Why does maneuvering speed vary

[flyifrvfr]

I know this question has been posed before But I'm going for my CFI checkride this monday and I know this question will be asked.Why does maneuvering speed change with gross weight. For instance a cessna 172 that weighs 1900 pounds has a maneuvering speed of 95 knots. While the same plane loaded to 2500 pounds has a VA speed of 105 knots. I know the Va speed decreases with weight and increases with weight but the question is why?

 

[BoDEAN]

You have a 250 lb linebacker.
If he hits a guy who is 200 lbs, it will not displace the 200 lb guy much.

But if you have that same 250 lb, and a 150 lb guy gets hit by him, he will be displaced a lot more. Same concept with the plane.

 

[flyifrvfr]

I did a keyword search and came across the same question which AVBUG answered. An Airplane flying at gross weight tends to fly at a higher angle of attack. More tail downforce is needed at higher gross weights which means at higher angle of attacks stall speed raises. Therefore a heavier plane will stall before damage can occur. Thanks in advance for anyone and everyone who answers the question, I should have searched before posting.

 

[Singlecoil]

Don't even start with that "tail down force increases at high angles of attack..." stuff with the inspector. You are leaving yourself an opening. I would keep it simple. V speeds vary according to the formula: gross weight v speed times (square root of(present weight divided by gross weight))
That formula doesn't work for Vne, but it does for stall speeds, climb speeds, approach speeds and Va.

 

[Timebuilder]

For once, Singlecoil and I agree. Don't go there.

On my checkride, I explained that a heavier airplane is more resistant to acceleration forces because of its greater inertia, and can be exposed to a higher airspeed without overstressing or permanently deforming the airframe because of this increased inertia.

He went right on to the next question.

 

[Singlecoil]

For once, Singlecoil and I agree. Don't go there.

...and that deserves a toast....<clink>

 

[midlifeflyer]

For once, Singlecoil and I agree. Don't go there.

Both the inertia and angle of attack explanations have merit.

"Increased tail force" issues aside, we do slow down to maneuvering speed because flying slower in a given airplane requires a higher AoA.

Decreasing the "AoA Gap" between existing AoA and critical AoA lets the airplane bridge the Gap without exceeding the maximum load factor.

Since a given airplane flies at a given airspeed at a lower AoA when it weighs less, the Gap is greater to begin with and the airplane needs to slow down more.

 

[ranger17]

The reason that Va increases with weight is that at Max gross weight the AOA is closer to your Critical AOA. This in turn means that you will stall the aircraft before you over stress the airframe. So at a lower weight there is lower AOA and you can displace the aircraft farther before it will stall at the same airspeed at Max Gross Weight, thus we have to slow down in order to increase our AOA so that we will stall before doing any damage. That is what Va is all about, having the aircraft stall before you do any damage to the aircraft. This is where your Vg Diagram comes in to play, use this on your checkride if the question comes up and explain how above Va you will exceed structrual limits before stalling.

 

[avbug]

Maneuvering speed represents a margin above stall; one will stall when making a full control deflection before exceeding structural limits of the airplane; you'll stall before you break.

Heavier weights, you stall faster. VA is higher; it represents a margin above the stall.

You can't explain it much more simply than that, and the examiner will be perfectly happy.

While he's like that, ask him if you can date his daughter. That's the acid test to see just how placated he really is.

Carry a first aid kit, in case he's not nearly so happy as he might appear.

 

[hydroflyer]

Part of the trick to this is finding an explanation that is worded in a way which YOU can understand. We all learn things differently so here's another try:

We all know the point of maneuvering speed is to have the airplane stall before it is subjected to loads that would exceed its design limits.

In order to stall, an airplane must exceed its critical angle of attack.

An airplane wing has a set amount of surface area to generate lift (not counting the flaps). Maximum lift will be generated at the critical angle of attack.

In order to generate excessive "g" force, the lift must be proportionally greater than the weight of the airplane (3 times more in the case of 3 g's)

Taking flyifrvfr's first example, and making the math a bit easier, say the 172 has a max limit factor of 3 g's. At 1900lbs, the wings would need to generate 6,700lbs of lift for a force of 3 g's to occur (3 x 1900). If the wing is at its critical angle of attack, it would need (say 96 knots for example) of speed to generate the 6,700lbs of lift.

Now raise that weight to 2,500lbs on the same airplane. Same wing, same critical angle of attack, but now in order to generate 3 times the weight, the wing must produce 7,500 lbs of lift, so it must go faster in order to do so such as 105 knots for example.

You can also view it as saying the wing can generate 7,500lbs of lift at 105 knots AND at the critical angle of attack. If you were flying at 2000 lbs and 105 knots, and that wing were to reach the critical angle of attack in flight for some reason and generate that full potential of 7,500lbs against the actual weight of 2,000 lbs, you would place the airplane under a load of 3.75 g's, well over the placarded limit. (7,500/2000 = 3.75) The only way to prevent that from happening is to limit the speed at which the airplane is traveling so you can control the maximum amount of lift that can be generated.

These numbers are just used to show relation and are not meant to be actual numbers of the 172. If this confuses you, then forget you ever read it. If not, maybe it can help you to see this issue in a different light. Good luck.

P.S. It is late and I am tired, so if I wrote something stupid that needs to be clarified, just ask and maybe I can figure out just what it was that I meant.

 

[midlifeflyer]

Part of the trick to this is finding an explanation that is worded in a way which YOU can understand. We all learn things differently so here's another try:

I think I'll join you. This is pretty much the same set of concepts, but from a diferent angle

==============================
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 gofrom 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.
==============================

 

[flyifrvfr]

I want to thank everyone who replied to this post. I will use this information on the checkride if the question is asked.

 

[skydraulic]

Keep your answer simple for the checkride. Pass it and then worry about how you're gonna explain it to your students. If you started talking about total lift and load factors I'd probably ask you something like...what is more stressful on the airframe: a 172 @ 2000 lbs. pulling +4.0 Gs or a 172 @ 2400 lbs pulling +3.5 Gs?

 

[Singlecoil]

Incidentally, you can still exceed the design limit load factor for negative g's when flying at maneuvering speed. It is only the positive g limit that won't be exceeded. You can visualize this by looking at a Vg diagram.

 

[hydroflyer]

I too agree you should keep it simple and short on the checkride. All these posts show some different ways to look at the same thing and I think I'll save some of these in case I have to use them in the future. Good luck on your checkride and let us know how it turns out.

 

[avbug]

You guys think too much.