Vd Speed Question

[Obidiah]

Hi All,

Can someone tell me if Vd (no not that VD) is a TAS or IAS value.

And perhaps a reference to how individual V speeds are established, I have read through various sections of FAR 23 but with reference to Vd it makes no comment regarding True or Indicated.

Thanks in advance.

[CharlieJuliet]

From memory, we tested an aircraft with a maximum Vmo of 230 KIAS (it varied with altitude) and flew up to Vd which was around 290 KIAS. So Vd must be indicated. I'm not sure what the relationship is, but will try and find out next week if no one else has come up with the answer beforehand.

[FlightTester]

As Vd is somewhere beyond Vmo, then it's a moot point to the general pilot whether it's in CAS or IAS. However, Vd is a KCAS number and it is then corrected to KIAS for display to the pilot on the standard instruments. It's also displayed in KCAS on the Flight Test Instrumentation, so that the TP and FTE can compare values.

[Mad (Flt) Scientist]

Actually, for a Part 23 aircraft, VD is defined as being an EAS term. This isn't really surprising, because it's a design/loads term not a flight/operations term, and for determining loads you want EAS.

From the current regs (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=c033b8fdd9ea57aff58579f0bd1048ef&rgn=div8&view=text&node=14:1.0.1.3.10.3.70.9&idno=14) (my emphasis):
§ 23.335 Design airspeeds.

Except as provided in paragraph (a)(4) of this section, the selected design airspeeds are equivalent airspeeds (EAS).

(a) Design cruising speed, V C.For V Cthe following apply:

(1) Where W/S′=wing loading at the design maximum takeoff weight, Vc(in knots) may not be less than—

(i) 33 √(W/S) (for normal, utility, and commuter category airplanes);

(ii) 36 √(W/S) (for acrobatic category airplanes).

(2) For values of W/S more than 20, the multiplying factors may be decreased linearly with W/S to a value of 28.6 where W/S =100.

(3) V Cneed not be more than 0.9 V Hat sea level.

(4) At altitudes where an M Dis established, a cruising speed M Climited by compressibility may be selected.

(b) Design dive speed V D.For V D,the following apply:

(1) V D/MDmay not be less than 1.25 V C/MC; and

(2) With V C min,the required minimum design cruising speed, V D(in knots) may not be less than—

(i) 1.40 V c min(for normal and commuter category airplanes);

(ii) 1.50 V C min(for utility category airplanes); and

(iii) 1.55 V C min(for acrobatic category airplanes).

(3) For values of W/S more than 20, the multiplying factors in paragraph (b)(2) of this section may be decreased linearly with W/S to a value of 1.35 where W/S =100.

(4) Compliance with paragraphs (b)(1) and (2) of this section need not be shown if V D /M Dis selected so that the minimum speed margin between V C /M Cand V D /M Dis the greater of the following:

(i) The speed increase resulting when, from the initial condition of stabilized flight at V C /M C,the airplane is assumed to be upset, flown for 20 seconds along a flight path 7.5° below the initial path, and then pulled up with a load factor of 1.5 (0.5 g. acceleration increment). At least 75 percent maximum continuous power for reciprocating engines, and maximum cruising power for turbines, or, if less, the power required for V C/ M Cfor both kinds of engines, must be assumed until the pullup is initiated, at which point power reduction and pilot-controlled drag devices may be used; and either—

(ii) Mach 0.05 for normal, utility, and acrobatic category airplanes (at altitudes where MDis established); or

(iii) Mach 0.07 for commuter category airplanes (at altitudes where MDis established) unless a rational analysis, including the effects of automatic systems, is used to determine a lower margin. If a rational analysis is used, the minimum speed margin must be enough to provide for atmospheric variations (such as horizontal gusts), and the penetration of jet streams or cold fronts), instrument errors, airframe production variations, and must not be less than Mach 0.05.

....

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as amended by Amdt. 23–7, 34 FR 13088, Aug. 13, 1969; Amdt. 23–16, 40 FR 2577, Jan. 14, 1975; Amdt. 23–34, 52 FR 1829, Jan. 15, 1987; Amdt. 23–24, 52 FR 34745, Sept. 14, 1987; Amdt. 23–48, 61 FR 5143, Feb. 9, 1996]

Part 25 is also EAS; the corresponding 25.335 starts with sentence:
§ 25.335 Design airspeeds.

The selected design airspeeds are equivalent airspeeds (EAS).

[John Farley]

Obidiah

In your original post did you mean TAS or EAS?

If you did indeed mean TAS then such a speed is only appropriate to navigation.

Airspeed limits that pilots are expected to observe are always IAS - the speed shown to pilots on the ASI.

There are some close relatives to IAS such as CAS and EAS but these are only of interest when doing detailed flight test analysis and from a normal piloting point of view can be considered to be the same as IAS.

[V1... Ooops]

Obidiah, I recently posted a rather lengthy reply to an earlier question of yours in this discussion: Vr Speed (http://www.pprune.org/flight-testing/334887-vr-speed.html).

BTW, Vd for a Twin Otter is 225 knots EAS. :eek:

[Obidiah]

Thanks V1,

I agree with your Vr speed response, thanks for the conformation. It was regarding the SC7

Vd speeds,

This came about following a recreational pilot spreading the word via a web forum that VNE is based on TAS where flutter considerations predominate. I don't believe it is, although flutter is TAS based, it is correlated back to an IAS value with supplemental speeds above 10,000 if required.

Just needed to do some research, Vd was part of that research.

[John Farley]

Obidiah

Why do you suggest flutter is related to TAS?

[Genghis the Engineer]

All structural limitations should initially be defined in EAS - the most obvious ones being Vd, Vf, Vne. However, they're clearly defined in operating limitations as IAS.



Flutter onset frequency is sort-of a function of TAS, but only tenuously. If you assume that the structural resonant frequency is fixed, then the critical figure is the Von-Karman vortex shedding frequency, which is defined by:

f= TAS * St / Diameter.

St however, the Strouhal number, which is a "black magic" derived figure based upon shape and Reynold number. Reynolds number in turn is defined by:

Re = density * TAS * Diameter / kinematic viscosity


So, it's not entirely unreasonable to describe flutter onset speed as a function of TAS - albeit that the inclusion of density means that you could play with the equations and make it an EAS function - but that's a bit tenuous.


The actual calculation of Vd is another black art practiced within design offices. Possible primary factors can include torsional divergence speed, wing drag loads, predicted flutter onset speed, canopy predicted birdstrike structural limit, predicted aileron reversal speed, and no doubt at-least a dozen other possible factors. The odds are that it'll actually be calculated for TAS at ISA S/L conditions - which in effect gives a result in EAS.

A reasonable textbook for a description of flight envelope derivation is Megson's "Aircraft structures for engineering students", although it needs tempering with the real-world knowledge of part 23 or better still if you can get hold of a copy, the old BCAR Section K.

G

[John Farley]

Genghis

Sorry to be so simple but surely the forcing function of flutter is the aerodynamic force? Take that away and things calm down! Mind you understanding stuff is not the same as calculating it.

[Obidiah]

As Genghis said without the maths :)

Personaly I have known of the relationship with TAS and flutter from my gliding days in the 80s. It can be observed by the presence of reduced VNEs for altitudes above 10,000' on the ASI.

As just an ordinary working pilot my simple explanation is related to the aeroelasticity of the wing and its natural frequencies combined with the degree of mass balancing of the control surfaces.

In essence the higher TAS (over a dynamic speed) in the presence of a gust (or input ??) can induce a frequency reaction in the wing which is unstable.

Nodes and antinodes and all that.

I dare say I shall be struck off this forum for such a crude explanation.

[John Farley]

As just an ordinary working pilot my simple explanation is related to the aeroelasticity of the wing and its natural frequencies combined with the degree of mass balancing of the control surfaces.

In essence the higher TAS (over a dynamic speed) in the presence of a gust (or input ??) can induce a frequency reaction in the wing which is unstable.

Nodes and antinodes and all that.

Cor! nothing like throwing in a few terms into the debate! Not that I would suggest you were trying to blind anyone with science.

As to variation of IAS limits with altitude I was always taught the mach number was the best way of explaining such matters.

Must be an age thing.

[Genghis the Engineer]

John and Obidiah,

Flutter is not a function of pure aerodynamic force - it occurs when the resonant frequency of a bit of structure co-incides with some other resonant input. The most common cited resonant input from our viewpoint is Von-Karman vortex shedding, the frequency of which changes with airspeed.

So, flutter in a particular bit of aeroplane will occur only at a particular range of speeds and its possible, in theory at-least, to accelerate through it. In practice, that's a daft thing to do of-course, since you'll then have to decelerate back through it again, and will also probably have to go through the worse than you saw at initial onset on the way.

It tends to be a high speed phenomenon, but to a large extent we mostly see it in those terms because aircraft structure is usually reasonally rigid, giving fairly high resonant structural frequencies, and thus high speeds to create the associated vortex shedding speed. Its safer to see it as a narrow speed range phenomenon but acknowledge that that speed is usually (but not always) high.

Gust response isn't, in my opinion, a significant issue save that if it was likely to occur anyway, turbulence effects may accelerate the initial structural motion.


A good instance of what I think was a long ignored case of mid-speed range flutter was the lateral flutter seen in a few VC10 beaver tails (the term given to the aerodynamic insert between the two podded engine exhausts) which was only noticed when the type started being used as a tanker - rather alarming some pilots trying to tank behind them. My suspicion now (I was only on the fringes of the problem, was very junior, and it was a lot of years ago) is that the aeroplane in airline service had happily accelerated and decelerated through it many thousands of times, but it only really became an issue when the aircraft started trying to fly at a constant AAR speed which had never previously been a stable speed for the type. (In the way of many flutter problems, a lump of lead solved it nicely!).

Another which I saw at the sharp end was elevator trim tab flutter in a pretty little kitplane called the Easy Raider, which we saw in a few examples and tended to sympathise in a very textbook manner with the longitudinal SPO. Damping it out by use of the control column and trimmer could allow you to fly through it and it didn't recur at higher speeds (fortunately in that case, it was essentially just a control cable tension issue and easily fixed - but it did occur wlel below Vh, let alone Vne).

G

[Genghis the Engineer]

As to variation of IAS limits with altitude I was always taught the mach number was the best way of explaining such matters.


I'd agree with you in most instances, but I don't think that this holds good if flutter onset is your major limiting factor.

G

[Obidiah]

Below is a small extract from an article written by Ken Krueger on the Rv series homebuilts, "Flying High and Fast" He seems to explain it better than I can.

Remember, as the airplane climbs, there are
fewer air molecules and less air pressure, so the needle
on The Gauge That Lies reads a lower speed,
even though the airplane is actually going just as
fast. That’s why True airspeed is faster than Indicated.
But flutter does not depend on Indicated Air
Speed/dynamic pressure. It is directly related to True
Air Speed — the velocity of the air passing by the airframe.

The velocity of the excitation force is the prime
concern, not the magnitude. It is very possible to exceed
this critical “flutter speed” without encountering
flutter if there is no initial disturbance. But if the critical
flutter speed is exceeded and then a disturbance is

encountered, the aircraft structure will begin to oscillate
in response to the velocity of the passing air.

This is not a typical resonance, where either increasing
or decreasing the speed will move the aircraft
away from the critical frequency and the vibration will
stop on its own. Going faster merely pumps more energy
into the system, increasing the amplitude of the
flutter. Go faster, flutter harder. Only going slower
and lowering the velocity of the air over the airframe
will solve the problem.

[Genghis the Engineer]

Whilst acknowledging that Ken Krueger is a very capable and respected aircraft designer - that piece is clearly written as "Janet and John learn about flutter" and not as a guide for aerospace professionals on how to analyse flutter.

I'd also still disagree that increasing speed beyond flutter onset will necessarily increase the risk of flutter - it will initially, but eventually the excitation frequency and resonant frequency will de-coincide (is that a real word or did I just make it up). I'm sure that if anybody feels the urge and has one available, this could be demonstrated very easily with a simple wind-tunnel experiment. Of-course, that doesn't change that doing so in a real aeroplane, isn't awfully clever.

G

[twochai]

BTW, Vd for a Twin Otter is 225 knots EAS.

Don't try this at home, folks!

There is, by definition, very little (perhaps no) margin remaining at, or beyond Vd. Under no circumstances should any airplane be intentionally taken beyond Vne (in the case of Part 23 certification) other than in a closely monitored and approved flight test environment, with full engineering instrumentation and support.

It is for this reason that Vd is not a published number.

[Port Strobe]

It is for this reason that Vd is not a published number

But for those that do manage to get hold of it, then it could be interpreted as V-dare? :E

[Genghis the Engineer]

But for those that do manage to get hold of it, then it could be interpreted as V-dare? :E

And possibly a fatal one. The test team should have identified Vdf, which is likely to be an intermediate speed which was the point at which some aspect of the aircraft's characteristics made them "unhappy". This might be co-incident with Vd, or it might be below that, which would make an attempt to fly to Vd (which is only technically a structural limit, not a handling limits) without that knowledge astoundingly undesirable.

G

[411A]

Interesting to note for many older piston types originally certified to CAR4b...

Vne and Vno is required to be reduced a certain amount per one thousand feet above a certain altitude, mainly for flutter onset effects found during original flight testing and pitot system installation errors.

Example.

B377 Stratocruiser

Airspeed limits Vno (Normal operating) 312 MPH (271 Knots) True Ind.
(Above 13,600' reduce speed 6 MPH for each additional 1,000')
Vne (Never exceed) 351 MPH (305 Knots) True Ind.
(Above 13,500' reduce speed 6 MPH for each additional 1,000')

By the way, the 'ole Stratocruiser was a joy to fly...!

[BarbiesBoyfriend]

So, Vd is best avoided, for health reasons.

They told us that in the army!:ok:

[Pilot DAR]

Seeing as we thread drifted into resonance, flutter, and flying through it, it is perhaps relevent that I relate one of my flight test experiences:

I was flight testing a Bellanca Scout with a Hoffmann four blade prop of reduced diameter (for quiet) replacing the big two blade metal neighbour annoyer.

While flying the aircraft from the front seat, things were fine. When I went for a ride in the back seat for a glider tow, I could feel a buzz in my butt. I looked all around, and noticed that the horizontal stab was either fluttering or resonating. I more think it was the latter, as we were flying at glider towing speed, (not even close to Vd!). This turned out to be to some degree associated with the prop, and it's different mass distribution. When we flew at lower or higher speeds, it was much better. Power setting did not affect it as much. When I flew the same aircraft with the original metal prop, it was fine. I found a CAA flight test report for a Citabria which presented the same observations, when a similar prop was flown. There are harmonics at work in that airframe, that go beyond a basic understanding.

Needless to say, I did not approve that Propeller on the Scout. Hoffmann later sent a solid (fixed pitch) propeller with the same geometery. When I cautiously test flew it, there was a marked improvement, and I did approve that installation.

Here's a video clip of the tail...

Video of Scout tail resonance - Photobucket - Video and Image Hosting (http://s381.photobucket.com/albums/oo252/PilotDAR/?action=view&current=MVI_8483.flv)

As for the original topic, Transport Canada will only allow me to approve Vne as 90% of the IAS to which I have flown (Vd IAS = Vne IAS x 1.1). When I test flew a Twin Otter for a nose boom installation, the 170 published Vne corresponded to a 187 KIAS Vd, which what I flew, with the shaker going at the front of the boom.

http://i381.photobucket.com/albums/oo252/PilotDAR/IMG_5942-1.jpg

Pilot DAR