Fat boy wants to soar...

On Mar 21, 2:05*am, Bruce Hoult wrote:
On Mar 21, 3:15*am, bildan wrote:

The idea that the gross weight limit isn't a hard rule is just wrong.
Aircraft manufacturers gain nothing by understating load capacity.
The certificated max weight is as high as can be safely allowed.

That is clearly not true. A lot of aircraft design is trading off one
desirable feature against another.

We're talking gliders not airplanes. I claim it's precisely true.
Trade-offs you mention were made by the engineers who signed the
airworthiness documents. Estimates made on the flight line don't even
remotely rise to that level of expertise.


In the case of powered aircraft the MTOW is the weight used to
establish the published takeoff run, the distance to clear a 50'
obstacle, the rate of climb, the service ceiling, and probably others.
If you're operating out of short strips then you want to know how much
load you can carry. If you're operating a cessna off a 4 km runway at
sea level with no obstructions then it will be perfectly safe to
operate somewhat over MTOW, especially if the extra weight is carried
in the wings.

Although an overweight glider could make an aero tow takeoffs
hazardous under some conditions for the same reasons.


In the case of, for example, our club's DG1000 basic trainers, the
aircraft is permitted to do unlimited aerobatics with a +7/-5 G rating
at MTOW. If that's not a requirement on a particular flight and you're
happy with the +5/-3 G like most other gliders then you could operate
at some higher weight.

Yes, you can fly an aerobatic glider at a heavier weight if flown
within standard class G limits. But, we were not talking about
aerobatic gliders.


There is really no excuse for operating outside the CG envelope. *The
JAR 110 Kg seat limit is a little restrictive for us chubby Americans
but, hey, it's an incentive to cut back on the bacon burgers and
fries.

Seat weight is one thing, CofG is quite another. With the tail ballast
box full, those same DG1000's are within the published CofG limit even
with two pilots well over 110 kg each.

We're not talking about 'CofG' we're talking about gross weight. The
seat is primary structure just like a wing spar and is only designed
to support 110 Kg. In many, if not most, gliders, if the seat
structure fails, the flight controls would be jammed - just as
catastrophic as a wing failure.

Bottom line, if you fly outside the airworthiness certification
limits, you're a fool claiming to be smarter than the engineers who
designed the glider. Yes, I know the BGA allows over gross flights.

Hi Bill

I agree on the - fly within the limits. But I must disagree on your view
that seats are designed for 110kg. There is no rule that says you have
to limit it to 110kg. Read CS 22.25(a)(2)- the a minimum assumed single
seat load is 110kg - and all the force multipliers work fomr that
assumption. Effectively - For JAR-22/EASA CS-22 they are designed to
withstand 110kg*40g - that's 4400kg...

So - no - a fat guy sitting on one is not going to break it. Although
some of them might deform enough to be a problem at reachable G levels.
This is the "test pilot" part of exceeding the design limits.

It's all here if you feel like reading.
http://www.easa.europa.eu/ws_prod/g/rg_certspecs.php

CS 22.625 Fitting factors - Says you have to multiply by 1.33 for seat
loads.
CS 22.561
CS 22.785 Seats and safety harnesses says it has to be strong enough to
handle the force multipliers specified for emergency landings and crash
resistance with the designed seat load. The minimum load for calculation
of strength required being 110kg.

Practically the force multipliers are so high the they design for 110kg.
Practically - the size of the cockpit naturally limits the amount of
load you can put on the seat pan. I sincerely doubt anyone is going to
manage a 200kg load on a single seat...

Of more interest structurally is the total mass of non-lifting
components. That puts bending load into the main spar - so it does
matter. Although - again the multipliers used are very high. You are
unlikely to exceed safe strength margins unless you are executing
aerobatic manoeuvres.

Bruce

bildan wrote:
On Mar 21, 2:05 am, Bruce Hoult wrote:
On Mar 21, 3:15 am, bildan wrote:

The idea that the gross weight limit isn't a hard rule is just wrong.
Aircraft manufacturers gain nothing by understating load capacity.
The certificated max weight is as high as can be safely allowed.
That is clearly not true. A lot of aircraft design is trading off one
desirable feature against another.

We're talking gliders not airplanes. I claim it's precisely true.
Trade-offs you mention were made by the engineers who signed the
airworthiness documents. Estimates made on the flight line don't even
remotely rise to that level of expertise.

In the case of powered aircraft the MTOW is the weight used to
establish the published takeoff run, the distance to clear a 50'
obstacle, the rate of climb, the service ceiling, and probably others.
If you're operating out of short strips then you want to know how much
load you can carry. If you're operating a cessna off a 4 km runway at
sea level with no obstructions then it will be perfectly safe to
operate somewhat over MTOW, especially if the extra weight is carried
in the wings.

Although an overweight glider could make an aero tow takeoffs
hazardous under some conditions for the same reasons.

In the case of, for example, our club's DG1000 basic trainers, the
aircraft is permitted to do unlimited aerobatics with a +7/-5 G rating
at MTOW. If that's not a requirement on a particular flight and you're
happy with the +5/-3 G like most other gliders then you could operate
at some higher weight.

Yes, you can fly an aerobatic glider at a heavier weight if flown
within standard class G limits. But, we were not talking about
aerobatic gliders.

There is really no excuse for operating outside the CG envelope. The
JAR 110 Kg seat limit is a little restrictive for us chubby Americans
but, hey, it's an incentive to cut back on the bacon burgers and
fries.
Seat weight is one thing, CofG is quite another. With the tail ballast
box full, those same DG1000's are within the published CofG limit even
with two pilots well over 110 kg each.

We're not talking about 'CofG' we're talking about gross weight. The
seat is primary structure just like a wing spar and is only designed
to support 110 Kg. In many, if not most, gliders, if the seat
structure fails, the flight controls would be jammed - just as
catastrophic as a wing failure.

Bottom line, if you fly outside the airworthiness certification
limits, you're a fool claiming to be smarter than the engineers who
designed the glider. Yes, I know the BGA allows over gross flights.

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