Maneuvering Speed and Weight 1

Can somebdy out there explain to me why is Maneuvering Speed G-limted and not Load-limited as a function of weight ?

Please see

http://www.ec.erau.edu/cce/faculty/sf335ch3_2.ppt

As far as I knew, a wing is, for this purpose, the limiting factor. A wing, as a beam, can only support a certain load before structural failure. Specifying this capability in Gs is just a very convenient way to do this.
It follows that a lightly loaded aircraft should withstand higher Gs than a heavily loaded aicraft, before breakup. The fact that highly respected engineers specify a lower Va for a lightly loaded aircraft baffles me. The explanation given in the textbook above is only true in my opinion, if a wing was designed to support a certain G load, regardless of the weight. In this last case, everything follows.
I am very confused. There must be another explanation

 

[rb1957]

i guess the answer to your question is "because that's the way it is defined" and then as you say "everything follows".

the presentation seems to be very clear is describing this.

Consider you have the same aerodynamic surface (wing) with two different gross weight airplanes. Clearly since the flight envelop is defined in terms of g the lighter weight plane will be loaded less, and possibly the structure could be lighter. And the heavier weight plane will require more speed to develop the higher lift (since CLstall is the same for both planes, the same wing is being used). it not counter intuitive at all ... lighter planes can fly slower (than heavier planes for the same g load).

i think the point to this definition is that you have to start somewhere. You could define the flight envelop by examining the structure and apply loads such that cause MS = 0. apart for the circular calculation path, this would open the door for unscrupious designers to design planes to very low loads (because they have very light (insufficient?) structures). this is actually the heart of designing airplane structures ... they are designed to a limited load, a load proven over time to afford an acceptable level of safety in service, but a load that can be exceeded in service (if things go pear-shaped).

any clearer ?

good luck with your studies

 

[Fabrico]

It is probably more along the lines of performance. A lightly loaded plane can and will do far more than a heavily loaded one.

Adding weight to a given plane is approximately the same as reducing wing and control surfaces for a given weight.

If a plane's weight is maxed out it simply will not perform high G maneuvers.

 

[be2]

I fly the Pa 28 from time to time. The maneuvering speed on this plane is lower for a lightly loaded condition. A lightly loaded Pa28 is a little livlier than with full load.It has to do both with the lower weight and the cg position.The cg on a Pa28 is usually further back when heavy loaded(as I recall it) and that should make the plane more unstable and quicker to respond. My personal view is however that the higher weight has more authority than the further back cg to make the plane slower in response. The flight handbook states allowable gs only in terms of full load. Naturally the wing can take more gs when plane is lighter but this is supposed to be a static condition, not a dynamic as when you jerk the controls hard. The speed (Va)at wich you CAN overload both the wing AND the controls simultaneousely is theoretically the same if you only consider a "static" load, but wing and controls loads aren´t really closely related.

 

[jsada]

Sorry to both of you, but your answers do not satisfy me yet.
You see, I am questioning wether a lighter than gross loaded airplane should be restrained at the same level of G limits as the fully loaded example.
Let´s imagine you are given the task to design the structure of the wing of an airplane that is projected to weigh at gross 15,000 lbs. Furthermore, let´s say, for simplicity, that you are asked to design the structure to support 6 Gs. You get to the task and come out with a wing structure good enough for 90,000 lbs load, right ? (90,000 divided by 15,000 = 6)
Now, let´s imagine that somebody is designing an acobatic airplane that is projected to weigh 10,000 lbs and sees your wing and asks you whether it will support 9 G´s. Your answer would be "yes, of course, because 90,000 lbs divided by 10,000 lbs happens to be exactly by coincidence, 9 times its gross weight.
What changed the G limits of the wing ? The answer is, of course, the lower weight of the aircraft. The G limits of any structure is dependant on the weight. The ultimate load factor stays as a constant ( 90,000 lbs in this example) , but the lower the weight, the more G´s a structure can support
Now, you see ?
Why should Va in the case of a lower-than-gross weight aircraft stay defined in G limits at the same value as the gross-weighed example ? It shouldn´t. It should be limited at the same load (90,000 lbs in our example). A structure does not "understand", or "feel" G´s it only "feels" a load expressed in lbs-force (or any other physical substitute).
If my theory is correct, a low loaded aicraft´s Va should be higher, not lower than the Va at gross weight.
I must be wrong, nevertheless, I am not a genius thatis goig to prove everybody wrong, so the explanation to keeping Va posted to the same G value should not be the one everybody gives.( By the way, my suspicion is that it is kept at the same value do to the fact that other components (engine mounts, tail, etc) have not changed weight, and that someone started giving a wrong explanation, mathematically correct, but wrng in the assumptions and that this explanation has caught on unfortunately without any questioning

 

[berkshire]

Jsada,
See if this helps.

http://www.flightlab.net/pdf/8_Maneuvering.pdf

B.E.

 

[rb1957]

you're right, of course, a lower weight airplane (with the same structure as a heavier weight airplane) can support a higher g load. BUT this isn't the point of the flight envelope.

the flight envelope exists to design airplanes to a consistent set of loads. I think the history of the flight envelope is that in the early days each designer was designing his (or her) plane their own way to their own cases and i suspect that this caused some "smoking holes in the ground".

In fact not all parts of the airplane are designed by the maximum weight, low weight cases can cause higher accelerations (which dominate over the low accelerations/high weight cases), other parts are designed by fatigue, heck "lighning strike" can constrain parts of the design. So I think your thought process is a little simplistic, an aerobatic plane is going to have many other design inputs that are different to a standard category plane.

btw, if you wanted a 15,000 lbs plane to support 6g would you need to design for 15,000*6*1.5 = 135,000 lbs (ultimate load).

 

[be2]

All of this has been more or less considered in the past. For Normal category airplanes (g allowable is about 3,8 and even if a lightly loaded normal category plane can take more gs it aint ordinarily supposed to be flown any harder. So, for simplicity allowable g´s are stated only for Max weight in the flight handbook.Its not often that normal category planes have g-meters installed so you´ll never know for sure how many g´s you do.

As I stated,in the case of the PA 28 the man speed is different for different weights. I think the determination of these speeds have been determined by experiment in the long time that these planes hve been produced. I don´t think someone actually calculated them with any great precision before the plane was test flown, since the actual stick forces and the general control caracteristics are at best educated guesses (or set by flight standards) at the design stage.

Some planes are certified in more than one category. The C152 for instance is "utility category" up to a certain weight and "normal category" above that weight. Utility category airplanes have a higher allowable g. Most planes I´ve flown have a fixed Va.

I don´t pretend to be an expert on this subject. My knowledge in stability and control is limited to static stability and steady maneuvering. I only want to learn more by participating in an interesting discussion and I´m willing to learn.

 

[Fabrico]

Re: "The G limits of any structure is dependant on the weight. The ultimate load factor stays as a constant ( 90,000 lbs in this example) , but the lower the weight, the more G´s a structure can support
Now, you see ?"

This might be where things fly south.

Load factor may stay constant but G limits imposed are not dependant on weight. Keep in mind that a plane flies in a gas not a liquid. A heavily loaded plane simply will not do sharp high banked turns, so little or no additional G load is created.

A lightweight stunt plane experiences very high wing loads because of what it is doing not what it weighs. If it weighed less it would turn sharper and still achieve the same G loads. If a plane were loaded to where it could barely fly, the only G force it could experience would be through it's landing gear. Its almost an even balance. In simple terms, payload is subtracted from attainable G load, not added to it.

 

[larryuav]

Jsada,

Your question is a good one and is not sufficiently answered in any textbook or FAR I've ever seen.

You are correct in that at a lower wt the wing can handle a higher g loading. However, this is my take on the subject:

1) Load factors on other aircraft components (engine mounts, avionics boxes, etc) are designed around the maneuver speed load factor (Nmax at a heavy weight). So although the wing can handle higher gs at lower weights, other components can not. This is why maneuvering speed limits you in terms of g loading, not total load (lbs). Only at max wt does it limit your total load.

Refer to FAR 23.371 and 23.423.

2) The definition of VA (maneuvering speed)is that it does not allow you to exceed a certain G-LEVEL. Yes, it so happens that at the max weight and above this g-level structural failure will result(just another reason not to exceed VA). But the bottom line is the definition is what it is: its a 'g-level' definition.

3) There is nothing on an airplane that will tell you what your maximum lift is (we're talking the good old days now - not the new technology stuff. Remember, these defintions and terms were created long ago). So there is no way that a pilot can know when the maximum load is exceeded. But, a simple accelerometer can be used for this purpose. And if the pilot never exceeds Nmax (regardless of weight), then he will never overload the aircraft. In fact at light weights, he will have healthy safety margins (this is a good thing).

 

[airplanenut]

Let’s say, Maneuvering Speed is roughly defined as the lowest speed at which the airplane can produce more then limit load (…abrupt and full control deflection….).
At any lower speed, the airplane will stall at a load factor below the limit factor.
Why is that?
Because G load depends on lift and lift depends on speed, so there will be a speed at which the airplane first is capable to produce limit G load. At any speed above, it will be able to produce more then limit load and is capable to hurt the structure. In short bad things start to happen….
Now how does the weight come in the equation?
Let’s look at a light airplane with a gross weight of 1300lb and a design limit G load of 4.
That means the wing and attachment is roughly designed for 4 times 1300lb or 5200lb total load (not counting tail down force, canopy reduction, inertia relief and other pla pla..) Maneuvering speed in this configuration is X mph.
Our pilot goes out and flies the airplane at 1000 lb (300lb) less and pulls 4 G (Wahoo - done it been there). He only applies roughly 4 times 1000lb to the poor wing and attach. That is about 1200 lb less (5200lb – 4000lb) total load then at gross weight.
That means he will be able to pull the 4 g at a speed less then X mph because he needs less lift to support 4000 versus 5200 lb- right.
Does that mean, he can pull 5.2 g safely (5200lb safe load on the wing divided by 1000 lb flying weight on this beautiful day?
No! Because there might be parts, components and pay load in the airplane, which have no idea how heavy the airplane is on this flight and are designed to a limit load factor based on the weight they support at this load. For example an engine mount – now the engine weighs always the same no matter how heavy the airplane is – right.
Also the loads our G pulling pilot imposes on the seat are based on the weight of the pilot and not the airplane. And so on…….
This means he will be able to overstress certain components even if he might not do any damage to the wing.
This is why the maneuvering speed is based also on weight.

 

[airplanenut]

Oh and I forgot. Yes Maneuvering speed based on weight is calculated.
In case of a light airplane it is reasonable accurate to say Va=sqrt(2 x n x W / cl max x S)which would be if in mph about Va= 19.77 x sqrt (n x W/S x cl max)and that is based on the general lift formula which is L = RHO/2 x v^2 x cl max x S. Total lift is assumed to be equal to weight and therefore Lift required to produce a g load n would be n x w.
(RHO is airdensity, v is speed, S is wing area, cl max is maximum lift coefficient)

 

[tmoritz]

I liked the way Rod Machado explained it. AOPA reprinted his explanation in their magazine.

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

Regards,


Tom Moritz
Mechanical Engineer
US Bureau of Reclamation

 

[aerodog]

I am not sure if this answers the original question but the analytical work for airplanes looks at several V-n diagrams, representing all the extreme conditions, gross weight, minimum flight weight, max payload/min fuel, max fuel/min payload.

What the pilot sees in the FAA approved Flight Manual ia a compilation of critical conditions condensed to the point if the pilot stays within the published data, safe flight can be assured.