Maneuvering speed vs. weight

I too have a similar background, and view on things.

OK, the part you are missing here, as I've been taught, is that :

as the wing increases AOA, other factors come into play.
Things like induced drag... which tends to diminish lift,
which is the major factor in a stall, and airspeed, which
at stall < AOA = 18 deg > is significantly lower due to induced drag
you can maintain controlled flight at a speed slightly faster than stall speed.

The greater the AOA, the greater the drag and the less the lift ( or load factor)
Therefore the load factor decreases with Increased AOA, and lowered airspeed,
resulting in significantly less G force on the wing.

Also factor in, the design safety ratio of 1.5 times the load specs.
In your example a rated load limit of 10,000 psi @ Va, is actually 15,000 psi.

And "Maneuvering Speed", is the maximum speed at which we can use abrupt control changes,
to limit further stresses on the wing, not the destruction point of the wing spar

Does that help you any?
(And I hope I have it right) too....

Charlie

 

The reason maneuvering speed is a function of weight is because stall speed is function of weight.

Here's me in a given airplane flying 5 knots above stall speed. I honk back on the stick and try to load up some G's. What happens? I stall before I can manage 1.1 G's - right?

Fly a little faster - now I can manage 2.0 G's before stalling.

When I finally get fast enough to manage 4.4 G's before stalling - I've hit maneuvering speed (in your example). As long as I don't go any faster that that, I can't overstress the wings. If I try - I'll stall first! (Exceed critical AOA)

If I increase the aircraft's gross weight I increase stall speed, and accordingly the speed at which I can induce 4.4 G'r from the airplane before stalling in a self-induced or externally induced (turbulence) maneuver.

Great huh? - how about the snap roll that occurs instead at 4.0 G's - that's why you need a knowledgeable instructor.

 

If you can convince me that the wing snaps at a different load based on any variables at all, please do so.




Does it make any difference to you if the weight is in the fuselage in the form of excess cargo, or in the the fuel cells in the wing in the form of excess fuel?

An updraft near a thunderstorm can produce an almost instantaneous increase in airspeed, especially if counteracted by an abrupt forward pitch to attempt to maintain altitude by a neophyte aviator. When the downdraft follows, it doesn't matter whether maneuvering speed is 172 or 175 when the airspeed is 230.

What is important is that you understand the principle if you are going to be a competent pilot. Good Luck.

 

AOPA magazine has a great article about this very subject this month. Required reading.

Art

 

Try this one:

http://www.aopa.org/members/ftmag/article.cfm?article=829

 

Here , I have to disagree with your statement.
Standing still on the ground, (acceleration "0 mph") The "Force" of gravity holds you from flying into space.
Gravity is the force of attraction, between two bodies in space, moving or not.

The imposed load, is what stresses the aircraft.

But.... how do you expect to achieve this "imposed load"... without the acceleration "factor"?


Maybe the issue here, is one of definitions, as opposed to loads or factors.

Charlie

 

Part of the problem with understanding what seem contradictory in maneuvering speed is that of following the established definitions involved. By definition, our light aircraft categories are based on G force limits. Normal, 3.8 G, Utility, 4.4 G, etc. Next, maneuvering speed is defined as "the maximum speed at which the airplane can be safely stalled". Working with those definitions, reading Rod Machado's article which I referenced above makes it plain to me.

The ratio of the pitch change required to stall the aircraft compared to the initial angle of attack indicates (for all practical purposes) the G load imposed. The heavy plane is flying at a high angle of attack to maintain a given speed so the ratio to the stall angle is small. The light plane is flying at a lesser angle of attack at the same speed so the ratio compared to the stall angle is much higher, hence imposes higher Gs. I will admit, as an engineer, I was curious about this as well. As long as I remember to work with the proper definitions, I have no problem.

I am really glad the question was asked since it made me get off my duff and do some research instead of just trying to work it out in my head. Now about that magnetic compass......

Skyraider: A small but significant definition problem. Gravity is exactly as you define it, but G is defined as the accelleration of (due to) gravity, and is defined as an accelleration of 32 feet per second per second (in and around Earth). It gives you "weight" even when standing on the surface (don't ask me to explain that, my physics is 50 years old). 0 mph is a speed, not an accelleration. Add a direction to that and you have a velocity. Like Vinne Bobarino, this stuff gives me a headache sometimes. (I wonder how he ever got to be a pilot!)

 

Thanks ABCandJRC ! I stand corrected.......... I think...

But....Yo, Mr. Kotter! ....

I used "0 mph" to indicate no speed.
If there is no speed, there can be no acceleration.
yet, I still have weight... and feel 1 "G" on my rump when sitting.
This is contrary to your above explanation

Here's a question you may be able to straighten out for me...
or at least forsooth, ponder upon, therewith.


If I am falling @ 32'/sec/sec (say for six seconds) {32, 64, 96, 128, 160, 192,}
that's 192ft/sec. at the sixth second.

Am I then and there, (by your above definition), experiencing
1)... 6 G's??
or is it only when there is

2)... an opposite force applied. (ie a change in direction..)

If the answer is 1)...
Then how can we recover from a fall (ie. a Hi Alt Low Opening [HALO] jump)
where terminal velocity (120 mph) is reached, without being unconsious,
or at least blacked out?

I tend to think (perhaps wrongly), that a "G", is defined as
"the unit of measurement of the change in the direction of a force, over time ".

We can withstand a 180 degree change in direction, in two minutes
(approx. 1 "G")
But not the same directional change in a millisecond ( approx. infinite "G's")
We'd end up squashed flat, wearing our boots, for a hat!

Yes, my Physics is also half a century (or more) old....

Very interesting conversation though.....


Vinne "Where's the aspirin" Barbarino

 

"I used "0 mph" to indicate no speed. If there is no speed, there can be no acceleration."

Actually, I think Newton gets in the way of that somehow but am not sure how to explain it.

"If I am falling @ 32'/sec/sec (say for six seconds) {32, 64, 96, 128, 160, 192,} that's 192ft/sec. at the sixth second.
Am I then and there, (by your above definition), experiencing 6 G's??"

No, it is still 1 G, you are accelerating at the Earth's acceleration of graviy. What you are describing is speed (actually a velocity but with an unknown direction except "down"). You are now going 32 fps, 64 fps, etc, ....speeds. You are still accelerating at 32 feet / second / second. You added that amount to get each of your numbers.

You are exercising portions of my gray matter that has either fallen asleep or is just plain died. I hope I am not misdirecting you.

In your example of changing direction in various times, you are actually describing acceleraton, but not the acceleration of gravity. I think a working definition of acceleration is the rate of change in velocity. That includes a change in the direction portion of velocity, so even if you maintained the identical "speed", a change in direction is defined as acceleration. It explains, though, the original problem about maneuvering speed. It is those accelerations (measured in units used to define the acceleration of gravity, Gs) that tear the wings off the Normal Category aircraft experiencing 5 Gs in turbulence.

My favorite physics problem from the old Sears and Zymanski (?sp?) that used to be the "standard" physics text was roughly this.

A physics student, determined to test the accelleration of gravity for himself, steps off a 10 story building, stopwatch in hand. As he steps off, Superman arrives on the scene. Assuming that Superman's accelleration is the accelleration of gravity, what must his initial velocity be in order to rescue the student before he hits the ground? How many seconds can elapse before even Superman can't save him?

Don't you love "word problems". This one works acceleration and velocity concepts to come to an answer. I used to be able to work that one. I know somebody will post the answers.

Here is an excerpt from http://www.krysstal.com/gravity.html

Lots of neat headaches there. The formula did not come through, so I will add (those) myself.

Newton's Second Law can be written:

A force acting on a body causes it to accellerate in the direction of the force.

The accelleration that a given force will impart on a body depends on the amount of matter that the body contains. This is called the body's mass. If a given force acts on two objects, the more massive object will be accellerated less; the less massive object will be accellerated more. This is common sense because it takes a stronger push to move a cannon ball that a football.
This is expressed by the famous equation:

(F=ma)

Where F is the force, m, the mass, and a the accelleration. This resistance to change in the state of motion is called intertia. All material bodies have inertia.

Newton was the first to distinguish between the mass of a body (something that does not vary) and its weight. Weight is a force acting on a mass. A person in space can be weightless but will always have the same mass. They are weightless because of the lack of a force acting on them. They continue to have mass because they are a material body with inertia.

On the earth the weight of a body (W) is its mass (m) multiplied by the accelleration of gravity, which has a value of 9.8 ms-2 and is given the symbol, g:

(W=mg)

The value of 9.8 ms-2 for the accelleration of gravity means that, as a body falls, every second it will be moving 9.8 m/s faster than during the previous second. This value was measured by Galileo.

 

I'm not trying to get into a "P" ing contest here, but....

velocity /vilossiti/ (pl. velocities)
noun 1 the speed of something in a given direction.
2 (in general use) speed.
ORIGIN Latin velocitas, from velox ‘swift’.


speed
noun 1 the rate at which someone or something moves or operates.
2 rapidity of movement or action.

accelerate
verb 1 When a vehicle or its driver accelerates, the speed of the vehicle increases:
2 If a person or object accelerates, it goes faster.

----------------------------------------------------------------------

A fair enough statement..

Sleepy Gray matter, granted.

If one gets into his car and starts off to go to achieve a predetermined speed of 55 mph
and in the first mile achieves 15 mph, the second mile 25 mph, and so forth.....
when he reaches 55 mph, I say he is no longer accelerating, you indicate he is. Yet you indicate, that he is only pulling 1 "G" while having accumulated six seconds of acceleration.

Yes, the only way to accumulate anything, is to add it together. When you've finished adding you have a sum.
Therefore 32'/sec /sec for six sec's = 192ft/sec...
no faster, or the numbers will continue to increase. (see above definitions)

My take here, is that once the "G" forces are achieved, and the mass is in equillibrium the apparent sensation of force is nullified. ( see Newton's first law, or ride an elevator.) You'll feel heavy for a moment untill equillibrium is reached.

But this, is a tangent to the original query.

We seem to equate "G's" with the pull of the earth (gravity) but as one changes direction of travel, the forces encountered also change. for instance:
Flying an F-16 heading 180, with the ability to do 90 deg banks @ over 600 mph,
1) the bank imparts no unusual "G" force,[i.e. flying knife edge] until the stick is pulled back, changing the direction of travel from 180, to say 270. The force of gravity hasn't increased, only the linear direction of travel has.
Yet the "G" force is enough to incapacitate a pilot......
same thing in a centrifuge.....

Yes, that does seem to explain the issue at hand... if only vaguely..

Once more, we seem to agree with another old timer...
And yes, I do so love a challenge.
Especially one that makes me think, outside the box...... Er ...envelope!

 

We are always accelerating at 1G as we sit or stand. Terror firma under our feet or butts keeps us from going further down to see the Devil. If you don't think that you are accelerating, step into a deep hole.

 

I think semantics is really getting in the way of things here.

1. As I stand/sit/lay on the ground, I am not accelerating. There is no net change in the direction or magnitude of movement - the reason is that the ground is providing a force exactly equal to, and opposite, gravity. I know, it's hard to wrap your brain around when you're used to thinking of "force" as something more active (i.e. throwing a ball). Nevertheless, the ground MUST be opposing the force of gravity or I would fall through it to the center of the earth. All forces are in equilibrium, and I am not accelerating.

2. A "G" is simply 9.8m/s/s (or 32ft/s/s). Nothing more, nothing less. It does not reference the source of the acceleration. The reason it is called a "G" is because it's something "standard" that we can all understand. It matters not if I am falling from a building, flooring it in the car, kicking in the afterburners, or yanking back on the stick in a 600kt 90deg banked turn in my F-16 (Gee I wish I really had one). Yes, gravitational acceleration is 1G. When I hop on the whirly centrifuge at the fair, the acceleration I FEEL is ~3 G's. It's not in the down direction, I'm accelerating OUT, but I perceive that I weigh 3x my actual weight.

3. "I tend to think (perhaps wrongly), that a "G", is defined as
"the unit of measurement of the change in the direction of a force, over time "."
This is indirectly true. Linking back to the simplest equation F=ma, we observe that ACCELERATION (a) is a change in velocity over time (either vector or magnitude). As FORCE (F) is a function of velocity, one can imply that there has been a change in force (in the recovering from a fall example, there is the addition of the force of the ground opposing gravity). If I fell from 18,000 ft, I would hit the ground with a given VELOCITY (terminal velocity takes into account many other factors including wind resistance, frontal surface area, etc). My deceleration (or negative acceleration) would then be the change in velocity (FAST to 0) over time (if I hit the ground, it would be almost instantaneous) thus implying a very large force opposing my downward movement. This would result in a very bad day for me.

4. "If one gets into his car and starts off to go to achieve a predetermined speed of 55 mph
and in the first mile achieves 15 mph, the second mile 25 mph, and so forth.....
when he reaches 55 mph, I say he is no longer accelerating, you indicate he is. Yet you indicate, that he is only pulling 1 "G" while having accumulated six seconds of acceleration."
In this example, there are several things going on. Just like a plane in flight experiences 4 opposing forces, the car does as well - it's just less evident since we don't really care about gravity as much in a car. Gravity is acting on the car at 1G. The road is resisting the gravity with a force equal to 1G. (analagous to weight/lift). The car is producing forward thrust while drag opposes it. If you accelerate in the forward direction faster than 32ft/s/s, then you'll feel more than 1G PUSHING YOU INTO THE BACK OF YOUR SEAT. There will still be a 1G force pulling you to the earth, but you don't notice it because of the opposing force balancing things out (thus no NET force and no downward acceleration).

5. "Quote:
Originally Posted by plasticpi
I understand it in terms of Gs perfectly. That's not where I'm getting lost. I'm getting lost because a G is a measure of acceleration, not of force.


Here , I have to disagree with your statement.
Standing still on the ground, (acceleration "0 mph") The "Force" of gravity holds you from flying into space.
Gravity is the force of attraction, between two bodies in space, moving or not."
Again, it's an argument of semantics. Gravity is, indeed, the FORCE causing us to accelerate toward the earth. A "G" is a measure of acceleration. It is the acceleration which results from the force of gravity and is a standard measure. If "G's" only referred to gravity and the acceleration it imposed, then the unit would be worthless because we would NEVER experience anything other than 1G unless we left earth. It is simply a unit of acceleration we can use to quantify acceleration experienced in other settings. G IS NOT EQUAL TO GRAVITY.

All the physics aside, Machado's article in Flight Training should pretty much clear everything up....