots so here goes. How do people actually determine gross weight of a airplane. Like when someone says there trying to get gross weight of a cub up to 2900lbs what determines how high it can be set? doug

As a teacher, I've learned that often the questions that seem the simplest are actually the deepest and hardest to answer, so don't apologize!

You may remember from high school science, that the force required to accelerate a thing is proportional to its weight. (Acceleration = rate of change of velocity. And to be precise, force is proportional to mass rather than weight, but the two work equally well for this purpose - so don't worry about the distinction).

Airplanes are certified to withstand certain accelerations in flight. Accordingly the forces required to accomplish those accelerations depend on the plane's weight.

Of course the airplane isn't a monolithic chunk, and each part exerts forces on other parts so that all the parts can accelerate together. Each part has to be able to sustain the forces necessary to accelerate the parts it's connected to.

Flight isn't the only consideration; the sudden accelerations of landing (my own landing accelerations are often LARGE) and rough ground must also be considered.

The accelerations are a given, resulting directly from flight and ground maneuvering events. So the required forces to accomplish those accelerations (and the required strength of components) then depends on weight. Too much weight = too much force to accomplish the acceleration for a given structural strength = structural overload and perhaps failure.

Does that answer?

Each part of a normal category airplanes structure has to be able to withstand a 3.8 positive G and a 1.52 negative G. A 2900# gross wt. airplane would have to be tested at and withstand a load factor of 11,020#(3.8 x gross wt.) and pass to be certified, same with neg. G's. A 60 degree banked turn is 2g's, what ever you and your aircraft weigh is now doubled. At 3.8 G's an airplane stalls. It's in some ways a safety measure to prevent structural damage. Identical airplanes stall at different airspeeds depending on the gross weight. Pilots talk about what a load a supercub can carry and they're right. A 1750 # gross wt. Cub is tested to 6,650 #'s, so if you fly off at 2300 #'s gross, you could actually make a 60 deg banked turn and only be at a load factor of 4,600 lbs. My advice would be when flying over gross fly NICE and don't increase the load factor anymore than neccessary. I'm going off memory here. But this should help.

Brad

Airplanes don't necessarily stall at 3.8 g.

At or below the aircraft's designated maneuvering speed the stall will occur at less than the designated maximum load factor (3.8 g for example), but at higher speeds stall will occur at higher load factors.

Remember that stall occurs at a certain angle of attack. And the lift of the wing (the load factor it is capable of delivering) is determined not only by by the angle of attack, but also by air density and the square of the speed.

Otherwise, I agree with Brad.

Hi Gordon,

I agree with you also. A heavily loaded airplane requires a greater angle of attack to produce the same lift as a light one for a given airspeed. So it's always closer to it's stall angle of attack. So it should stall at or before 3.8 G's to prevent structural damage, at given airspeed, light or heavy. I'm thinking of a high speed stall or an abrupt maneuver at low airspeed, it stalls before the 3.8 for safety I assume. Best bet don't Stall.

Thanks,
Brad

OK, so when all these experimental planes are at oshkosh, how did they determine there gross weights? doug

Years ago I was told that a general rule of thumb, the gross weight was determined mostly by the structural strength of the landing gear. Most aircraft on floats has a higher gross weight than on wheels, because the float gear is stronger than the land gear. This might not apply to experimental cuz you can call it anything you like even if it may be incorrect !!

Actually, there are a large number of variables that dictate the maximum certificated weight of an airplane.

First, if you are dealing with an experimental homebuilt category airplane, the builder determines the maximum gross weight. This can be based on most anything, though if it's radically outrageous, the FAA probably won't buy it.

Now, if you are talking about a CERTIFIED aircraft, right now the thing that seems to have more influence than anything else on gross weight is NOISE certification. If it's too heavy, it will be closer to the microphones during the climb, and will fail, due to excessive noise. The alternative may be to reduce prop rpm, or install a larger, slower turning motor. Or, restrict the gross weight so that the airplane is higher as it passes the microphones.

Secondly, the load factors described by others apply to the aircraft's primary structure AND to all its installed components, such as a battery box. Acceleration to the load limit cannot fail ANY Components on the airplane.

That said, I think more work goes into beefing up the landing gear assemblies to pass the drop tests than almost anything else.

Finally, there are literally dozens of other criteria that can limit max weight, such as engine cooling capability, climb performance, and so forth.

In other words, there are MANY factors that are involved in setting max weight for a certified aircraft.

MTV

So what gross weight is acceptable in a PA-18A
you talk of 2900 pounds. The books talks of mauw as being 1800 pounds. I have recently put on an airglas fuel belly pod and concerned I can't take a passenger with full fuel i.e. 68 gallons
thoughts appreciated
frsnk Persson

Most of the time a Super Cub can lift whatever you put in it. There is a kit to give you a 2000 lb gross weight.

Exceed your gross weight, and the most problematic thing is your insurance - it is probably no good if you were provably over weight.

A heavy stock Supr Cub will have a useful load somehere just north of 400 lbs, or less than a J-3. Best to keep the empty weight low. One way to do that is to start with factory empty weight and compute everything. Most will disagree with that statement, but when you try it you will see the legal advantages.

Some airplanes do not perform at all well when overloaded. Use some serious caution when packing thqt 172 for a family vacation at Big Bear.

opinion, all of it.

Gordon Misch said:

Flight isn't the only consideration; the sudden accelerations of landing (my own landing accelerations are often LARGE) and rough ground must also be considered.

Click to expand...

Now I know what to call my landings. Unplanned Accelerations!!

tempdoug said:

OK, so when all these experimental planes are at oshkosh, how did they determine there gross weights? doug

Click to expand...

They weigh their girlfriends and adjust accordingly

Glenn

phoenix said:

So what gross weight is acceptable in a PA-18A
you talk of 2900 pounds. The books talks of mauw as being 1800 pounds. I have recently put on an airglas fuel belly pod and concerned I can't take a passenger with full fuel i.e. 68 gallons
thoughts appreciated
frsnk Persson

Click to expand...

2900# for a stock 18A is seriously overdoing it. Top Cub from CC is 2300 I believe. The 2000 lb stc I have is still 1900 landing. Keeping within C of G is way more important than being a few lbs over.

In my 1750# GW Cub with aux tanks. If I put the full 60 gals in it there is useful load left for my butt and a toothbrush.

Legally.

phoenix said:

So what gross weight is acceptable in a PA-18A
you talk of 2900 pounds. The books talks of mauw as being 1800 pounds. I have recently put on an airglas fuel belly pod and concerned I can't take a passenger with full fuel i.e. 68 gallons
thoughts appreciated
frsnk Persson

Click to expand...

Frank,

What "books" are you getting the Maximum All Up Weight (MAUW) for a SUper Cub of 1800 pounds? The stock Super Cub in the Normal Category was limited to 1750 pounds on wheels, and 1760 on floats.

You need to sit down with the weight and balance certificate for your airplane and calculate a weight and balance. If you need some assistance, find an instructor to give you a hand.

More than likely, you will not be able to legally carry a passenger, and maybe not even yourself with that much gas aboard.

You may want to consider modifying the airplane with the Wipaire 2000 gross weight kit. It can be done to a covered airplane.

Operating in excess of legal weights may draw the ire of government types, and if anything bad happens, your insurance company may not be happy to find out that you're operating the airplane in excess of legal parameters.

MTV

He got it from the NTSB.

phoenix said:

So what gross weight is acceptable in a PA-18A

Click to expand...

Just going by memory (which is severely lacking these days), but 2050 sticks in my mind for some reason. But that is in restricted category.
For passengers, unless you get the 2000 GW, 1750 is acceptable, unless you live in Alaska .........................

Plus, if it can be shown that you flew way over gross and hurt your passenger, the insurance may not cover the millions they sue you for for negligence.

2,070 lbs PA-18A even with the 125 hp model. Restricted category as S2D noted.
DENNY

And, before you assume that that Restricted category weight offers the same assurances as the Normal category, please read up a bit on Restricted category, and what it does and doesn't afford you.

MTV