Overweight takeoff / flight

John Gaquin wrote:

An old pilot once told me, when I was a young pilot, "...sumbitch flies a
hell of a lot better overweight than it does outta gas..."

That postulates a situation in which I those are my only two choices. I'm
betting that I can live my entire life without putting myself in that situation.

George Patterson
A man who carries a cat by the tail learns something that can be learned
no other way.

"Koopas Ly" wrote in message
om...

I would start by considering the increase in weight as comparable to
an increase in load factor. Hence, all your aoa-related speeds would
increase by the square root of the load factor. Vs, Vx, Vy, Vglide,
etc. would all increase. Va would also go up.



I take issue with Va. At first thought, it should go up as sqrt( m/m0)
with m the new weight and m0 the maximum gross at which Va is
quoted. This since at a higher Va, we can maintain the same AOA
as we did at m0, so the G forces at stalling AOA never exceed the
design limitations.

BUT, there are 2 things (at least) which contribute to the setting of
Va in the first place. One is the limitation of 'heavy things in the plane',
such as a bag of sand in the baggage compartment. If this is the limiting
factor, then Va should indeed scale as sqrt(m/m0). However, there is
also the 'torque on the wings' (low wings) or 'force on the wings' (struts
on Cessna). If you are pulling 3.5G with a higher gross weight, you'll
be exerting more force than was designed for at certified gross.

So to be safe (hah!, we're talking about overloading dammit), then
unless you know exactly which type of failure limits Va in the first
place, you'd be best off using Va for certified gross and not scaling
it up.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

BUT, there are 2 things (at least) which contribute to the setting
of Va in the first place.

Neither one of the things you mentioned is given in Part 23 as a
requirement for Va. Part 23 uses the speed solely to provide the
design requirements of the elevators, ailerons, and rudder.

Since the forces on these control surfaces will not vary with weight,
you certainly can't scale it up.

In article , Greg Esres
wrote:

BUT, there are 2 things (at least) which contribute to the setting
of Va in the first place.

Neither one of the things you mentioned is given in Part 23 as a
requirement for Va. Part 23 uses the speed solely to provide the
design requirements of the elevators, ailerons, and rudder.

Since the forces on these control surfaces will not vary with weight,
you certainly can't scale it up.

G-forces are directly related to weight. Since the size of the control
surface is directly related to the forces exerted on it, control
surfaces are dependent on weight.

G-forces are directly related to weight. Since the size of the
control surface is directly related to the forces exerted on it,
control surfaces are dependent on weight.

Sorry, you lost me. The forces exerted on the control surfaces are
going to depend on airspeed and angle of deflection.

The size of control surfaces is irrelevant in this discussion, since
they are fixed at design time.

"Greg Esres" wrote in message
...
BUT, there are 2 things (at least) which contribute to the setting
of Va in the first place.

Neither one of the things you mentioned is given in Part 23 as a
requirement for Va. Part 23 uses the speed solely to provide the
design requirements of the elevators, ailerons, and rudder.


Well, right conclusion even if you don't agree with my method.

Va might be used in the design of the control surfaces, but I
was eluding to how Va is established in the first place.

And certainly one of the limitation is ensuring that you can't
exceed 3.5G at Va by yanking the yoke back. Isn't that
what Va is all about?

Since the forces on these control surfaces will not vary with weight,
you certainly can't scale it up.

Now you've lost me. If that were the case, Va would be
the same at any aircraft weight, which it certainly isn't.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

Isn't that what Va is all about?

Conventional wisdom says so, but there is no requirement in Part 23
that says this must be true. Part 23 only uses this speed in its
requirements for control surfaces.

In my view, the most correct definition of Va will be it's the speed
above which you cannot make full or abrupt control movements, due to
control surface integrity.

New airplanes are supposed to come with a new Vo speed, which DOES
require that the airplane stall before exceeding the load factor.

Here's a copy from a draft copy of an AC 23.something that I found.
The AC was intended to make this clear to test pilots, but I don't
think the draft was ever finished:

------------snip-----------------
VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane
structural limits nor should it be interpreted as a gust penetration
speed. Only if VA = Vs sqrt(n) , will the airplane stall in a nose-up
pitching maneuver at, or near, limit load factor. For maneuvers where
VAVS n , the pilot would have to check the maneuver; otherwise the
airplane would exceed the limit load factor.

Amendment 23-45 added the operating maneuvering speed, VO in §
23.1507. VO is established not greater than VS sqrt(n) , and is a
speed where the airplane will stall in a nose-up pitching maneuver
before exceeding the airplane structural limits.

------------snip-----------------


Va would be the same at any aircraft weight, which it certainly
isn't.

It is in some airplanes. My Piper arrow doesn't scale it with weight.

Moreover, Part 23 says that Va is *only* defined at max gross. Some
manufacturers do publish Va's at lower weight, but that appears to be
at their option. As written, it doesn't match Part 23 definition.

"Greg Esres" wrote in message
...

In my view, the most correct definition of Va will be it's the speed
above which you cannot make full or abrupt control movements, due to
control surface integrity.

This is way interesting & I've got the FAR's in front of me
now to get to the bottom of this.

First, I can't find a specific definition of "Design maneuvering speed" in
the FAR's, but my personal working definition is almost like yours.
I'd substitute "without risk of structural failure" for talk of control
surface integrity. Since control surface failure is indeed structural
failure, my definition would seem more restrictive than yours.

It looks like Va is mentioned twice in pt 23.

In 23.335 we get Va must be = Vs sqrt(n), with n the load
factor. We also get "Va need not exceed Vc" which
makes no sense to me, at least as far as a regulation goes.

Then, in 23.423 we see Va used in establishing the characteristics
of the (horizontal) control surfaces. Note that this doesn't
say this is how you calculate Va, it says you must use this speed
in the design of control surfaces to achieve certain rates of
response when they are used and/or to make sure you don't
break anything..............I suppose that manufacturers
do such a poor job of designing control surfaces that
they have to restrict Va just to meet this certification
requirement.....Well, bugger me Greg, looks like you're right!


New airplanes are supposed to come with a new Vo speed, which DOES
require that the airplane stall before exceeding the load factor.


Since control surfaces seem to be the limiting factor, I'd assume
that manufactures would design them for as low a Va as possible,
consistent with 23.335. So they'd choose Va = Vs.sqrt(n).

Vo does differs a little from pt 23 certification requirements, in that
Va isn't exactly Vo, because Va calculations assume that airfoil
lift does scale linearly with AOA and as the square of airspeed
when in fact these are only approximately true.

I'd bet that Vo and Va are pretty close. Allowing for the 1.5 safety
factor, I bet they're indistinguishable.

Here's a copy from a draft copy of an AC 23.something that I found.
The AC was intended to make this clear to test pilots, but I don't
think the draft was ever finished:

------------snip-----------------
VA should not be interpreted as a speed that would permit the pilot
unrestricted flight-control movement without exceeding airplane
structural limits nor should it be interpreted as a gust penetration
speed. Only if VA = Vs sqrt(n) , will the airplane stall in a nose-up
pitching maneuver at, or near, limit load factor. For maneuvers where
VAVS n , the pilot would have to check the maneuver; otherwise the
airplane would exceed the limit load factor.

Isn't this just a warning that Va "may not be less than Vs.sqrt(n)", and
so could be higher?

Va would be the same at any aircraft weight, which it certainly
isn't.

It is in some airplanes. My Piper arrow doesn't scale it with weight.

Moreover, Part 23 says that Va is *only* defined at max gross. Some
manufacturers do publish Va's at lower weight, but that appears to be
at their option. As written, it doesn't match Part 23 definition.

I don't see that in pt 23. I see it being defined as 'may not
be less than' some expression involving gross weight parameters,
but there is nothing to say that this applies only to gross
weight (to be pedantic). Nor does 23.423 - which we both
agree partially defines Va - say anything about the weight
of the plane during the certification maneuver.

I'd remind you how we got here. The suggestion was that
Va, should be scaled upward in an overloaded airplane. We
both claim that it should not. I'd also scale my maneuvering
speed downwards if underweight just to stay within load
factor limits, and I bet you would too. To my mind, the laws
of physics trump the FAR's. (and my Va is indeed pretty close to
Vs.sqrt(3.5)). After all, pt 23 just tells me how to certify a
plane, not how to fly it.

I'd claim that Va shouldn't be increased because it is really
the minimum of a number of different speeds where things
start to fall apart, and without further data we don't know
which one does the limiting.

Interesting discussion.


--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

"Greg Esres" wrote in message
...
Since the forces on these control surfaces will not vary with weight,
you certainly can't scale it up.

Huh? You have to scale Va with weight. Even within legal configurations, a
specific Va is valid only at a specific weight, with lower weights resulting
in lower Va and higher weights resulting in higher Va.

Just because you went outside the design/certification envelope, that
doesn't change the nature of Va.

Pete

a specific Va is valid only at a specific weight, with

Show me a Part 23 requirement that says so.

Todd Pattist has lectured on this a couple of times, and he's right.