Vmax

[EESDL]

Have to give a 15min lesson on Monday - it's been a while since my rotary pof so would like to confirm that it's the onset of the pressure wave on the forward blade that is the ultimate constraint?

[Irlandés]

Or retreating blade stall... depends on the machine and the IAS/Rotor RPM relationship.

Some machines will get RBS before they get Advancing Blade Compressibility and vice versa.

Open to correction of course.

Vmax??? Oh, and I thought you wanted to talk about motorbikes! ;)

Irlandés

[Nick Lappos]

Vne is seldom set by stall or compressibility, it is usually set by the maximum qualified speed that the manufacturer tests to, based on where he stopped the testing. If the maximum level flight speed as limited by engine power is 150 knots, there is little to gain by making Vne 210 knots in a dive, so most test organizations simply cut off the speed where it is convenient to test, perhaps 10 to 15 knots above Vh.

Limits are interesting things, most limits are not set where the item turns to mush or breaks apart. Most limits are set where the tests were done, because the qualification is based on test, and cannot go beyond what was tested. In effect, the Vne is usually just the maximum tested, and represents a reasonable margin above where the aircraft cruises at max power.

Vne is sometime set where the handling gets poor, or where the margin to withstand an engine failure is minimal.

[Weight and Balance]

To add to Nick's excellant reply:

Most manufacturers will select maximum demonstration speeds (that then determine Vne's) fairly conservatively. Nobody wants to destructive test the whole friggin' helicopter - and crew.

To return to the original question, some of the theoretical limits used to set these conservative max demonstration speeds include advancing blade mach number and retreating blade stall. So, in a sense, the value of Vne may be based on these factors. A more correct way of stating it may be to say that the designers aim to get all relevant speeds (Vh, Vne, demonstration speeds, etc.) to fall below the point where things like tip speed or blade stall become of concern.

In the commercial world, Vh is often limited by operating cost concerns (how much engine and how much gas the customer can buy), and Nick is correct to state that Vne is then set some small margin above Vh. This then gives the designer some of the boundries to work within when selecting the parameters that will determine the rotor's high speed performance (airfoil, blade twist, etc.). Other boundries will be set by things like target costs and weights, and selected blade manufacturing method.

So, it's kind of a chicken and egg thing. Yes, blade stall and mach number problems may occur above Vne, but thats because Vne was picked to be below these (and other) problems. A good general definition of Vne would be the highest speed at which no speed-related problems are expected. Hope that clears things up!:)

By the way, for several after-market bubble window installations on AStars, Vne is set at 10 % below the speed at which the window blew off on test.;)

[TwinHueyMan]

I've got a question along these lines...

What restricts the VNE from being higher on a Bell 212 when 230s with teetering rotors have maximum speeds upwards of 25-30 knots faster than the 212? While I'm not completley sure of the main rotor RPM difference between the two I don't see it being enough to threaten the 212 with RBS and not the 230s... the 212 has a good bit of power too, especially on a light weight flight. Is the fuselage unaerodynamc enough to limit the airspeed so heavily?

The 206L seems to share the 212's low speed as well, while the 206B is more towards the 230 end of the spectrum.

Is the 230 vastly different in some way to the other teetering rotor helicopters?

Mike

[SASless]

Twin....it has been my experience in 212's that it is the co-pilot's dental work and damage thereto that limits the Vne of the machine.....for some reason it always rides rougher on the left side it seems. I would venture to guess....part of the Vne limit is accounted for by mast limitations as much as any other reason.

[tone-uncage-fire]

Vd = Vdestruct (ie, when catastrophic failure occurs)
Vne = Vd - 10%
Vmax = The manufacturer (or operators) limit; often the upper limit to which the acft was flight tested.

-------------------------------

It is my understanding that the acronym 'Vne', to which we most often refer, is actually Vmax. (The ins & outs of which have been suitably detailed)

-------------------------------

At a pure (PofF) level the Vmax of a helicopter is limited by:

1. RBS or,
2. Advancing blade compressibilty or,
3. Engine power or,
4. Physical fwd limit of cyclic movement

[widgeon]

A question:-

As I understand most companies bill on a per flight hour basis.( at least in VFR utility market )

On a helicopter the faster you go the more fuel you use and the more stress the dynamic components take ( more speed = higher operating cost) .

Do most operators tell the pilot what speed he should fly or do they leave it to his discretion ?.

[S76Heavy]

We cruise at 80% Tq. Keeps the wear and tear to reasonable standards, and usually gives us a cruising speed close to Vno.
Works pretty well.

[Weight and Balance]

to add to tone-uncage-push the button:

On the commercial side, Vd is dive velocity. I've been to Vd several times, in several models, and even a knot or two beyond (depending on the recovery technique). I ain't destructed yet (much touching of wood, crossing of fingers, hail marys, etc.).

According to FAR 29.1505, Vne is defined as

"i) 0.9 times the maximum forward speeds established under § 29.309;

(ii) 0.9 times the maximum speed shown under §§ 29.251 and 29.629; or

(iii) 0.9 times the maximum speed substantiated for advancing blade tip mach number effects under critical altitude conditions. "

29.309 sets a speed based on structural considerations, which would include loads in the control system. 29.251 and .629 set a speed based on freedom from flutter and vibration anywhere in the helicopter.

Every civil certification agency I know of will only accept so much analysis for these speeds. In the end, yah gotta go fly it. This is Vd.

By the way, Transport Canada gets really picky, and won't accept that Vd is 10% over the Vne you finally get. You have to fly to 1/.9, or 11% over Vne.

Any manufacturer can actually select a lower target Vne, as Nick discussed before. This gives you a lower Vd, and you can still say that you have established a maximum speed under 29.309 et al, without getting anywhere close to destructing.

[Nick Lappos]

tone-uncage-fire,

the definitions weight and balance has provided are quite right. The theory you present is nice, but not grounded in practicality. I helped author a NASA report on limits, and had to carefully state that limits are mostly based on where the qualification test stopped. A limit is the maximum value for which safety and operability can be assured. A limit might be physical (stuff breaks above the limit) it might be practical (why bother to test farther, nobody goes there) and it might be economically set (since nobody wants more why pay to test more).

Of course, we poor folks have no way of knowing what reason was used, and nobody has a real idea how much above limit you can go before we read about it in the morning papers!

[Weight and Balance]

Nick said:

"Of course, we poor folks have no way of knowing what reason was used, and nobody has a real idea how much above limit you can go before we read about it in the morning papers!"

Right on! Please don't misunderstand anything said here to mean that the helicopter may be OK above Vne. Some theory guy back in the design office may think so, but he's probably not with you when you get the urge to find out.;)

[tone-uncage-fire]

Thanks chaps, always good to hear informative info........ I agree entirely with your sentiments - but a lot for rotary PofF is theoretical still (ie not grounded in practicality)

I know that much of the PofF I was taught as a student is no longer considered correct. I guess one day someoone woke and and said "well actually, it doesnt work like that at all!"

A bit like the debate over 'hookes joint effect'. Is it a real or apparent dragging force? And if its only apparent then why is it considered a force at all?

Or why the Huey tailrotor was orginally on the wrong side....and spinning wrong way. Did they not realise or not understand the PofF? (Actually Nick, Id love to hear your take on that)

This, along with Tech Log, has got to be the most educational forum on pprune. Thanks chaps/chapesses, keep it coming.

[Nick Lappos]

tone-uncage-fire,
The difference between an apparent force and a real one is this:

A real force applies a true force, a push that comes from somewhere, and must be reacted. If I stand on firm ground and push you sideward, you must react that force or accelerate sideward.

An apparent force comes from the dynamic situation, and is a product of the acceleration state. It is actually imposed by the item's inertia and the aceleration of the situation. If you are doing 100 miles an hour in your car, and turn the wheel, you create an apparent force that seems to push you to the side, but this apparent force is really the resistance of your mass to the new path you are forcing it to take (pull your point of view out of the car to a point high above the earth, and see that you are forcing the car to scribe a circle).

This is true of the acceleration/deceleration of a hook's joint, where the blade is closer and then farther from the mast during its trip around, so an apparent force is needed to "create" that acceleration.

Regarding the Huey's tail rotor spin direction, only a member of the original design team can explain the reason for its original direction. Everything on a design is done for clear reasons, but problems can change the weightings of the original arguments, and force changes.

On Monday morning, the game seems so very clear, but on Sunday afternoon, things seemed a bit more complicated!

[Irlandés]

I once read (a long time ago) the following line...

"There's no such thing as centrifugul force, it's an engineer's phantom, there's only inertia."

Never understood it until now. Good explanation Nick!

Irlandés

[Nick Lappos]

Irlandés,

Thanks for the nice words, but now you've made me feel guilty for the wisecrack I put in the altitude record thread!!

I truth, these posts always teach me things, often from the observations and questions of the sharp PPRUNERs and sometimes from the need to come away from Engineering jargon try to explain something in regular English.

Nick

[Irlandés]

No worries Nick. Anyway as they say back in old country 'If they're not making fun of you, then they don't like you!'. What the saying doesn't add however is 'just because they're making fun of you, doesn't mean they like you'. :D

[idle stop]

Tone-
Beg to differ on the UK definitions:
Vmax: highest IAS achievable in level flight at maximum power/torque.
Vno: Vne - 10 (%?) UK idea is that this gives a sensible buffer to avoid exceedence of Vne.
Vne: max permitted certificated IAS.
Vdf: max possible IAS before 'orrible things likely to 'appen (but flown to during certification process: that's what prototypes are for!)

[EESDL]

Thanks!
Will explain tip speed etc and relate it to solutions with fixed-wing (they're all fw now) and the fact that these luxurys are not open to rotary designers to such an extent. Quick example using pi x D, RPM etc showing speed in still air and margin that gives for forward flight. Should be enougth for this particular bunch of plank drivers.
Thanks again