Question about Va speeds

[B-rad]

Thanks for the info. I didn't know that. I'll get future B-rad to read up on it so I can know it for myself.

[photofly]

I know people like to moan about the futility of this kind of thread, but I find it instructive to go and look things up and write about them.

[Driving Rain]

I know people like to moan about the futility of this kind of thread, but I find it instructive to go and look things up and write about them.

+ 1

[Steve Pomroy]

As it turns out, there are several speeds at which there is a (direct or indirect) required structural tolerance for turbulence (and Va is not one of them!). Two of these speeds are always published in the flight manual—and marked on the ASI—for normal and utility category aircraft: Normal Operating Speed (Vno), and Never Exceed Speed (Vne). Also related to turbulence, but not always published, are Vb (Gust Penetration Speed), Vc (Design Cruise Speed) and Vd (Design Dive Speed).
Gust Penetration Speed, or Vb, is required to be determined and published for commuter category aircraft, but is usually unpublished for normal and utility category aircraft. Vb is the maximum speed at which the aircraft structure can withstand a 66 foot per second (fps) gust perpendicular to the flight path. How much is 66 fps? It’s about 39 knots, and that’s a helluva gust!

Well it's nice to know my work is appreciated:). The article this comes from can be read in it's entirety here: http://www.flightwriter.com/2010/10/tub ... ation.html.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

[Driving Rain]

My sincere apology Steve. I've edited that post as it should have been posted in the first place.

[dr.aero]

Steve...

I just read your articles on Va and the Vb, Vc, and Vd speeds - they seem quite informative!

I'm still not convinced that it's possible to be at Va, input full up elevator, and overstress the wings. This is in part due to FAR 23.391 which, I've pointed out in this thread, seems to me to state that it can't be possible to overstress the airplane in any of the maneuvers stated in 23.397 - 23.459. The other part is due to what's written in the Metro III FAA approved AFM: http://i.imgur.com/KFNYnJE.png

"...individual application of full available aerodynamic control will not overstress the aircraft..."

It clearly states that it will not overstress the aircraft - not just the control surfaces and their supporting structures.

The Metro III has a stall speed at 16,000 pounds of 104 KCAS. The limit maneuvering load factor is 3.02g, which happens to correspond to the minimum required by FAR 23.337(a)(1). The calculated stall speed at 3.02g is 181 KCAS or 180 KIAS - that is 5 knots slower than the published Va. As you've pointed out, there are other airplanes that also have slightly higher Va speeds than what the airplane would stall at if at it's maximum positive limit load.

Regarding testing maneuvering loads for "each horizontal surface and its supporting structure", FAR 23.423(a) states: "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."

If you're cruising at a particular speed and suddenly pull back on the pitch control, the airplane will immediately pitch up and the speed will simultaneously, and immediately, decrease. Varying the speed at which the pilot pulled back would change the decrease in speed when reaching a particular load factor. If the test pilots who flew the Metro III were flying at 185 KIAS and then suddenly pulled back on the control column, I see it quite feasible that they could lose 5 knots of speed by the time they reached 3.02G.

The same could be said for the Grob and Piper airplane where you showed that they had a Va speed which was a few knots or MPH higher than what the stall speed would be at their positive limit load factor.

FAR 23.423(b) talks about requirements for a "checked maneuver" performed by the pitch control surfaces. Interestingly, it specifically specifies that for the requirements of paragraph (b) - "The deflections and timing of the “checked maneuver” must avoid exceeding the limit maneuvering load factor." That would be either the positive or negative limit maneuvering load factor.

Paragraph (a) doesn't specifically mention that detail but I would refer you to FAR 23.391 again for that.

Unless there has been an oversight by the FAA in their approval of the type certificate and AFM for the Metro III airplane, it does seem that the Va speed published for the Metro III will ensure that the aircraft is not overstressed if there is an "individual application of full available aerodynamic control".

[Steve Pomroy]

My sincere apology Steve. I've edited that post as it should have been posted in the first place.

No worries, and thanks for the link!

[Steve Pomroy]

I just read your articles on Va and the Vb, Vc, and Vd speeds - they seem quite informative!

Thanks! I try.

The other part is due to what's written in the Metro III FAA approved AFM: http://i.imgur.com/KFNYnJE.png

"...individual application of full available aerodynamic control will not overstress the aircraft..."

It clearly states that it will not overstress the aircraft - not just the control surfaces and their supporting structures.

While that's interesting, you should keep in mind that manufacturers are within their rights to build an aircraft that exceeds the certification standard. The standard is a minimum. I suspect that every aircraft exceeds that certification standard somewhere in all those requirements. But that's not something I'd bet on unless the manufacturer specifically says so in the POH/AFM, which they appear to have done in this case. Far more interesting to me is your earlier example of the Cessna Cardinal, which appears to have a Va that is lower than the allowable minimum. I'm not sure how that fits into the regs.

The regulations you've referred to include the following note:

The deflections and timing of the “checked maneuver” must avoid exceeding the limit maneuvering load factor.

In other words, the test must be conducted so that the limit load factor is not exceeded. This does not mean that it cannot be possible to exceed the limit load factor.

Regarding testing maneuvering loads for "each horizontal surface and its supporting structure", FAR 23.423(a) states: "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."

If you're cruising at a particular speed and suddenly pull back on the pitch control, the airplane will immediately pitch up and the speed will simultaneously, and immediately, decrease. Varying the speed at which the pilot pulled back would change the decrease in speed when reaching a particular load factor. If the test pilots who flew the Metro III were flying at 185 KIAS and then suddenly pulled back on the control column, I see it quite feasible that they could lose 5 knots of speed by the time they reached 3.02G.

The structural loads on the control surface happen first, at Va. Angular and linear accelerations happen next. Further, the on-paper analysis of elevator loads at Va are done assuming constant airspeed. It's true that during a test done as you describe, the airspeed will decrease, but so does your stall speed when pitching rapidly. That is, your stalling angle momentarily increases during rapid pitch due to unsteady flow. This is why stall testing is done with a maximum deceleration of 1 knot per second, it gets us our worst case (maximum) stall speed. So the requirement to remain within the limit load factor during Va testing may be met by pilot technique instead of design features.

Unless there has been an oversight by the FAA in their approval of the type certificate and AFM for the Metro III airplane, it does seem that the Va speed published for the Metro III will ensure that the aircraft is not overstressed if there is an "individual application of full available aerodynamic control".

No oversight required. As noted above, the manufacturer can build an airplane that exceeds the standard.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

[dr.aero]

Steve...

In other words, the test must be conducted so that the limit load factor is not exceeded. This does not mean that it cannot be possible to exceed the limit load factor.

Yup, good point.

I understand that most aircraft are built beyond the requirements - it would cost significantly more money and time to build an airplane that meets the minimum requirements and nothing extra! The only thing I'm having trouble with is the interpretation of FAR 23.391 - everything else you say makes sense and I agree with it. Maybe I'll see what the FAA has to say with regard to it.

Thanks!

[2R]

Just in case any students are lurking in FLIGHT TRAINING and are still puzzled as to what Va is.
Va and Vb are the airpeeds you should know if you are flying near turbulance.
If you are over Va you can seriously damage the airplane or fall out of the sky when the control surfaces or wings fall off.Below Va the control surface should stall out before you can damage the aircraft.
Some aircraft Va speed are a lot lower than normal cruise speeds.Mooney's, Piper Malibu,and of course the fork tailed doctor killer had a Va speed approx thirty knots lower than cruise.
If you are not below Va for the weight of the airplane in turbulance your wings can rip off,as did many early aircraft.Or just some expensive repairs before the next flight.
Some aircraft are designed to stall above a certain wing loading to protect the structure of the airplane ,but pilots being pilots can still damage the airplane causing expensives wrinkles or grief to those who do not respect the speeds listed in the POH,AFMs.
READ THE POH/AFM of the airplane your are flying and respected the airspeeds listed.The Va can be one of the most expensive numbers if it is ignored.
Just remember boys and girls if it gets bumpy SLOW DOWN as those speed bumps in the sky can scare your passengers and make your next inspection a very expensive one.
Passengers never remember the extra two minutes the flight might take ,but they will remember you if you hit the bumps hard and you will lose a customer, or worse a friend who may not want to fly with you again.

Blue skies

[Daveee]

Va decreases with reduction in weight... but also is the max. speed at which a stall can be performed in controlled flight without causing airframe damage.

When heavy loaded an A/C can handle strong(er) turbulence (or more abrupt manoeuvering) at higher airspeed, than it can when lightly loaded. Lets see it this way... a light a/c has less inertia, and as consequence is more "easily" tossed around by turbulence or rapid control movements. A heavier a/c is not so easily displaced and therefore less susceptible to high G-loads that can exceed the airframe limitations.

As a rule I use the for every 20% reduction in weight I decrease my Va 10%, or Va=VS 1.7 (Vs reduces with reduction in Weight).

[photofly]

Accident investigation into a PA28 ArrowII that lost a chunk of wing through structural failure while performing a recovery from an unusual attitude, manoeuvring at a speed below Va. Four dead.

http://www.aaib.gov.uk/cms_resources.cf ... 025533.pdf

[FL-510]

First post ever on the site....

Okay, Va! Va when you think about it could be considered as a stall speed. It is the speed where you could fully deflect the yoke to get nose-up movement to the stall WITHOUT EXCEEDING g's of the aircraft. Ex. c-150 +4.4g ...
The speed does increase because it takes a larger force ( compensated with speed ) to obtain 4.4g's with a heavier weight of the aircraft.

This might be obvious, but your actual Va in your aircraft decreases during your flight...

[Steve Pomroy]

Va and Vb are the airpeeds you should know if you are flying near turbulance.
If you are over Va you can seriously damage the airplane or fall out of the sky when the control surfaces or wings fall off.

Vb is a good reference speed for turbulence if it's published. Va is not. Vb is normally higher than Va, but it is not required to be. Vno also provides us with a turbulence limit (via Vc, which Vno must be equal to or less than). At Vb, the airframe can withstand a 66 fps gust. At Vno (Vc, actually), the airframe can withstand a 50 fps gust.

Below Va the control surface should stall out before you can damage the aircraft.

This is not guaranteed.

If you are not below Va for the weight of the airplane in turbulance your wings can rip off,as did many early aircraft.Or just some expensive repairs before the next flight.

Whether or not your wings get ripped off depends on both airspeed and turbulence intensity. There is no magic speed, above which hitting a single bump will destroy your aircraft and below which turbulence can be as bad as you like. That's why there are three turbulence design speeds, with the higher speeds corresponding to less severe turbulence allowances. None of these speeds is Va.

FL-510, welcome.

Va when you think about it could be considered as a stall speed. It is the speed where you could fully deflect the yoke to get nose-up movement to the stall WITHOUT EXCEEDING g's of the aircraft. Ex. c-150 +4.4g ...

No, really, this is not true. The minimum standard value for Va is referenced to the stall speed. You can exceed the LLF of the aircraft at Va.

In Advisory Circular 23-19A: AIRFRAME GUIDE FOR CERTIFICATION OF PART 23 AIRPLANES, the FAA provides some clarification of Va. Here's a snippet:

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 √n will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

[dr.aero]

Steve...

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 √n will the airplane stall in a nose-up pitching maneuver at, or near, limit load factor. For airplanes where VA>VS√n, the pilot would have to check the maneuver; otherwise the airplane would exceed the limit load factor.

I think that's exactly what I was looking for. Thanks!

[New_PIC]

Just in case any students are lurking in FLIGHT TRAINING and are still puzzled as to what Va is.
Va and Vb are the airpeeds you should know if you are flying near turbulance.
If you are over Va you can seriously damage the airplane or fall out of the sky when the control surfaces or wings fall off.Below Va the control surface should stall out before you can damage the aircraft.
... <snip> ...
Blue skies

Another point this student picked up on: I now have a better understanding of why my instructor stressed so much that recovery from a spiral dive should be separate and distinct control movements. Thanks all.

PS: I'll try not to resurrect too many old threads as I catch up on this forum.

[Steve Pomroy]

Another point this student picked up on: I now have a better understanding of why my instructor stressed so much that recovery from a spiral dive should be separate and distinct control movements.

That's a good point. There's also another good reason to separate your roll and pitch during a spiral dive recovery:
Your published limit load factor is based on symmetrical maneuvers. During asymmetric maneuvers (pitching with roll and/or yaw and/or slip), your limit load factor is reduced to 2/3 of your symmetric limit. So your normal category aircraft (3.8 g LLF) can only handle 2.5 g during a rolling pull. this reduces your margin to structural failure during a potentially aggressive maneuver.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero