Maneuvering speed vs weight

[matthewlai]

FAA says maneuvering speed INCREASES with weight. Why?

This question has been bugging me ever since my PPL days (which is only a few months ago).

I gave the FAA answer on the written test, and passed, but since I'm an engineer, I'm going to figure this one out.

It doesn't make sense to me at all.

Maneuvering speed is the speed at which full and abrupt movement of any ONE control surface can be applied without overstressing the airframe.

Or in other words, the control surface will either stall before a force big enough to overstress the airframe can be generated, or end of travel will be reached.

Why does it INCREASE and not DECREASE with weight?

The stress on an airframe is the sum of 2 forces - gravity pulling the plane down, and lift pulling the plane up.

Lift generated by the wings is proportional to airspeed squared, and the angle of attack (roughly, before stall). That means, at the same speed, and same angle of attack (critical angle of attack), it doesn't actually matter how heavy the plane is. The wings will ALWAYS generate the same amount of lift (the plane could be climbing or descending depending on the weight, but we don't care about that).

Force of gravity, on the other hand, is proportional to mass.

What that means is, at the same airspeed and critical angle of attack, a heavier airplane will stress the airframe more.

So why is it that maneuvering speed increases with weight?

[djpil]

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

[matthewlai]

Ah! It makes sense if the stress limit is in G's.

But then the next question would be, why are airplanes rated in G's, and not absolute forces?

A heavy airplane and a light airplane pulling the same G definitely stress the airframe differently?

[The500man]

Think about the load factor. It's lift divided by weight. The same applies to Vs. Some of these things often seem quite confusing but Va is similar to a stall speed (gust + buffer) in that it relates to load factor; if I remember correctly!

[matthewlai]

Yes, the question is, why is load factor used to rate an airplane, instead of force?

[Mark 1]

What you say is true if the only criterion was wing bending and shear.

But G-limits are correct for engine mount, seat loads, baggage loads etc. which won't necessarily be lower for reduced gross weight.

Similar arguments apply to consideration of wing fuel weight as that reduces wing root loads but has no effect on the mass-loading effect of the powerplant.

[piperboy84]

FAA says maneuvering speed INCREASES with weight.

This one has always baffled me also, cant wait for a response in laymans terms that I actually understand .

[AviatorTB]

My understanding is follows:

Your max weight plane has a Va of Xias. Flying at that speed and weight, the difference between your alpha and the alpha to stall the wing is less than can be commanded by full up elevator. As your weight goes down with burned fuel, the same lift requires less alpha, therefore the margin expands. So, unless Va is consciously and correctly reduced by the pilot, he can damage the plane with sudden, full up elevator.

I'm choosing up elevator just as one example of a control input.

I'm sure one can get much more specific and wrapped up in symbols if one likes.

[matthewlai]

Your max weight plane has a Va of Xias. Flying at that speed and weight, the difference between your alpha and the alpha to stall the wing is less than can be commanded by full up elevator. As your weight goes down with burned fuel, the same lift requires less alpha, therefore the margin expands. So, unless Va is consciously and correctly reduced by the pilot, he can damage the plane with sudden, full up elevator.
That is the most common explanation I hear, and I don't think it's right.

At the maximum travel, the airplane will always stall (with very few exceptions), regardless of airspeed or weight. Elevator more or less sets the angle of attack, and usually has the authority to go past the critical angle of attack.

That means, the maximum lift produced up full up elevator is the same regardless of airplane weight.

[AviatorTB]

A pokey little Cirrus 22 will easily do 150kias and its Va is 133. I can assure you that even at 120kias, using full elevator in the Cirrus will result in a rapid climb. Sure, in time it will stall, but not very quickly. Now imagine the wing loads at 150 when suddenly commanding full up elevator.

If you're straight and level, there's only 1G on the frame. When in a constant speed climb, there's only 1G on the plane. However, to get from S&L to that CS climb, you have to accelerate (change directions) and that loads the wing. If you are above Va and you do it too quickly or with too much authority, the frame goes ouch.

[24Carrot]

I think Mark 1 has made the main point.


Imagine you are an engine component, attached to some part of the aircraft by a bracket. Let's say the bracket can support up to 4 times your weight.

Now, how much do you weigh? Your normal weight times the G-factor. If the G-factor goes over 4, the bracket will break or deform.

But for any given aerodynamic force, the G-factor is inversely proportional to the whole aircraft's mass.

[Tarq57]

Yep, Mark 1 has the correct answer.

Not all limits necessarily pertain to just the wing structure. In fact, in overload conditions, typically the wing is the last item to fail, just after the tailplane/elevator.

[sapperkenno]

A lighter aircraft will react quicker to control inputs as it has less momentum than a heavier aircraft at the same airspeed. I'm sure this ties in somehow but my head's not with it today!

[phiggsbroadband]

Hi Mathewlai, Its all to do with how the plane reacts to different All Up Weights (AUWs.)

If you throttle back to just 70 knots with just yourself on board, then pull fully back, you will not be able to pull more than say +2G.

Try the same at max AUW (full of passengers and full fuel.) and you will not get +2G but maybe just 1.5G. To get the 2G, you will need to fly faster when fully loaded.

Increase the speed to beyond Va and it is then possible for you to exceed the +4G limit of most planes, with full deflection of the yoke. Again Va will be higher when fully loaded, by a factor that I think is proportional to the squre root of the increase in weight.

[englishal]

A pokey little Cirrus 22 will easily do 150kias and its Va is 133. I can assure you that even at 120kias, using full elevator in the Cirrus will result in a rapid climb. Sure, in time it will stall, but not very quickly. Now imagine the wing loads at 150 when suddenly commanding full up elevator.
You've never done accelerated stalls then. Part of the FAA CPL is to demonstrate accelerated stalls, and what you do is fly along straight and level BELOW Va for your weight, then yank back on the elevator very quickly. The aeroplane doesn't go into a rapid climb as you would expect but stalls. But critically as you are below Va, none of the forces on the aeroplane exceed any design limits so you don't cause any damage.

The heavier the aeroplane is, the easier it is to stall, which is why Va rises with increased weight.

[keith williams]

Va is simply the stall speed at the limiting load factor.

Like any other stall speed it is proportional to the square root of the weight.

If the weight increases this causes Vs, Va and all of the other stall speeds to increase.

If we know the value of Va at any given weight we can calculate the new Va at any different weight using the following:

Va at new weight = Va at old weight x Square root of (New weight / Old weight)

[shumway76]

The reason is:

So that your aircraft will stall before it can be overstressed due to g loads when you abruptly maneuver the aircraft.

Fly faster than maneuvering speed on a lightly weighted aircraft and abruptly maneuver the aircraft, you will overstress the airframe.
Fly slower than maneuvering speed on a lightly weighted aircraft and abruptly maneuver the aircraft, it will stall first before it can be overstressed.

[jimpl]

I have read that IN FLIGHT the Va airspeed does not vary with wing loading (i.e. G-load). This notion holds that Va varies with aircraft MASS not aircraft WEIGHT. Admittedly, on earth MASS and WEIGHT may be essentially the same value but loading an aircraft in flight with the increased load factor of a level turn (or a pull-up) would increase the apparent WEIGHT of the aircraft and its contents but would not increase the aircraft's MASS. Therefore, this notion holds, Va would not vary.

As an aircraft flies along using its fuel the aircraft's MASS would be reducing and thus so would the aircraft's Va airspeed. Not by a lot I'd guess.

[megan]

Therefore, this notion holds, Va would not vary.From the relevant flight manuals for Va speeds.

Cessna 207 Basic weight in 2200 pounds region.
3800 pounds 130 KIAS
3050 pounds 117 KIAS
2300 pounds 101 KIAS

Cessna 172N Basic weight 1400 pounds region
2300 pounds 97 KIAS
1950 pounds 89 KIAS
1600 pounds 80 KIAS

I would opine that a 17 knot spread represents a substantial proportion of the 172 speed capability, at 75% power, 2000 PA, standard temp, cruise is 116 KTAS. Ignore Va at your peril.

[onionabroad]

..in laymans terms... although this may have been posted in the old 2013 thread...

the reason the Va increases with weight, is that it's just to do with stalling speed.... it's the speed you want the aircraft to stall instead of break following a sudden control deflection..
as weight goes down, so does the stalling speed. If the Va stayed the same but weight went down, then there's the chance that a sudden maneuver would bend something instead of stall the aircraft.


Higher weight, means higher stalling speed at various loads... so Va is higher.
Lower weight, lower stalling speed at various loads.... Va is lower.

[jimpl]

From the relevant flight manuals for Va speeds.

Cessna 207 Basic weight in 2200 pounds region.
3800 pounds 130 KIAS
3050 pounds 117 KIAS
2300 pounds 101 KIAS

Cessna 172N Basic weight 1400 pounds region
2300 pounds 97 KIAS
1950 pounds 89 KIAS
1600 pounds 80 KIAS

I would opine that a 17 knot spread represents a substantial proportion of the 172 speed capability, at 75% power, 2000 PA, standard temp, cruise is 116 KTAS. Ignore Va at your peril.

-----------

Yes! But this describes changing the MASS of the aircraft by loading it differently - and, if I understood the article, this certainly would change the Va of the aircraft. This is not the point of what I had read. I think the point of what I had read is that G-load, such as in level turns or in pull-ups, increases the apparent WEIGHT of the aircraft but not its MASS - and it said Va varies with aircraft MASS not aircraft WEIGHT.

Whatever. The important thing is to fly safely in whatever way the pilot understands that.

[custardpsc]

What Shumway said - nicely put

The reason is:

So that your aircraft will stall before it can be overstressed due to g loads when you abruptly maneuver the aircraft.

Fly faster than maneuvering speed on a lightly weighted aircraft and abruptly maneuver the aircraft, you will overstress the airframe.
Fly slower than maneuvering speed on a lightly weighted aircraft and abruptly maneuver the aircraft, it will stall first before it can be overstressed.

Which can be expanded in to the discussion about angle of attack needing to vary at the different weights to achieve the same speed

[megan]

it said Va varies with aircraft MASS not aircraft WEIGHTMASS is not an aviation used term, Weight in aviation terminology is what the aircraft weighs ie maximum take off weight, maximum landing weight, maximum ramp weight, maximum zero fuel weight. Pulling "g" is regarded as apparent weight at that moment in time in aviation, though as you describe it is also weight per unit mass..

[double_barrel]

It also took me a while to get my head around the real meaning of Va - My take: it's the maximum speed at which you will stall before you break the aeroplane if you do something silly. And it's easier to recover from a stall than glue a wing back on. Since stall speed goes up with weight, so does Va

But I saw an interesting Youtube video on this subject a while ago and cannot find it now. It was trying to make the point that GA pilots and big jet pilots view Va completely differently. GA pilots view it as a maximum, commercial pilots regard it as a minimum. ie they can always break their aircraft with violent control inputs before they stall. Does anyone happen to have seen the video or know if what I am saying makes any sense?

[ACMS]

My son was asked “why” Va reduces with reducing weight in his RPL test, he couldn’t explain why. He knew it was 111 at max weight and 88 at min weight.

He was asked a lot of pre flight questions on the usual stuff from the RPL test form and got nearly all of it correct over 1 1/2 hours but he was told he needed to give 100% correct answers to the ground questions before the flight….so because of that he failed and didn’t leave the ground…..


An RPL TEST for goodness sake, not an ATPL test..

Go figure.

[Fl1ingfrog]

I would have failed before I'd sat down because I have no idea what an 'RPL' test is.

[Genghis the Engineer]

Recreational Pilot Licence. Roughly equivalent to the UK NPPL or European LAPL in some non-European countries.


I think it's been adequately covered, but the basic principle remains - Va is the point where the stall curve and g-limits co-incide. So, slower than that, the stall will protect the aeroplane from exceeding g-limits following a severe pitch input or vertical gust, above that the stall will not do that and the aeroplane can pull more g. Whilst the total load is what matters to the mainplane, all sorts of other structure (seats,. engine mounts, harnesses, any random box of electronics) are certified against their actual weight and a maximum g so it remains the total g that matters, not the total load. Heavier aeroplane, stall speed goes up, therefore so does Va. So long as you are below MAUM and within the g-limits, the total certified strength of the mainplane can't be exceeded.

G

[scifi]

Remember once you have pulled back beyond the critical angle of attack, the wing stalls and produces much less lift, so has less forces acting on it. L no longer = D, in fact L might be just half of what it should be for level flight.

[Vessbot]

Ah! It makes sense if the stress limit is in G's.

But then the next question would be, why are airplanes rated in G's, and not absolute forces?

A heavy airplane and a light airplane pulling the same G definitely stress the airframe differently?

As others have speculated, the overall load limit could be set by items whose breaking point does not change with airspeed (as does the max load provided by the wings) so it occurs at a certain acceleration (aka G). And for the G to be kept the same, a difference in weight must be matched by a difference in lift, which can only be provided (if we’re already at max AOA) by a difference in speed. Unlike the the breaking point of a wing root, which occurs at a certain force (aka lift) of which the maximum of a certain value, occurs at one speed only.

As an aside, I find it kind of amusing how many replies you got (even almost a decade later) which didn’t understand your question, didn’t catch on to the difference in constant-G vs. constant-force limits, and simply blurted out the basic PPL 101 explanation of why the constant G speed changes with weight (one of which, amazingly, pointed you to “think about the load factor” as if your original post wasn’t already based on it, at a level far beyond the basic quip that was the rest of that post).

Goes to show how so much of what passes for reasoning is really just word association. Hear a thing mentioned in a question, and just blurt out the soundbyte that you associate with that thing… no engagement with what the question is actually asking about that thing, how it relates to the other items in the related conditions, etc.

[megan]

So long as you are below MAUM and within the g-limits, the total certified strength of the mainplane can't be exceededOne item that often gets lost in this debate is the effect of rolling "g", that is, rolling while pitching, Genghis might give a view from the engineering perspective as to how designers address the issue in design of aircraft structure, the only aircraft I flew that had documented rolling "g" limits put the limit at two thirds of the symmetrical limit.

[David J Pilkington]

Some relevant reading material that some may be interested in.

The correct definition of Va is explained at https://www.aopa.org/news-and-media/all-news/2011/january/20/saib-aims-at-better-v-speed-understanding with a link to the USA FAA's special airworthiness information bulletin on the subject.

Light aircraft certified to FAR 23 amendment 45 or later will not have Va (design maneuvering speed) in the manual, it will be Vo, the operating maneuvering speed, as explained in this extract from FAA AC 23-19A.
https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/604x306/vaac2319underlined_4a15dadcf59c15a962b29b922e1e2289f49a6b6a. png

[Genghis the Engineer]

One item that often gets lost in this debate is the effect of rolling "g", that is, rolling while pitching, Genghis might give a view from the engineering perspective as to how designers address the issue in design of aircraft structure, the only aircraft I flew that had documented rolling "g" limits put the limit at two thirds of the symmetrical limit.

You are quite right. The concept of Va is only strictly relevant for a symmetric load case (although as it happens the moveable control surfaces are also certified for full deflection up to that speed).

This is one, probably the main, reason why upset recovery drills involves pushing to unload, THEN rolling to the wings parallel with the horizon, THEN pitching back to the level flight attitude. And never combining those three bits of handling technique - otherwise you have potential to create a complex load case for which the aeroplane was not designed, and may cause a structural failure at lower speeds.

G

[Vessbot]

But I saw an interesting Youtube video on this subject a while ago and cannot find it now. It was trying to make the point that GA pilots and big jet pilots view Va completely differently. GA pilots view it as a maximum, commercial pilots regard it as a minimum. ie they can always break their aircraft with violent control inputs before they stall. Does anyone happen to have seen the video or know if what I am saying makes any sense?

This confuses two different things, which is easy to do since they unfortunately ended up with the same name. The GA “maneuvering speed” that the thread has been talking about, aka Va, is a maximum maneuvering speed, defined as the stall speed at the limiting G, and with the purpose of knowing that we can’t possibly exceed the limiting G below this speed (with a symmetrical pull only, yada yada)

The airline thing is minimum maneuvering speed, often shortened to simply maneuvering speed (hence the confusion) that simply protects against stall as long we stay above it. It’s stall speed, plus a safety buffer, plus an adjustment for increased G from turning (vertically unaccelerated). Often given as a simplistic formula, like on my airplane as Vref+10, that should cover the real math while being easily done in the cockpit

[Genghis the Engineer]

Terminology can affect mindset, without a doubt.

Another speed, Vc, or "cruising speed" is used in many airworthiness standards to define a "maximum" at which level flight is routinely done, and then is used for various airworthiness considerations. But I've come across plenty of instances, particularly in the USA, where people have read that, and assume it's a target and then thrash their engines to death trying to cruise at what is really just a structural maximum term.

It might be best to just call them Va, Vc, Vh, etc. rather than give them fuller names, then anybody using them is less likely to make incorrect assumptions about their significance based upon the name.

G