The problem is this will not happen immediately. Metal has memory in its crystal structure. It will remember that it was once subjected to excessive loads, possibly from vibration. The memory of this may make the component fail (possibly the tail cone) long after you have forgotten about it...

Max Speed flutter
 

Graviman,

You are quite right in your description of the vortex dynamics, but for the moment the precise effects are beyond the current model, although it seams feasible : putting some 2 order dynamic behaviour in the dynamic AoA/Lift/stall curve, cfr for instance Leishman.


As far as your comment is concerned about the R44-I : what will happen first? Well I am not sure about this. It clearly starts vibrating and I didn't want to foul around with this. I'll try a dynamic sim-scenario

The derating of the R44 creates a phenomenon that at high altitudes relatively speaking "more power" is available so hitting limits is easier.


ShyTorque

In the type you are flying (I think) max cruise is 154 against vne 168. I have no personal experience with the type, but pilots flying tell me it can be pretty unconfortable. Apparently you can also get VNE, but the max cruise must be there for fatigue reasons ?

Also very true to keep quite some margins when expecting gusts (for instance flying over mountain ridges with losts of wind), the max speed gust I got was more than 50 knts over the Mount Saint Baume in Southern France, even with a precautionary reduction to 90 this provoked almost instant 140+ knts. Couldn't lower collective more for risk off overspeed, so just shot up in the air to brake off... Because of very low AoA in that case no retreating blade stall occurred.

d3

Try a Lynx Mk7 as it was designed, ie no TOW launchers. VNE S&L, no problem. :ok:

And the latest 109E's: but not at max gross ;)

D3, take another look at post #8. :ok:

Shy Torque,

Not many Types have "La Cicciolina" to remind them of the Vne with a polite "Airspeed, Airspeed!" warning. Your ##*** is quite something as it can bust the Vne straight and level with more than 10%PI to spare!

However, like yourself, I prefer not to hear her polite witterings and so stay well below the Vne!

Fine heli though, isn't it?!!

22

MightyGem

Those were the days. Mk 1 not so good though, until the GT arrived :D

:D In a lynx s&l.

Can be acived in a mk27 and mk 90b though light on fuel:)

I take it that that's a Navy type lynx?

Max level flight speed Versus OGE performance (food for thought)
 

Military detailed specifications typically required hot high OGE hover performance. This is diametrically opposed to maximum forward flight speed. The Blackhawk called for OGE performance at 4000 ft and 95º F. To achieve the OGE specified hover performance Sikorsky incorporated 18º of equivalent twist in the main rotor blades. This effectively power limited the forward flight speed to approximately 155 kts. Rotor system drag drives up the power required and becomes the limiting factor in forward flight. Similarly the CH-53E incorporated main rotor blade equivalent twist of 20º to achieve hot and high OGE hover performance. At 137% torque (approx 13000 HP) the CH-53E is capable of 192 kts Vh. This was demonstrated for the US Navy in 1994to substantiate data for a new generation transport helicopter. By comparison, the CH-53A with 11ºof equivalent twist was capable of this same airspeed at 7500 HP. The MH-53E in another example of just how much rotor system drags plays in the equation. Sikorsky built the MH-53E Sea Dragon with large fuel tanks effectively doubling the flat plat area of the fuselage while only reducing the maximum Vh speed by 3 kts.:ok:

the true meaning of maximum speed
 

I think some people here don't quite undertand what Vne (under US/civil rules) actually means. The UK MoD (and in the past the CAA) does things slightly different.

the Vne is the maximum speed for which the aircraft is certified and can be defined by any criteria, it is not of itself a true measure of the actual absolute capability of the aircraft design, it certianly has no relationship with Vh (maximum level flight speed using max continuous power)

For any particluar type it could be defined by retreating blade stall considerations, component fatigue life considerations, handling characteristics, vibration or ride quality considerations.

There is no requirement in airworthiness terms that means Vne has to be achievable in level flightf or any particular weight and any performance inferrence shouldn't be made

For UK MoD aircraft the equivalent value is normally defined as Vno with a 10%MARGIN above that to Vne, however the requirements to be met at MoD Vne are way different to those for US/Civil certification (they are less as Vno pretty much means Vne (JAR)). Deliberate flight at speeds above Vno is normally prohibited but there is not a significant penalty if the value is inadvertantly exceeded (say due to a gust or AFCS runaway), but the UK MoD have different aircraft is service with Vmax (which means what?), Vno, Vne limitations so its actually as clear as mud what the limiting factors are.

DM

DM

I suppose you mean the Puma HC1?

Vmax is not a limit, more of a planning figure and is the speed normally expected to be achieved at the maximum allowed collective pitch (the Puma not having been fitted with a torque guage).

No, not Navy. The Mk1 (AH1) was the first mark the army got and had a few problems with a lack of power and TR authority. The Mk1 GT was a Mk1 with better engines and the precursor to the Mk7, which is an outstanding machine given its age.

Vmax is the maximum speed which can be achieved in level flight at maximum engine (probably max continuous) power ie where your power required curve crosses the power avaliable line. Unfortunately the term is often used in the military when talking about a flight envelope limit.

For example the Sea King has a FE limit of about 127 kts but the Vd (design limit is 157 kts) the Vne is apparently calculated by reducing the Vd by a safety factor of 1.1 bringing you into the low 140's. The British Military Release to Service then reduces that further by another safety factor to get to 127 kts. Generally speaking, the power at the FE limit is about 65% matched Tq and is nowhere near max power available.

As for the Lynx - it's Vd must be around 212 kts since that is what Westlands managed in the record. Apparently it only starts to get interesting in handling terms above 180 kts. I have had one (Mk 7 no TOW) at 165 S&L, smooth as silk with loads of power left.

The answer to the original question is that for many years only symmetrical aerofoil sections were used on helicopters but now improvements in blade construction allow several different sections to be used in one blade. The problems are all to do with pitching moments on a cambered blade as the AoA changes producing instability and vibration and therefore undesireable control loads.

Prouty's favourite saying regarding blade design is 'what you gain in the hover you lose in forward flight and what you gain in forward flight you lose in the hover'. Designing one blade that performs perfectly in both flight regimes still seems to be the holy grail of the helicopter designer.

Crab, at some point i think the market requirements will be such that helicopter designers are forced to consider active blade twist. Whether this is a speed dependent twist or a once per revolution twist will depend on what can become proven technology. The constraints are blade construction fatigue durability and practical actuator installation.

This is why X2 is important, as it opens the door for the next step.