Va: maneuvering speed ad nauseam

(Koopas Ly) wrote:

What is the critical "item" that will break at 3.8 g's on a light GA
airplane like a C172? Is it the engine mount? I see that's it's
often mentioned as the "weakest link".

It's my opinion that the definition of Va taught to private pilots
leads to a false understanding that the wings will break at 3.8 g's.
The nuance we've discussed is not stressed. Why? I don't know.

I don't know. The engine mount is just one to pick as an example - but
as others have said it isn't the most likely. It could be almost
anything - on aircraft with tip tanks it could be the attachments for
those for example. All up I don't think it matters - these are just
possible reasons why Va doesn't get higher as weight decreases.

You may be right, but the aerodynamics taught to private pilots is
rather simplified in general. I did find out about it in my training,
but it was probably a result of asking the same question you are
asking - why does it decrease not increase.

Physics is something some people find very difficult, so requiring
that everybody understands why Va decreases with decreasing weight may
be too difficult.

The engine mount on most light aircraft is designed to withstand 9
G's minimum.


Where is that specified? Does Cessna provide max. g limits for items
on the aircraft or are max. g limits for these items explicitly
specified in FAR 23?


And as I said earlier, the 3.8 figure is based on gross
weight. Reducing gross would allow them to take a higher G figure but
the same net force.


Agreed. This may sound like a stupid question, but how do you define
"gross weight"?


Cessna also states in the 172 POH that it's designed to 150% of
the G figures given, or 5.7 G's. I think the 3.8 figure would be the
yield point, where things begin to bend, and the 150% figure would
break them entirely. Or something like that.


You're correct. An airplane has to sustain ultimate loads, typically
50% beyond limit load (highest in-service load to be experienced) and
show no failure (fracture). An airplane has to sustain limit loads
without permanent deformation (yielding). At least, that's for FAR 25
aircraft.



Dan

"Koopas Ly" wrote in message
m...

I am sure most know the typical textbook definition of Va...goes
like..."the minimum speed at which the wing can produce lift equal to
the design load limit" or "the speed at which the pilot can use full
control deflections without over-stressing the airplane".

[Not answering Alex's questions but...]

maybe that's better as "*one* full control deflection".

http://www.dft.gov.uk/stellent/group...ty_025533.hcsp

is not unlike

http://www.ntsb.gov/events/2001/AA587/default.htm

though closer to home for those of us who fly light aircraft.

Julian Scarfe

(Koopas Ly) wrote

Agreed. This may sound like a stupid question, but how do you
define "gross weight"?

Ah yes...a grossly misused term and.....one not used in the FAR.

Section 23.25: Weight limits.
(a) Maximum weight. The maximum weight is the highest weight at
which compliance with each applicable requirement of this part
(other than those complied with at the design landing weight) is
shown. The maximum weight must be established so that it is --

However a web search shows that "gross weight" is used mainly by
state motor vehicle codes as follows:

Gross Vehicle Weight or GVW
The combined weight of a commercial vehicle and its load.

And the military:

Definition of: gross weight
(DOD, NATO) 1. Weight of a vehicle, fully equipped and serviced for
operation, including the weight of the fuel, lubricants, coolant,
vehicle tools and spares, crew, personal equipment, and load. 2.
Weight of a container or pallet including freight and binding. See
also net weight.


Using "gross weight" for aircraft can be misleading....for example:

The Boeing-707 had a maximum "taxi weight" of 336,000 lbs, and a
maximum "takeoff weight" of 333,100 lbs. And....its maximum
takeoff weight might be limited to the maximum landing weight plus
the weight of the fuel burned during the flight or to the maximum
weight that would allow compliance with the second segment climb
requirements.

What was its "gross weight"?

Pilots who use the term "gross weight" aren't very well versed in
the FARs.

Bob Moore

Greg Esres wrote in message . ..
The engine mount on most light aircraft is designed to withstand 9
G's minimum.

Where do you get this figure?


Canadian Aviation Regulations 523.561 gives some numbers for
protection of occupants in an emergency landing (a crash). Seats and
belts, for example, have to withstand 9 Gs forward, 6 Gs downward, 3.0
upward for Normal and 4.5 for Aerobatic. Items of mass within the
cabin must be able to withstand 18 Gs forward.
The structure itself must be able to withstand, in the event of
"complete turnover" during a crash, 9.0 Gs forward, and some lesser
numbers in other directions. This structure would include engine
mounts.
Somewhere else in the many sections are more numbers specifying
componemt strengths. The FARs would have an equivalent section. The
minimum legal flight load strengths for any Normal Category airplane
is 3.8 Gs. Note that flight loads and "emergency landing" loads are
not the same, but the airplane must meet those requirements.
A 3.8 G engine mount would be most dangerous. In training students
in our Citabria, we have seen well over 3 Gs on the G meter in botched
landings. The wings and tail don't much care about that impact, since
they don't experience any air-load increases, but the gear, fuselage
and engine mount do. Imagine, for a minute, the effect of an engine
departing the airframe in such a spot: airspeed at or near stall
speed, an airplane that's suddenly 350 pounds lighter, and a CG back
around the trailing edge of the wing. People killed, maybe, for the
lack of one pound (or less) of 4130 tubing?


Dan