Maneuvering speed vs weight

[jimpl]

I think this is right but not quite on point

Originally Posted by megan

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.

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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 WEIGHT

MASS 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]

Originally Posted by 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?

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 exceeded

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.

[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/...-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.

[Genghis the Engineer]

Originally Posted by megan

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]

Originally Posted by double_barrel

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