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  #1  
Old 01-08-2019, 05:26 PM
JackinMichigan's Avatar
JackinMichigan JackinMichigan is offline
 
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Default What determines max gross weight?

I recently completed max gross weight testing. I loaded my RV-10 with 500 extra pounds and got the CG to within 1/2" of max aft. I fully expected it to act a lot more sluggish, but apart form needing more nose down trim during climbout I couldn't tell any difference in performance. She jumped off the runway and climbed like a witch, just as she always has. It seemed like the plane could have easily taken on another 200 pounds without issue.

Also, cold days at low elevations will generate a lot more lift than hot days at high elevation, yet the max weight is the same for all conditions.

So what determines max gross weight? Is structural? Is it arbitrary?

My father had a friend that was a flight engineer on a B-29, and he said they would load those planes thousands of pounds beyond the max recommended weight by Boeing. They would have to fly in ground effect for a couple hundred miles till they burned off enough fuel to climb.
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  #2  
Old 01-08-2019, 05:56 PM
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RV6_flyer RV6_flyer is offline
 
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Quote:
Originally Posted by JackinMichigan View Post
I recently completed max gross weight testing. I loaded my RV-10 with 500 extra pounds and got the CG to within 1/2" of max aft. I fully expected it to act a lot more sluggish, but apart form needing more nose down trim during climbout I couldn't tell any difference in performance. She jumped off the runway and climbed like a witch, just as she always has. It seemed like the plane could have easily taken on another 200 pounds without issue.

Also, cold days at low elevations will generate a lot more lift than hot days at high elevation, yet the max weight is the same for all conditions.

So what determines max gross weight? Is structural? Is it arbitrary?

My father had a friend that was a flight engineer on a B-29, and he said they would load those planes thousands of pounds beyond the max recommended weight by Boeing. They would have to fly in ground effect for a couple hundred miles till they burned off enough fuel to climb.
A lot of things go into "setting" gross weight on an EXPERIMENTAL airplane.

1. How does it fly at the set weight?
2. What are the stall characteristics?
3. What are the spin characteristics?
4. How much of the Engineers designed in safety factor are you willing to remove?
5. How much of the Engineers designed landing gear safety factor are you willing to remove?

These are just some of the things to consider.

The designer set a recommended gross weight. IF you are willing to remove some of the designer's safety margin and still stay within the designer's CG range, you may be able to operate at a higher gross weight than the designer recommended proving you are willing to compromise some of the safety the designer built in.

I do not recommend changing the designer's CG range of any aircraft built according to plans.
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Last edited by RV6_flyer : 01-08-2019 at 05:57 PM. Reason: spelling
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  #3  
Old 01-08-2019, 05:58 PM
scsmith scsmith is offline
 
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Normally it is structural. There are cases where take off weight is adjusted for altitude or temperature, but not usually for general aviation aircraft, because it is important to keep things simple.

The amount of lift is determined only by angle of attack and dynamic pressure, which equates to indicated airspeed (with some minor caveats). So hot, high, cold, low, all that changes is the true airspeed needed to get to the same indicated airspeed. (and you should know that from your private license training)

Available engine power is also affected by those things and influences take-off and climb performance, and it is up to you to adjust your take-off weight to achieve the necessary performance to depart and climb from a particular place.

Respect the recommended maximum gross weight from Van's. It is based on structural calculations and a safety margin which accounts for analysis uncertainty, material properties variations, assembly quality/variation, environmental effects, etc. That safety margin is important for all the above reasons; it is not for you to say, well, there is a 50% margin, so I will take 25% of it with a higher gross weight.
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  #4  
Old 01-08-2019, 06:01 PM
Desert Rat Desert Rat is offline
 
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For a certificated airplane, it's determined by structural limitations, I'd imagine that vans follows a similar process
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  #5  
Old 01-09-2019, 12:03 PM
sblack sblack is offline
 
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You need to be able to pull a given amount of G without any permanent deformation of the structure and the structure needs to take 1.5 times that amount of G without anything breaking. In the certified world those limit numbers are:

CATEGORY/LIMIT LOAD FACTOR
Normal*/3.8 to 1.52
Utility (mild acrobatics, including spins)/4.4 to 1.76
Acrobatic/6.0 to 3.00

This is an oversimplification of a complex question, since there are numerous static and dynamic load cases to consider, but this gives you and idea of one of the most important which is the wing bending.

You also need to meet minimum climb requirements but RVs are such good performers that this is usually never limiting.

What load factors were used for the RV series of airplanes I don't know. I suspect the limit load factors are documented somewhere. Obviously the lighter you are the more G you can apply before the structure fails, so that is why there is a different weight limit for aerobatic flight than for normal operations.

So getting back to your original question, yes, you can overload your airplane, but you won't be able to pull as much G before the wings fall off or before you encounter other problems, like structural deformation on a hard landing. That could be a problem if you have an upset of some kind and you need to pull out - the normal safety margin won't be there. These numbers are not carved in stone, but they have been established over 100 yrs of airplane design and deviating from them is not recommended and has killed lots of people. In wartime they push those limits since they are willing to take more risk - people shooting at you distorts the whole risk equation.

Since apparently you don't have an intimate knowledge of these subjects (few pilots do), you would be well advised to respect these limitations in order to stay safe and avoid damage to your airframe. On the other hand, if you takeoff 10 lbs over weight, your airplane is not going to explode and in a few minutes you will have burned down to your max gross weight. It's up to you - you are free (except for breaking the law of course) to go take what ever risk you feel comfortable with, but be very aware of what you are doing and if you have passengers you really need to respect that they presume you are not taking undo risks.
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  #6  
Old 01-09-2019, 04:01 PM
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N941WR N941WR is offline
 
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Once the strength requirement is met, do performance criteria come into play?

For example, assuming the airframe is strong enough, will "they" determine that at a given GW the aircraft will only climb at 200 FPM (or some arbitrary number) and use that number as the max GW?
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  #7  
Old 01-09-2019, 05:19 PM
flyinga flyinga is offline
 
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As far as I know, there are no climb gradient limitations for a part 23 single engine airplane. A C150 at max gross weight and a high density altitude will have a negative climb gradient but that is not a limit. it would obviously be stupid and possibly fatal to do it but not a limit.
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  #8  
Old 01-09-2019, 05:41 PM
Tankerpilot75 Tankerpilot75 is offline
 
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While your cg may be good for takeoff with an over loaded aircraft have you checked it out (along with handling characteristics) at a very low fuel condition? Center of gravity moves aft as you burn fuel so what is in cg limits with full tanks can quickly get out of limits as you burn gas.

Trust me, it gets ugly fast when you’re past the aft cg limit. If that ever happens, remember full nose down trim helps!
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