Maneuvering Speed vs Weight

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ChrisB
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Maneuvering Speed vs Weight

Post by ChrisB »

Why does maneuvering speed increase when weight decreases in the RV6?
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maverick12
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Re: Maneuvering Speed vs Weight

Post by maverick12 »

Never flown the plane but perhaps something do with a shift in CofG position along with a design factor and hence easier to exceed load factor limits.
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photofly
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Re: Maneuvering Speed vs Weight

Post by photofly »

From the comment on that sheet, it looks like there's a misunderstanding of what Va is.

Va is a design speed, and it doesn't change with weight. It's the speed the manufacturer approves full use of one (and only one) control surface, without damage. It isn't connected to stalling, except is has to be not less than VS1 √n, per 14 CFR § 25.335.

The speed at which the aircraft will "break before it stalls" depends on weight, and that's what's listed here. I also think those figures are the wrong way around.
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Re: Maneuvering Speed vs Weight

Post by dialdriver »

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scdriver
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Re: Maneuvering Speed vs Weight

Post by scdriver »

photofly wrote: Sat Feb 26, 2022 3:22 pm From the comment on that sheet, it looks like there's a misunderstanding of what Va is.

Va is a design speed, and it doesn't change with weight. It's the speed the manufacturer approves full use of one (and only one) control surface, without damage. It isn't connected to stalling, except is has to be not less than VS1 √n, per 14 CFR § 25.335.

The speed at which the aircraft will "break before it stalls" depends on weight, and that's what's listed here. I also think those figures are the wrong way around.
A couple of questions:

First, my understanding of Va is that the full use of control surface for establishing the speed is based on the load factor when the wing reaches critical angle of attack (i.e. stalls), so wouldn't both definitions be the same? Albeit with the load factor/critical AOA being a bit more informative.

Second: CFR25.335 which you referenced holds that Va must be established at design weight. It doesn't say that this has to be just one weight?

Fundamentally, because Va is related to load factor limits (normal/utility etc categories) the speed at which full control movement can be done has to vary with weight, because AOA at cruise depends on weight. I do agree with you however, I think those numbers from the op might be the wrong way around.
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digits_
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Re: Maneuvering Speed vs Weight

Post by digits_ »

When determining Va, is there a defined speed with which you move the control? This would likely have a major impact on the result.
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Re: Maneuvering Speed vs Weight

Post by Conflicting Traffic »

scdriver wrote: Sat Feb 26, 2022 11:10 pm First, my understanding of Va is that the full use of control surface for establishing the speed is based on the load factor when the wing reaches critical angle of attack (i.e. stalls), so wouldn't both definitions be the same? Albeit with the load factor/critical AOA being a bit more informative.
No. The minimum required speed for the manufacturer to use as Va is determined as the stall speed at the limit load factor. But this is just a way to have a reference to base the speed on -- and manufacturers can use a higher speed if they wish. The speed itself defines the strength requirements of the control surfaces.
scdriver wrote: Sat Feb 26, 2022 11:10 pm Second: CFR25.335 which you referenced holds that Va must be established at design weight. It doesn't say that this has to be just one weight?
The regulations don't specify, but physically the peak air loads on the control surfaces don't change with weight.
scdriver wrote: Sat Feb 26, 2022 11:10 pm Fundamentally, because Va is related to load factor limits (normal/utility etc categories)
The minimum allowable value of Va is determined in part by load factor limits, but other than setting a regulatory standard, Va is not "related to load factor limits". Physically, any speed could be used to define the structural requirements of the control surfaces. The regulations as written relate that speed to the stall speed and limit load factor.
scdriver wrote: Sat Feb 26, 2022 11:10 pm the speed at which full control movement can be done has to vary with weight, because AOA at cruise depends on weight.
That doesn't follow. You're thinking in terms of stall-before-break, but this isn't the basis of Va (it is a widely held misconception though). Va is the maximum speed at which you can apply a single full deflection of a single control surface without breaking the control surface. The airspeed and the deflection and aerodynamics of the control surfaces determine the air loads on the control surfaces. Since the deflections and aerodynamics of the surfaces are fixed at manufacture, the airspeed at which full deflection is required determines the structural strength requirements of the control surfaces. The air loads on the surfaces don't change with weight.
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Conflicting Traffic please advise.
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Re: Maneuvering Speed vs Weight

Post by Conflicting Traffic »

digits_ wrote: Sat Feb 26, 2022 11:20 pm When determining Va, is there a defined speed with which you move the control? This would likely have a major impact on the result.
Yes. Maneuvering speed is determined based on the assumption of instantaneous full deflection. It's the initial static air loads that are maximum. Once the aircraft starts pitching/rolling/yawing, the air loads are reduced as the relative airflow at the control surface changes. During flight test, the test pilots can use a "checked maneuver" to test Va. This means that they apply full deflection as fast as they can, and then are allowed to remove it immediately in order to avoid structural damage to the non-control surface parts of the aircraft.
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Conflicting Traffic please advise.
photofly
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Re: Maneuvering Speed vs Weight

Post by photofly »

Here's a question for you all you really smart aerodynamics people. Here's the flight manoeuvring envelope from FAR23.333 (https://www.law.cornell.edu/cfr/text/14/25.333):
manoeuvre_envelope_orig.png
manoeuvre_envelope_orig.png (97.78 KiB) Viewed 2480 times
The notes in 25.335 about Va say:
(c) Design maneuvering speed VA. For VA, the following apply:

(1) VA may not be less than VS1 √n where -
(i) n is the limit positive maneuvering load factor at VC; and
(ii) VS1 is the stalling speed with flaps retracted.
...
To my untutored eye it looks like the diagram doesn't match the text (nor the text in 25.331(c)(1) which describes the elevator displacement test.) The condition on Va in 25.335 means that when flying at Va and applying full elevator it must be possible to equal or exceed the positive load factor limit (without stalling).

The stall-before-break corner is (uncontroversially) at A, and the manufacturer's nominated Va must be equal to or faster than that corner.

So I think the diagram should look like this:
manoeuvre_envelope.png
manoeuvre_envelope.png (98.38 KiB) Viewed 2473 times
This is the only way that the test in 25.331(c)(1) makes any sense:
(1) Maximum pitch control displacement at VA. The airplane is assumed to be flying in steady level flight (point A1, § 25.333(b)) and the cockpit pitch control is suddenly moved to obtain extreme nose up pitching acceleration. In defining the tail load, the response of the airplane must be taken into account. Airplane loads that occur subsequent to the time when normal acceleration at the c.g. exceeds the positive limit maneuvering load factor (at point A2 in § 25.333(b)), or the resulting tailplane normal load reaches its maximum, whichever occurs first, need not be considered.
Am I wrong? PilotDAR? Any thoughts?
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Re: Maneuvering Speed vs Weight

Post by AirFrame »

The simplest answer to the original question is: It doesn't.

There is more going on here though. A number of things about this chart are just plain wrong. First, there's no mention of VNE being True airspeed instead of Indicated airspeed, but that may be a discussion for another time.

Second, 1900lb isn't in the operating envelope for an RV-6. That's 20% over the design gross weight for the airframe. I'd love to know how they justify that.

Third, it mentions full control deflection (has to mean elevator, although it's not stated) producing 6G at VA. Well, at 1600lb, pulling 6G is a fail. The aerobatic gross weight for an RV-6 is 1375 lb...

I haven't done the math but I suspect VA at 1375lb and 6G (aerobatic category) is comparable to VA at 1600lb and 4.4G (utility category).

A good discussion of manoeuvring speed, explaining why it decreases with weight, is here.
https://www.aopa.org/news-and-media/all ... 0decreases.
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Re: Maneuvering Speed vs Weight

Post by scdriver »

Conflicting Traffic wrote: Sun Feb 27, 2022 1:29 am
scdriver wrote: Sat Feb 26, 2022 11:10 pm First, my understanding of Va is that the full use of control surface for establishing the speed is based on the load factor when the wing reaches critical angle of attack (i.e. stalls), so wouldn't both definitions be the same? Albeit with the load factor/critical AOA being a bit more informative.
No. The minimum required speed for the manufacturer to use as Va is determined as the stall speed at the limit load factor. But this is just a way to have a reference to base the speed on -- and manufacturers can use a higher speed if they wish. The speed itself defines the strength requirements of the control surfaces.
scdriver wrote: Sat Feb 26, 2022 11:10 pm Second: CFR25.335 which you referenced holds that Va must be established at design weight. It doesn't say that this has to be just one weight?
The regulations don't specify, but physically the peak air loads on the control surfaces don't change with weight.
scdriver wrote: Sat Feb 26, 2022 11:10 pm Fundamentally, because Va is related to load factor limits (normal/utility etc categories)
The minimum allowable value of Va is determined in part by load factor limits, but other than setting a regulatory standard, Va is not "related to load factor limits". Physically, any speed could be used to define the structural requirements of the control surfaces. The regulations as written relate that speed to the stall speed and limit load factor.
scdriver wrote: Sat Feb 26, 2022 11:10 pm the speed at which full control movement can be done has to vary with weight, because AOA at cruise depends on weight.
That doesn't follow. You're thinking in terms of stall-before-break, but this isn't the basis of Va (it is a widely held misconception though). Va is the maximum speed at which you can apply a single full deflection of a single control surface without breaking the control surface. The airspeed and the deflection and aerodynamics of the control surfaces determine the air loads on the control surfaces. Since the deflections and aerodynamics of the surfaces are fixed at manufacture, the airspeed at which full deflection is required determines the structural strength requirements of the control surfaces. The air loads on the surfaces don't change with weight.
That’s a good explanation thanks. You said manufacturers can use higher than stall-before-break for Va, but does that ever happen? Since POHs always have Va values for different weights this has to be stall-before-break, so I assume the technical “Va” is not published as maneuvering speed in the poh is that correct
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Re: Maneuvering Speed vs Weight

Post by CpnCrunch »

This isn't an "official" Rv6 POH (which probably doesn't exist). It's just the POH created by the guy who built that particular plane. It seems unlikely that a manufacturer would screw up Va in this way, but I could certainly believe a builder could make this screw up.
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Re: Maneuvering Speed vs Weight

Post by photofly »

scdriver wrote: Sun Feb 27, 2022 9:18 amYou said manufacturers can use higher than stall-before-break for Va, but does that ever happen? Since POHs always have Va values for different weights this has to be stall-before-break, so I assume the technical “Va” is not published as maneuvering speed in the poh is that correct
A few years ago I posted on this website the results of a short survey of various light aircraft flight manuals comparing the published Va with the stall speed x square root of load limit. As I recall they were all pretty close, and the Piper Archer had the biggest discrepancy, of a few knots.

Steve Pomroy also posted a couple of examples here:
http://www.avcanada.ca/forums2/viewtopi ... 10#p799910
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Re: Maneuvering Speed vs Weight

Post by T1DPilot »

I'm going to take this from my experience with another aircraft the Zlin 242L, which had a Aerobatic, Normal and Utility category.

I looked online to see if there was a full version of the POH you are referencing and I found it here: https://www.jaair.com/wp-content/upload ... N468AC.pdf

So the general rule of thumb you mention is correct, that Va decreases with a decrease in weight, the opposite also applies.

When we decrease weight from the normal category to the utility category our "G" limit also changes thus allowing us to stress the aircraft more. With that higher G limit we will still stall the aircraft before we reach that G limit. The same happens when you go into the aerobatic category, now we can do +/-6 G compared to the +2/-1.5 G in the normal. Category changes can cause in increase in Va, but within those categories Va decreases with weight decrease.

So what that part is saying is that at 1900 lbs (GW for Normal) the Va is 100 knots, at the 1650 lbs (GW for utility) the Va is 115 knots, this is all due to the different G limits of each category.
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Re: Maneuvering Speed vs Weight

Post by wreckage_brother »

Here are some excepts from Chapter 5 of the FAA Pilot’s Handbook of Aeronautical Knowledge. Here, the design maneuvering speed is defined as the maximum speed at which an airplane can be safely stalled.

https://www.faa.gov/regulations_policie ... ak_ch5.pdf

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photofly wrote: Sun Feb 27, 2022 6:58 am The stall-before-break corner is (uncontroversially) at A, and the manufacturer's nominated Va must be equal to or faster than that corner.
I don't understand how a Va being faster than the corner speed would work. You say Va is the speed the manufacturer approves full use of one control surface without damage.

So let's say [VS1*√n] is 110 knots and the manufacturer nominates a Va of 140 knots. If you apply full elevator control between 110 and 140 knots, the maximum load factor would be exceeded even though Va is not reached.
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photofly
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Re: Maneuvering Speed vs Weight

Post by photofly »

wreckage_brother wrote: Tue Mar 29, 2022 2:42 am
photofly wrote: Sun Feb 27, 2022 6:58 am The stall-before-break corner is (uncontroversially) at A, and the manufacturer's nominated Va must be equal to or faster than that corner.
I don't understand how a Va being faster than the corner speed would work. You say Va is the speed the manufacturer approves full use of one control surface without damage.

So let's say [VS1*√n] is 110 knots and the manufacturer nominates a Va of 140 knots. If you apply full elevator control between 110 and 140 knots, the maximum load factor would be exceeded even though Va is not reached.
Va applies to the rudder and ailerons too. They don't generate g force.

In respect of the elevator: the g-force is neither instant nor simultaneous with elevator application: to increase g-force the aircraft needs to rotate nose-up in pitch to increase the angle of attack of the wings and cause them to develop extra lift, which takes time. Meanwhile the load on the elevator is maximal as soon as the control surface hits the stop and before a pitch rate is developed.

It is therefore possible for full elevator application to break the elevator at a speed above [VS1*√n] before the g-load reaches anything like the limit.

25.331(c)(1) makes it explicit that it is the elevator strength after control deflection but prior to the g-force reaching its limit, that counts:
Airplane loads that occur subsequent to the time when normal acceleration at the c.g. exceeds the positive limit maneuvering load factor (at point A2 in § 25.333(b)), or the resulting tailplane normal load reaches its maximum, whichever occurs first, need not be considered.
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Re: Maneuvering Speed vs Weight

Post by tsgarp »

photofly wrote: Sat Feb 26, 2022 3:22 pm From the comment on that sheet, it looks like there's a misunderstanding of what Va is.

Va is a design speed, and it doesn't change with weight. It's the speed the manufacturer approves full use of one (and only one) control surface, without damage. It isn't connected to stalling, except is has to be not less than VS1 √n, per 14 CFR § 25.335.

The speed at which the aircraft will "break before it stalls" depends on weight, and that's what's listed here. I also think those figures are the wrong way around.
That is not correct. Please see here: https://www.faasafety.gov/files/notices ... Review.pdf
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photofly
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Re: Maneuvering Speed vs Weight

Post by photofly »

Which bit of what I wrote do you think is not correct?
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Re: Maneuvering Speed vs Weight

Post by tsgarp »

photofly wrote: Fri Apr 15, 2022 9:07 am Which bit of what I wrote do you think is not correct?
Your statements are at odds with the FAA publication (and all the other definitions of Va that I've seen, except Wikipedia) in the following areas.

From the FAA at the link I provided:
VA is the aircraft’s design maneuvering speed. Flying at or below VA, means that the airplane will stall before the structure
is damaged by excessive loads.

Another important thing to understand is that VA changes with the aircraft weight: VA decreases as weight decreases, and it
increases as aircraft weight increases.
For those who really want to get a handle on Va (and all the other theoretical aspects of aerodynamics) read Kershner's "Advanced Pilot Manual" or "Aerodynamics for Naval Aviators". Both of the those sources are much better than Avcanada. Or you could also check out
download.png
download.png (4.5 KiB) Viewed 1483 times
. Their online resources are pretty good.
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Last edited by tsgarp on Fri Apr 15, 2022 11:09 am, edited 2 times in total.
photofly
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Re: Maneuvering Speed vs Weight

Post by photofly »

I think if you really want to get a grip on what Va is, you should turn to 14 CFR § 25.335, where it is defined. Seeing as Va is a legal term, to the extent any written source contradicts the law, that written source is wrong - be it Wikipedia, Kershner, an anonymous FAA commentator, or any other website.

If you’re prepared to look in detail at 14 CFR 25.335, and you think I’ve misunderstood it, I’m all ears. If you can’t be bothered to do that, waving airily about other books and websites is irrelevant.
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Last edited by photofly on Fri Apr 15, 2022 11:13 am, edited 1 time in total.
DId you hear the one about the jurisprudence fetishist? He got off on a technicality.
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