Manuevering Speed

mhcasey

mhcasey

Well-Known Member
Alright, I confess that I've never learned why Va is lower in lighter airplanes than in heavier. I recently found the explanation that a lighter airplane will accelerate faster than a heavier aircraft with abrubt control input, thus pulling higher g's, and since aircraft are certified based on G loads, the lighter aircraft will need to slow down to ensure it will not be accelerated too quickly, thus pulling too many g's.

But, the lower weight of the aircraft allows it to pull more g's without overloading ths wings, right? I'm fairly certain that many planes can be considered regular or utility, based upon weight.

So what's the deal?

Thanks,

Mike
 
mhcasey said:
Alright, I confess that I've never learned why Va is lower in lighter airplanes than in heavier. I recently found the explanation that a lighter airplane will accelerate faster than a heavier aircraft with abrubt control input, thus pulling higher g's, and since aircraft are certified based on G loads, the lighter aircraft will need to slow down to ensure it will not be accelerated too quickly, thus pulling too many g's.

But, the lower weight of the aircraft allows it to pull more g's without overloading ths wings, right? I'm fairly certain that many planes can be considered regular or utility, based upon weight.

So what's the deal?

Thanks,

Mike
Click to expand...


Mike:

The issue is complicated, because there are really two kinds of maneuvering speeds. The Va defined by the FAA is called Design Maneuvering Speed. The intent of the speed is solely to protect the control surfaces from departing the aircraft if you deflect them at or below a certain speed. Here is an excerpt from Part 23, regarding the elevators:
Each horizontal surface and its supporting structure, and the main wing of a canard or tandem wing configuration, if that surface has pitch control, must be designed for the maneuvering loads imposed by the following conditions:
(a) A sudden movement of the pitching control, at the speed VA, to the maximum aft movement, and the maximum forward movement, as limited by the control stops, or pilot effort, whichever is critical.
(b) A sudden aft movement of the pitching control at speeds above VA, followed by a forward movement of the pitching control resulting in the following combinations of normal and angular acceleration:

etc.

​
There is similar language for the ailerons and the rudder. The loads imposed on these surfaces depends solely on calibrated airspeed. If these surfaces will stay attached at 95 knots max gross, then they will stay attached at any weight.

There is another type of maneuvering speed that you will find in aerodynamics books. This speed has nothing to do with control surfaces, but is solely a speed above which it is impossible to exceed the load factor limit of the airplane. This speed is defined as

Maneuvering speed = sqrt(n) * Vs

Where n is the load factor limit (such as 3.8) and Vs is the stall speed. The reason that the load factor limit cannot be exceeded is that you will stall the airplane before you reach it.

Many manufacturers have made Va and this aerodynamic maneuvering speed the same, but the FAA doesn't *require* it. In fact, they say that sqrt(n)*Vs is a lower limit, not a max limit. Here's what Part 23 says:
(1) VA may not be less than VS * sqrt(n) where -
(i) VS is a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, C(na); and
(ii) n is the limit maneuvering load factor used in design

​
So Va can be set well above the speed that would protect the aircraft from stalling before it reached its load limit factor.

The FAA has defined another speed, Vo, which matches the aerodynamic maneuvering speed. Here's how it's defined:
§ 23.1507 Operating maneuvering speed.
​
The maximum operating maneuvering speed, VO, must be established as an operating limitation. VO is a selected speed that is not greater than VS times the square root of n established in § 23.335(c).

​
See the difference in definitions?

Now, your question also had to do with can a lighter airplane withstand a higher load factor limit when it's light. If the wings were the only consideration, then the answer is yes, because the wings only see load, not load factor. However, there are many fixed weight items in the airplane, such as the engine, that may only be built to withstand some load factor times the weight of the component, and this limit will not change with overall changes in weight.
 
By the way, for the skeptics, here's an excerpt from AC23-19, AIRFRAME GUIDE FOR CERTIFICATION OF PART 23 AIRPLANES. This AC never made it out of draft mode, so you probably will have a hard time finding a copy of it:

(c) The design maneuvering speed is a value chosen by the applicant. It may not be less than Vs sqrt(n) and need not be greater than Vc, but could be greater if the applicant chose the higher value. The loads resulting from full control surface deflections at VA are used to design the empennage and ailerons in 14 CFR part 23, §§ 23.423, 23.441, and 23.455.
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 VA>VS sqrt(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.
 
Take a look at some descriptions/explanations of the VG diagram also, that will help.
 
Or a really simple and quick explanation.

Take a bathtub fill it with water. Place a Cork at one end (floating) and a piece of lumber (floating) at the other end. Stick your finger in the water and move it back and forth to make waves. Wich of the two objects move the most? The cork, right?

There you go.

The heavier an aircraft is, the more load factor (i.e. turbulence) - in this case represented by the waves - it can withstand before exceeding it's designed limit. Va is also called "penetration" speed or the speed at which you slow to when you enter turbulence.

Like I said .. a very quick explanation.
 
pilot602 said:
The heavier an aircraft is, the more load factor (i.e. turbulence) - in this case represented by the waves - it can withstand before exceeding it's designed limit.
Click to expand...

I don't feel that explanations using analogies such as this fairly communicate the nature of maneuvering speed. The implication is that heavier aircraft in a particular level of turbulence have an absolute greater protection from exceeding the load factor limit. This is not true. It depends on how fast it's travelling, which is not true of your cork or lumber.

The effect of the greater weight rests on the resultant AOA of the main wing.

If you have an aircaft of, say, 2000 lbs, it might be cruising at a 3 degree AOA. This aircraft may stall at 18 degrees AOA. If it encounters an severe updraft, then this aircraft might pull 18/3 = 6 g's before the wing stalls. This exeeds the ultimate load factor on the airplane; you may have structural failure.

If this same airplane were twice as heavy at the same airspeed, it would be flying at a 6 degree angle of attack. If it encounters a severe updraft, it can generate, at most, a 18/6=3g load factor before it stalls, which it can handle easily.

So the better explanation as to how weight affects maneuvering speed is it limits how much excess lift the wing can generate.
 
tgrayson said:
I don't feel that explanations using analogies such as this fairly communicate the nature of maneuvering speed. The implication is that heavier aircraft in a particular level of turbulence have an absolute greater protection from exceeding the load factor limit. This is not true. It depends on how fast it's travelling, which is not true of your cork or lumber.

The effect of the greater weight rests on the resultant AOA of the main wing.

If you have an aircaft of, say, 2000 lbs, it might be cruising at a 3 degree AOA. This aircraft may stall at 18 degrees AOA. If it encounters an severe updraft, then this aircraft might pull 18/3 = 6 g's before the wing stalls. This exeeds the ultimate load factor on the airplane; you may have structural failure.

If this same airplane were twice as heavy at the same airspeed, it would be flying at a 6 degree angle of attack. If it encounters a severe updraft, it can generate, at most, a 18/6=3g load factor before it stalls, which it can handle easily.

So the better explanation as to how weight affects maneuvering speed is it limits how much excess lift the wing can generate.
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:yeahthat:

I didn't even bother reading your first response. It was way over my head, but this agrees with what I learned. A heavier plane does not have enough room between the AoA and critical AoA and will stall. A lighter airplane at a lower AoA has a long way to go before reaching critical AoA and that leaves more room to break the plane.
 
tgrayson said:
I don't feel that explanations using analogies such as this fairly communicate the nature of maneuvering speed. The implication is that heavier aircraft in a particular level of turbulence have an absolute greater protection from exceeding the load factor limit. This is not true. It depends on how fast it's travelling, which is not true of your cork or lumber.

The effect of the greater weight rests on the resultant AOA of the main wing.

If you have an aircaft of, say, 2000 lbs, it might be cruising at a 3 degree AOA. This aircraft may stall at 18 degrees AOA. If it encounters an severe updraft, then this aircraft might pull 18/3 = 6 g's before the wing stalls. This exeeds the ultimate load factor on the airplane; you may have structural failure.

If this same airplane were twice as heavy at the same airspeed, it would be flying at a 6 degree angle of attack. If it encounters a severe updraft, it can generate, at most, a 18/6=3g load factor before it stalls, which it can handle easily.

So the better explanation as to how weight affects maneuvering speed is it limits how much excess lift the wing can generate.
Click to expand...

You try going into what you just wrote with a primary student and see how much luck you have. Hell go into that with someone who has been flying a little bit and see how much luck you have. Give 'em the analogy so they have something to picture and then you can build on it.

Instructing isn't about proving how intelligent you are to the student. It's about getting sometimes tough, always foreign, concepts into the students' brains at a level they understand and can build on.

Like I said .. a very quick explanation.
Click to expand...
 
pilot602 said:
You try going into what you just wrote with a primary student and see how much luck you have...Hell go into that with someone who has been flying a little bit and see how much luck you have.
Click to expand...

I've done both many times over 8 years of instructing. It works, they understand, and it's accurate. You might give it a try.

pilot602 said:
Give 'em the analogy so they have something to picture and then you can build on it.
Click to expand...

Analogies are sometimes useful, but in the end, they're lies. These lies are sometimes impediments to true understanding.

pilot602 said:
It's about getting sometimes tough, always foreign, concepts into the students' brains at a level they understand and can build on.
Click to expand...

Students can understand many things if the instructor is capable of explaining them. Usually the dumbed down explanations that instructors give students are based on the instructor's limitations, rather than the student's.

The explanation I gave takes maybe 5 minutes and the usual response is "Hey, that's easy!"
 
flyguy said:
:yeahthat:

I didn't even bother reading your first response. It was way over my head, but this agrees with what I learned. A heavier plane does not have enough room between the AoA and critical AoA and will stall. A lighter airplane at a lower AoA has a long way to go before reaching critical AoA and that leaves more room to break the plane.
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I like the AoA explanation as well as a fair compromise between the "over the head/lost interest" explanation and the "too simplistic and inaccurate one." My own version of the AoA explanation is (FAQ-in-training):


==============================
This explanation was derived from a number of online and offline sources.

Let's go back to the definition of maneuvering speed. Euphemistically, it's the speed at which an airplane will stall before it breaks due to a gust or abrupt control movement.

Putting it in slightly other terms, it's the speed at which the wings can suddenly go from their existing angle of attack to their critical angle of attack without increasing the load factor (G-force) beyond the aircraft's design. For normal category aircraft, that design maximum is 3.8 G.

Let's fill this out with some numbers. We are flying an airplane that stalls at 15º AoA. At it's normal 120 KT cruise, it's AoA 3º.

What happens if we suddenly change the AoA from 3º to 15º? Because there is (roughly) a one-to-one relationship between increase in AoA and increase in load, we have just increased the ~1-G cruise load on the wings by a factor of 5 G. Too bad we suffered structural damage at 3.8!!

What we're really trying to do to protect ourselves is increase our AoA so that the gap between our AoA and the critical AoA is smaller. How do we do that? We slow down. When we slow down while maintaining level flight, we reduce power and increase pitch, which increases our AoA. So, let's say that flying our hypothetical airplane level at a 90 KTS takes a 5º AoA. Even that small change means that suddenly bridging the AoA gap only involves a 3-G increase, below the 3.8 G damage point.

Why the slower speed for lower weight? Well, in general, a lighter airplane can maintain level flight at a particular airspeed with a lower angle of attack. So the cruise to critical AoA gap is larger at lighter weights. So we need to slow down more to get our cruise AoA where we need it to be to keep the gap manageable.
==============================
 
pilot602 said:
You try going into what you just wrote with a primary student and see how much luck you have. Hell go into that with someone who has been flying a little bit and see how much luck you have. Give 'em the analogy so they have something to picture and then you can build on it.

Instructing isn't about proving how intelligent you are to the student. It's about getting sometimes tough, always foreign, concepts into the students' brains at a level they understand and can build on.
Click to expand...

Gotta go with TGrayson on this. In fact, to put it in a bit harsher way (yes, I know it might sound mean), but the truth is that, for the most part, the student is only limited by the limitations of the instructor. That's the way I always approached it when I was teaching, and I never had any trouble with students understanding, what seem to be, difficult concepts. Yes, I do agree, it's a lot more work for the instructor!
 
Midlife, there's a high likelihood of me copy-pasting that into a lesson plan when I start my CFI stuff in a few months.

I'm fairly certain I can find this in the POH, but I'm stuck in Dallas right now (I live/fly in Austin) and am afraid I'll forget to look it up later. So, is the published Va, aka, the one in the POH and on checklists, based on max gross, or lighter? Do larger aircraft have several published Va's?

Thanks again,

Mike
 
mhcasey said:
So, is the published Va, aka, the one in the POH and on checklists, based on max gross, or lighter? Do larger aircraft have several published Va's?
Click to expand...

If only one Va is published, it's at max gross, because that's how the FAA defines it.

My King Air Manual only publishes one Va; it doesn't scale it by weight because the FAA definition doesn't require or expect it to.
 
mhcasey said:
Midlife, there's a high likelihood of me copy-pasting that into a lesson plan when I start my CFI stuff in a few months.
Click to expand...
I'd be honored.

I'm fairly certain I can find this in the POH, but I'm stuck in Dallas right now (I live/fly in Austin) and am afraid I'll forget to look it up later. So, is the published Va, aka, the one in the POH and on checklists, based on max gross, or lighter? Do larger aircraft have several published Va's?
Click to expand...
They don't have to be that heavy to have several. Take a look at a POH for a 172S.

For just about all of the V-speeds given in the simple POH (at least the ones based on load - what about speeds like Vlo and Vfe?), if only one speed is given it's the speed at max gross weight.
 
pilot602 said:
try going into what you just wrote with a primary student and see how much luck you have.
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This is "technical talk".

Each student is different. Some brand new students can get their heads around this kind of stuff, and others cannot - or will not.

There is no "one way". In any case the general plan should be "from simple to complex." Take the student as far as he/she can go, but do not overwhelm.
 
Instructing isn't about proving how intelligent you are to the student. It's about getting sometimes tough, always foreign, concepts into the students' brains at a level they understand and can build on.
Click to expand...

I could wax poetic about how great an instructor I am, or how intelligent I am or any other topic but the one at hand. The point is, attack me all you want, but the above quote is all that matters at the end of the day. There are millions of ways to achieve that end. :)
 
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