Va - Manoeuvring speed? ... what manoeuvring speed?

[facthunter]

It IS. But does the salesperson mention it? It's also one thing to know the speeds (or colour on the dial) but another to make good use of the information. Flap speed can be very limiting. If you are maintaining a 5 or 8 knot gust reserve, you have a flap extend problem which can be structural very quickly. Has anyone ever been told what to do if ONE flap does the wrong thing. You WILL notice the effect.. Nev

 

 

[metalman]

For me ,I err on the side of caution, in my plane I cruise at the top of the green, always ,on bumpy days ( I don't fly on any day with a turbulence SIGMET) I'll pull it back to 70-75 kias , and if it takes me a bit longer to arrive I can "live" with that, I've got plenty of hours in tecnams and I fly the same if it's bumpy I stay in the green ,infact I can't think of a plane I'd fly that I wouldn't treat this way.

 

I usually estimate VA and then try to knock a bit more off ( the difference between light and heavy in my plane is pretty small , 60-70 kg ) it but having seen the Cessna info with a 15kias difference between light and MTOW I'll be taking a bit more interest in having an accurate number.

 

Matty

 

 

[Dafydd Llewellyn]

Here's the corresponding extract from CAR 3 (the design standard for the Cessna 172, 177 etc):

 

§ 3.184 Design air speeds. The design air speeds shall be chosen by the designer except that they shall not be less than the following

 

values:

 

Vc (design cruising speed) = 38 Ö W/S (NU) = 42 Ö W/S (A)

 

except that for values of W/S greater than 20, the above numerical multiplying factors shall be decreased linearly with W/S to a value of 33 at

 

W/S=100: And further provided, That the required minimum value need be no greater than 0.9 Vh actually obtained at sea level.

 

Vd (design dive speed) =1.40 Vc min (N) =1.50 Vc min (U) =1.55 Vc min (A)

 

except that for values of W/S greater than 20, the above numerical multiplying factors shall be decreased linearly with W/S to a value of 1.35 at

 

W/S=100. (Vc min is the required minimum value of design cruising speed specified above.)

 

 

Vp (design maneuvering speed) = Vs (Sqrt) n (my insertion - the software did not convert the square root sign) where:

 

Vs =a computed stalling speed with flaps fully retracted at the design weight, normally based on the maximum airplane normal

 

force coefficient, CNA.

 

N= limit maneuvering load factor used in design, except that the value of Vp need not exceed the value of Vc used in design.

 

Therefore, there is scope for the designer to declare different values of design manoeuvring speed at different weights - but this is normally not done nowadays in my experience. It complicates the flight envelope vastly with negligible practical benefit.

 

 

 

The flight envelopes specified by some "simplified" recreational design standards do not require gust loads to be separately considered; therefore for those aircraft Vc and Va have to be essentially the same. The green band (where marked) on the ASI normally goes to Vc; but some design standards do not require ASI colour-coding. Aircraft such as the Sabre and the Foxbat (and the early Cessna 210s - I don't know about the later ones) are quite capable of cruise speeds significantly above Vc, which in Australian summer conditions is downright dangerous.

 

 

[Bob Llewellyn]

For a 600kg aeroplane with an aspect ratio of 7.0 and a (clean) stall speed of 45kts, the speed at which a design gust will cause a 4G event is ~120kt by CAR 3; ~103kts by the ASTM (LSA); or ~90kts using the LSA formula and the results of the old DCA Australian gust spectrum survey.

 

Note that the 45kt minimum stall speed is unique to Australia; for a fully ASTM-compliant unflapped LSA aeroplane (40kt stall), the 4g event speed becomes ~101kts (CAR 3); ~91kts (LSA / FAR 23); and ~75 kts using the LSA formula with the DCA mean gust.

 

Generally speaking, the 100hp class of LSA aeroplanes designed for cruise (rather than STOL) have higher power-to-weight ratios than "traditional" aeroplanes, and so push the edges of their gust envelopes. Of course the actual figures change with aspect ratio, twist, and planform; but not by very much.

 

I was going to post a couple of graphs, but the site won't let me...

 

 

[Dafydd Llewellyn]

For a 4 - G limit-load aeroplane, twice the flaps-up stall speed is a pretty realistic approximation.

 

 

[Oscar]

I suggest that, in some cases, 'push the edge' is a somewhat kindly phrase. Take the Pipistrel Virus 912 (figures from the website): 75% power cruise: 121 kts. Vb: 76 kts. ( No Va given and no indication of whether those speeds are KIAS or CAS, draw your own conclusions). 45 kts difference - or looked at another way, a 37% reduction in speed required between smooth and rough conditions. I understand that they are a very nice aircraft to fly, but jeez, you'd feel a bit as if you were getting out and walking in turbulence.

 

 

[motzartmerv]

121 kts cruise? Typical website fluff..

 

 

[poteroo]

I suggest that, in some cases, 'push the edge' is a somewhat kindly phrase. Take the Pipistrel Virus 912 (figures from the website): 75% power cruise: 121 kts. Vb: 76 kts. ( No Va given and no indication of whether those speeds are KIAS or CAS, draw your own conclusions). 45 kts difference - or looked at another way, a 37% reduction in speed required between smooth and rough conditions. I understand that they are a very nice aircraft to fly, but jeez, you'd feel a bit as if you were getting out and walking in turbulence.

In practice - use a target speed of midway between Vs and Va if you encounter severe turbulence. Your Va is measurable on each and every flight by conducting a clean stall as soon as you're at height, and x2 the observed IAS - takes care of the weight factor.

 

The C172 POH posted by Nobody is an essential wake-up call for flying schools. Students head off on solo navs, with a placarded 105KIAS as their Va - when the real value is probably more like 92-95KIAS at the lower weights they fly at. The other worrying operational habit that I see is where pilots deploy flap at too high airspeeds for the conditions. It can be as rough as bags - yet pilots deploy flaps right on Vfe. (load limits are halved once you extend flaps!)

 

I'd draw everyones' attention to the V-G diagrams posted back on P1. What one needs to consider is the 50fps gust lines provided. If you look to the vertical axis - there is a load factor involved at each speed on the horizontal. If the aircraft strikes severe turbulence of strength enough to apply 2-3G to the airframe, and the controls are then manhandled too coarsely......bingo, you have exceeded the total loading. Hence the good sense advice to ride with the gusts rather than fight them, (ask for an altitude 'block' if in CTA). An aircraft can be well under Va, but still suffer a destructive loading due to both gust and pilot inputs. Note that there is a graded zone just outside the flight envelope boundary wherein the structure may be damaged - but not necessarily destroyed. This perhaps explains damage on the VA ATR-72 at Albury, and to aircraft which have been flown through Cb's and survived. A C402 flew through a cold front between Kalgoorlie and Perth back in the 70's and just about every structure was deformed - it never flew again AFAIK.

 

happy days,

 

 

[rgmwa]

Van's POH numbers (CAS) for the RV-12 are probably fairly typical for an LSA +4/-2 limit load factor design:

 

Vso 41 kts at gross

 

Vs1 45 kts at gross

 

Va 90 kts at ?? - not specified

 

Vno 108 kts

 

Vne 136 kts

 

Typical cruise speeds are quoted in the 100-115kt range.

 

Wheel and leg fairings are good for another 4-5 kts.

 

rgmwa

 

 

[Oscar]

121 kts cruise? Typical website fluff..

Merv - once again, I agree with you (that's twice this year alone, but don't consider it the start of a bromance just yet..)

 

Still, at least the Pipistrel figures are available: I note that the Flight Design brochure for the CTLS has 7 pages of illustrations of the available colour schemes, but gives just two V-speed figures: VH and VNE.

 

What does that say about the target market for the aircraft that I think claims to be the 'world's most popular'?

 

In fact - when one tracks down the CTLS POH - the Va at max. weight is a very reasonable 99 kts (reducing to 90 at lightest weight) for a VH of 120 kts, so probably a cruise of around, what - 110 kts? Vb is good - above VH by a healthy margin.

 

So, perhaps the FD people can be excused for not bothering to quote the relevant figures in their advertising, since the thing is less likely to fly one into trouble than some others. Quite possibly, this is fair enough - but if the things are selling at a very high rate, does that mean that there are a lot of people out there as buyers who do not care - or possibly, do not understand - that these flight limitations are important in real-life?

 

And if that IS the case: is pilot education lacking in getting the message across that one has to accept and apply the structural limits when flying? HITC in post #48 recounted an instance of an educated, sensible and generally cautious pilot basically ignoring the limitations predicated by the conditions because 'Jabirus are built tough'. I think (and don't jump on me here as a 'Jab. lover', most people agree about the ruggedness of the airframe at least) that that is pretty much 'conventional wisdom' - but it's not something one should use as an excuse to ignore the limits Jab. place on their aircraft and endanger oneself and one's pax. Hilux utes are 'built tough' - but they can be broken and so can Jabs. 'Bugger' is not useful if some of the airframe has departed..

 

 

[djpacro]

A minor point but I think that you will find that the 172 example in an earlier post (being a model S) and the 177 were certified to FAR 23 not CAR 3. I don't see much difference of relevance between these requirements.

 

FAR 23 requires "Compliance with the flight load requirements of this subpart ....At each weight from the design minimum weight to the design maximum weight" and the text of 23.335 is clear to me that the minimum value of VA is a function of that design weight.

 

Of course, the regs specify a minimum VA to be used in design so OK to specify a single value of VA. Of course, still need to show compliance with 23.423 "A sudden deflection of the elevator control, at VA, to (1) the maximum upward deflection,". Per my earlier post, at a light weight, that is likely to result in exceeding the limit load factor declared for the maximum design weight in that category - that is OK too if compliance was shown to all the loading requirements but the pilot probably won't know such details.

 

Just silly people like me have an accelerometer anyway.

 

 

[facthunter]

I don't think it's silly. I call it a GEE!!!. meter. Gives you an idea what the airframe has to put up with. I'm always gentle though. It records the highest figure in any given sequence when you reset it. Nev

 

 

[Head in the clouds]

....Just silly people like me have an accelerometer anyway.

I've downloaded a couple of the free g meter/accelerometer apps to my Android phone. They work fine and I've compared them against a calibrated mechanical(?) g meter and get exactly the same reading.

 

Might be something that folk should consider doing as I'd reckon that most people wouldn't really know what 2 or 3 g actually feels like. There was a g meter in the glider I did aero and spin training with and after that I had a much better idea how much load I was putting on an airframe.

 

 

[Dafydd Llewellyn]

A minor point but I think that you will find that the 172 example in an earlier post (being a model S) and the 177 were certified to FAR 23 not CAR 3. I don't see much difference of relevance between these requirements.FAR 23 requires "Compliance with the flight load requirements of this subpart ....At each weight from the design minimum weight to the design maximum weight" and the text of 23.335 is clear to me that the minimum value of VA is a function of that design weight.

 

Of course, the regs specify a minimum VA to be used in design so OK to specify a single value of VA. Of course, still need to show compliance with 23.423 "A sudden deflection of the elevator control, at VA, to (1) the maximum upward deflection,". Per my earlier post, at a light weight, that is likely to result in exceeding the limit load factor declared for the maximum design weight in that category - that is OK too if compliance was shown to all the loading requirements but the pilot probably won't know such details.

 

Just silly people like me have an accelerometer anyway.

Yes, the C177 was certificated to a very early version of FAR 23 - and I agree there's little practical difference between the two; the gust load formulae differ somewhat, and so does the nominal design gust velocity - but the resultant wing & tail loads are about the same. The effect of having a single Va based on the MTOW is that you must design things like the engine mount for the load factors it gets at minimum weight. I do not use the methods of FAR 23 Appendix A & B to calculate the basic loads; those formulae constrain the configuration and the wing aspect ratio too much. When one comes to calculating the basic loads from first principles, it is necessary to calculate the balance and checked manoeuvre loads, as well as the gust loads, for all the corners of the flight envelope, for maximum and minimum weight, and forward and aft CG. So there are hundreds of load cases in the spreadsheet, and the critical cases for the engine mount (for example) are not necessarily the same as those for the main lift truss or the tailplane. OR you can design the engine mount for the case that happens to be critical for the wings, and step-down the value of Va for lighter weights, so that (assuming the pilot observes this - which is quite unrealistic) in theory the same design load will suffice for all weights.

Similarly, you can work out the gust loads for Vc at the maximum design altitude, and have a single value of Vc - OR you can step-down Vc at increased altitude, to keep the gust loads from exceeding the sea-level values. Whenever I see either a range of values of Va according to weight, or a range of values of Vc according to altitude, I know that I'm looking at a structurally pared-to-the bone aircraft. A design that does not do these things has a little more "hidden safety margin" - and a lot of pilots place unthinking reliance on that.

 

This thread ignores the fact that, at the limit load, the structural strength of a new airframe must not suffer any permanent deformation. It must be able to carry 1.5 times that (for a metal aircraft) without breaking. A composite structure also has a "material variability" factor on top of that. These factors confer the appearance of robustness (provided you do not have a minimum-tolerance specimen in the critical part of the structure) that some pilots abuse. It is stupid to do so, because even if the aircraft you happen to be flying is above the minimum tolerance, applying unnecessary stresses to it cause accelerated fatigue damage. A ten percent increase in stress level roughly corresponds to a halving of the fatigue life. The fatigue life of most recreational aircraft is not required to be specified (s0 far); do you REALLY want to find out how short it can be?

 

 

[GAFA]

Have a look at the numerous videos that have been posted on YouTube from RA-Aus pilots. These videos clearly show there is a lack of understanding of VA as many of the videos clearly show the pilots crusing and descending with the airspeed right up in the middle of the yellow arc and at times the pilots are turning/rolling rapidly as well as flying very close to turbulence generating clouds.

 

 

[poteroo]

Have a look at the numerous videos that have been posted on YouTube from RA-Aus pilots. These videos clearly show there is a lack of understanding of VA as many of the videos clearly show the pilots crusing and descending with the airspeed right up in the middle of the yellow arc and at times the pilots are turning/rolling rapidly as well as flying very close to turbulence generating clouds.

Definitely. Makes you wonder why ASI's are not clearly marked with a Va range. (maybe a fluoro orange arc or similar?). It's very misleading to have the 'smooth air only' begin at Vno because if you hit some rough stuff over that speed the initial pilot response is to pull back on the stick to begin the slowdown. This just increases the airframe loadings at waaaaay over Va. The analog ASI's that come from VANS have a blue line at the MTOW Va number - but this probably isn't enough to catch the eye. And it's not just RAAus pilots....... If you want to see some really scary IAS then become an instructor doing BFR's in RV's!

 

happy days,

 

 

[facthunter]

I was going to mention that many raise the nose to wash off excess speed. This can be just what you don't want if you are in turbulence or likely to enter it. Be prepared to make big power changes to achieve the speed reduction, depending on how "clean" your aircraft is. Once you have the desired speed restore an appropriate power level and concentrate on "loosely" holding the desired attitude. Power and attitude gives the result you want. The airspeed will vary due to the turbulence. Don't chase it. The desired speed should be roughly in the centre of the indicated range. Nev

 

 

[djpacro]

... The effect of having a single Va based on the MTOW is that you must design things like the engine mount for the load factors it gets at minimum weight. ...... Whenever I see either a range of values of Va according to weight, or a range of values of Vc according to altitude, I know that I'm looking at a structurally pared-to-the bone aircraft.

Yes indeed, particularly significant in later variants of a type where max weight and power are substantially increased. One example I mentioned previously is one of the least robust aerobatic types around, having been developed from an older, lighter design. The other example has a reputation as being extremely robust however the later model with essentially the same airframe has some fairly serious ADs related to the structure as a result.

 

I don't think it's silly. I call it a GEE!!!. meter. Gives you an idea what the airframe has to put up with. I'm always gentle though. It records the highest figure in any given sequence when you reset it. Nev

An electronic accelerometer with a permanent memory is now a common feature to detect abuse. For one aeroplane I tell people they have bought the wings if they exceed the limit load factor. For the other I don't care what they do. (PS: today was talking to the new owner of your old aeroplane - it has gone to a good home.)

[motzartmerv]

For the other I don't care what they do.

Dave?

 

 

[djpacro]

Dave?

True.

 

 

[pmccarthy]

The placard in my Texan gives

 

Vmo (Max operating) 108

 

Va (Min manoeuvring) 78

 

Hmmm....does that mean if I want to manoeuvre I must fly faster? Something lost in translation from the Italian?

 

 

[JUSTNUZZA]

Here is my V-N diagram.

 

Va = 89

 

Vc = 115

 

My actual cruise 122

 

Vne = 145

 

Vd = 162