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VNE / Velocity Never Exceed
#11
FYI factoid: Ray Prouty once told me that in wind tunnel tests, retreating blades actually don't stall because it takes a few milliseconds
for a stall to occur, and by that time the blade is no longer retreating.

That result quite surprised those running the tests as the lift just kept on rising well past where a stall should have occurred.

The center of lift does migrate instantly which still causes blade twisting
and instability, so it's effectively a stall as far as flying is concerned.
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#12
Good point Hoyt. I actually would prefer not to use the term Retreating Blade Stall because of what you pointed out about what R.W. Prouty said. I don't remember if R.W. Prouty had a name for it but the better way to explain what is really happening is that the rotor system gets to a point where it can no longer aerodynamically compensate for dissymmetry of lift... but the term Retreating Blade Stall is pretty much an institutional term. Things do change over time. I know that now the term Gyroscopic Precession has been replaced with Phase Lag.
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#13
It suggests a question you'll undoubtedly be able to shed light on:

In my limited rotary time ( about 250 hours ), Flying a Rotorway Scorpion,
I wanted to see what happened as the speed increased beyond what I felt comfortable flying at
( about 80 mph) ( I don't think Rotorway had any reliable VNE figure, probably BJ was just guessing + the
design was always changing anyway. ).

After, I'd guess about 90-100 MPH, it just felt uncomfortable flying it, more stick shake and other things I can't put my finger on,
so the question is: wouldn't just the feel of rotorcraft be enough to tell when you're going too fast, and not to worry about any official
VNE specification?

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#14
Hoyt, I think what you have encountered may have to do primarily with high speed blade tracking among other things. On any given helicopter tracking will probably not remain perfect as angle of incidence/angle of attack changes from the minimum extreme to the max. It's usually very easy to track blades on the ground with the collective full down but once the angle of attack is increased and the aircraft is pulled into a hover the blades may or may not stay in track. Furthermore, as airspeed is increased and dissymmetry of lift is introduced changes in tracking may occur. The maximum change in tracking will probably occur at Vh (max level speed) just because it is the highest angle of attack from where the blades were tracked on the ground with no angle of incidence/attack. If the focus is tracking the blades at Vh then it is very likely that they will be out of track in the hover. So, most helicopters are tracked to get a smooth ride at cruise, an acceptable ride in the hover and the least emphasis placed on how well they ride at Vh. Stiffer blades with a symmetrical airfoil and no twist (washout) will generally track better throughout the entire speed envelope. The Mosquito has fairly stiff A-Symmetrical blades with no twist and I have noticed most fly smoothly at high speed if they hover smoothly.

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#15
Thanks, I forgot about tracking. It's easy to see if they're not tracking, but I don't remember if they were or not.
The question is, though, is why bother witha VNE number at all if you can tell by flying if you're going too fast for the current conditions --
just back off if it feels wrong.
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#16
Hoyt, Using the feel of the aircraft to discern if you are near Vne or not is not a good idea at all. There are machines that are in track and fly smooth at a hover, cruise, Vh and Vne. The reason we have an airspeed limitation for Vne is so that we have a standardized line in the sand not to cross for safety. Observing airspeed, power, temperature and rpm limitations is an implied part of being a pilot. On the other hand since the Mosquito is a personal machine you may do with it as you wish. If you chose to remove the Vne marking from your airspeed indicator and fly it clear of the actual Vne by feel, that is your business and it is not my place or anyone elses to tell you otherwise... but please make that decision public so just in case you crash we can all factor that into the theories we'll be coming up with as we guess what caused your crash.
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#17
I'm no expert but I have been told that when you encounter "retreating blade stall" it is sudden and violent. So you could be cruising along feeling good just below this critical speed and encounter an extra 10 knot gust and be there before you know it! Speaking to people that have experienced it they say your not there for long but it is not pleasant when it happens and while you can recover from it I don't think it's a place you want to go. Smile
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#18
The complexity of rotor craft aerodynamics and all the parameters and variables involved is mind boggling.

No wonder it takes forever to design, test and determine operating limits for a new helicopter like the swift.  

If it took any less time then perhaps shortcuts are being taken, so the fact that the Swift is slowly reaching its release should be reassuring that everything is being taken care of in detail to ensure the best possible outcome.

I always knew that Helicopter flight was more black magic than nature would normally allow.

Man has truly dominated physics in order to make such a diverse collection of parts behave to his wishes and carry him aloft.

Kudos to John for sticking with it to create amazing machines of the air.

I'm curious about the whole history of Johns work, successes and failures etc and how it led up to todays Mosquito.

I recall reading Popular Mechanics when I was in my teens onward and seeing ad's in the back for DIY Helicopters of some sort and I wonder if perhaps I was seeing ad's for John's early work, perhaps early AIR versions?

Is there a thread or Bio where I can read the story of John U's journey?
Ray
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#19
Lots of failures and a few successes along the way! Mike Messex has a great chart listing the steps in the development of the Mosquito. Starts way back in 92. Hard to believe its been that long!!
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