Vne: What Happens If Exceeded?

[PW_Plack]

This question came up in Doug's thread about Ken Rehler's accident, so I'm starting a new thread for these questions.

How should a gyroplane's Vne be determined? Is it the speed at which...

(a) disymmetry of lift or retreating blade stall starts banging the teeter stops,

(b) the drag on the fuselage or control surfaces threatens to break something,

(c) an arbitrary limit is set based on general past experience, or

(d) other?

(I realize the answer may differ depending on the expertise of the manufacturer, or the specific design of the airframe.)

If we fly at a speed close to Vne, will the momentary increase in airspeed caused by gusts or turbulence be sustained enough to pose a risk, or will the relatively small mass of the machine and inertia in the rotor avoid these problems? (I recall pilot comments about occasional bumps through the controls when flying through turbulence...is this what did it?)

Is Vne typically stated with any margin at all to accommodate gusts, or is it a hard limit? (I recall reports of GBA testing the SparrowHawk prototype at 10 MPH above the stated Vne, or +10%.)

If you built a machine of your own design, how did you determine what this number should be?

[Doug Riley]

Various unwanted events can determine VNE. There's no one thing that always happens as you go too fast in a gyro.

My personal gyro VNE is reached when the rotor 2/rev gets too rough. This used to happen at above 80 in my old Air Command, and occurs at 95 or so in the tandem Dominator. Many Dom. pilots fly faster than that, but vibration always causes me to imagine something fatiguing or shaking loose.

Windshields and pods may start to distort at high speeds.

In dive-testing one of the old Pitcairn-Cierva machines, rotor instability cropped up in a fast dive. These blades, I believe, were not aerodynamically balanced with trailing-edge reflex, and so the advancing blade would "tuck" when the pitching moment became strong enough to overcome the stiffness of the blade structure.

Gyros in which a net nose-down airframe pitching moment is counter-balanced by rotor thrust will fly nose lower and lower as you speed up. Such a gyro can get to a speed at which the slightest forward twitch on the stick is enough to cause a pitchover. It's this type of design that has made many old-time gyro pilots fear speed. HTL machines with inadequate HS, and airframes with nose-down aerodynamic tendencies fit this catagory. This dangerous tendency can and should be designed out of any machine.

The "classic" symptom of overspeed in a gyro is running out of flap-hinge range, resulting in "tapping" of the flap (or teeter) stops as each blade goes through its retreating position and on back.

[Harry_S.]

Good post, Doug.

I would go with an addendum..."As a novice pilot, do not even think about pushing your machine beyond the published VNE."


Cheers.:)

[bones]

Paul you said;
I won't say at this time whether I agree or not. I will just ask the question.

WHY????

Paul your always saying that you look at things from a new pilots eyes right, well as you know th most dangerous time is when they/us think we're GOOD pilots about the 15-30 hr mark now if they are reading about this type of stuff all the time what is the first thing that they will try once they have that feeling of invincability, my guess would be to open it up to see what the top end is, that was all i meant.:cool:

When i said the low and slow thing on the other thread, it was just what we say over here, there is no reason not to go as high as you like in any gyro that you feel safe to do so in..:p

[Aussie_Paul]

Paul you said;
I won't say at this time whether I agree or not. I will just ask the question.

WHY????

Paul your always saying that you look at things from a new pilots eyes right, well as you know th most dangerous time is when they/us think we're GOOD pilots about the 15-30 hr mark now if they are reading about this type of stuff all the time what is the first thing that they will try once they have that feeling of invincability, my guess would be to open it up to see what the top end is, that was all i meant.:cool:

When i said the low and slow thing on the other thread, it was just what we say over here, there is no reason not to go as high as you like in any gyro that you feel safe to do so in..:p

That's fine Mark. You have your personal comfort zone as, do I, and it smart to keep within that zone.

I thought that you meant that a gyro was not capable of operating safely at the higher speeds. I was hoping that you may have had a reason that I had not thought about.

The gyro is in its own element at the low end speed range due to the non stalling characteristics that make a gyro the safest flying machine ever devised.

Thanks for your input.

Aussie Paul. :)

[Chopper Reid]

The VNE of a gyro has always interested me as well as supposed cruising speed. Just how does one get to these conclusions ?
We read various posts and also gyro manufacture's adds as to top speed and see many interesting figures pop up. I guess the latest claim to fame has been the UFO in which a speed of 100 mph was mentioned.

Speed unless its verified by GPS over a two way course is not worth the paper its written on. Also ,the air at the time of testing has much to do with top speed .

[RayPierce]

Various unwanted events can determine VNE. There's no one thing that always happens as you go too fast in a gyro.

My personal gyro VNE is reached when the rotor 2/rev gets too rough. This used to happen at above 80 in my old Air Command, and occurs at 95 or so in the tandem Dominator. Many Dom. pilots fly faster than that, but vibration always causes me to imagine something fatiguing or shaking loose.

Windshields and pods may start to distort at high speeds.

In dive-testing one of the old Pitcairn-Cierva machines, rotor instability cropped up in a fast dive. These blades, I believe, were not aerodynamically balanced with trailing-edge reflex, and so the advancing blade would "tuck" when the pitching moment became strong enough to overcome the stiffness of the blade structure.

Gyros in which a net nose-down airframe pitching moment is counter-balanced by rotor thrust will fly nose lower and lower as you speed up. Such a gyro can get to a speed at which the slightest forward twitch on the stick is enough to cause a pitchover. It's this type of design that has made many old-time gyro pilots fear speed. HTL machines with inadequate HS, and airframes with nose-down aerodynamic tendencies fit this catagory. This dangerous tendency can and should be designed out of any machine.

The "classic" symptom of overspeed in a gyro is running out of flap-hinge range, resulting in "tapping" of the flap (or teeter) stops as each blade goes through its retreating position and on back.

Todd Day of Aiken pushed his Air Command (CLT & tall tail) at near the VNE.
His machine pitched suddenly nose vertical.
Tail was pointing at the ground. Somehow the craft recovered.
He said he thought he was dead. He talked to Ernie Boyette about it and Ernie said at the VNE the blades would do something. I wish Todd was on the internet to explain what Ernie had told him.
I think he discussed it with Steve McGowan also..

Doug....what you describe is exactly opposite of what Todd experienced.
??????/

[gyromike]

Ray,

Did he have McCutchen Skywheels on his machine?

I remember some discussion about the Skywheels doing this because the were not mass balanced on the quarter-chord point.

Or something to that effect.

[C. Beaty]

The rotor limited top speed of a gyro is about 35% of rotor peripheral speed but that’s not chiseled in stone; some of the 1930s gyros were flown as fast as 50% of peripheral speed.

It is also something that’s quite easy to demonstrate with the older flat-bottomed blades.

Bensen and Rotordyne blades can be started at very high pitch settings.

I’ve cranked the pitch setting of Bensen type blades right to the stops and they can still be started. With Bensen blades at the highest possible pitch, top speed was about 20 mph with the stick on the forward stop. The gyro refused to go any faster and simply screwed itself up and down with power setting.

Others that had Rotordyne blades found about the same thing at high pitch settings.

The stall of the retreating blade begins at the root and spreads outward as speed increases but unlike a helicopter which stalls at the retreating tip, stall is gradual rather than abrupt.

There are plots of retreating blade stall, determined by photographic studies of tufts attached to the blades of a Kellett KD-1 on the NACA site with more than 60% of blade stalled.

The Bensen standard had the mast and rotorhead tilted back at a 9º angle with ±9º of flap travel and also ±9º of control travel*. With the nose high attitude characteristic of low speed flight, it was impossible to lower the rotor disc enough to keep it from climbing. I don’t recall ever contacting the flap stops.

*That’s with a symmetrical “U” block. The original Bensen rotorhead with offset pitch pivot had less forward travel than 9º but I don’t recall the exact figure. It is also possible that HTL gyros which fly more and more nosedown as speed increases could contact the flap stops before running out of forward stick travel.

[Jazzenjohn]

In the old Bensen book the vne was set by the builder by going faster and faster, 5 knots at a time, and observing if the stick hit the forward stop, the controls started to diverge, or if other handling problems arose. The vne was then reckened to be 5-10 knots lower than the last good flying speed. That's how I remember reading it anyway. Rando has a copy.

[Fleamer]

Hmmmm

I am surprised that we don't have some wind tunnel testing of these things. Perhaps a business opportunity for someone to build one and rent it out - or better (from my perspective) a group/association could pitch in and build one and charge less!
With only about 20 hours or so on a gyro twenty years ago - but lots of other flying craft experience - I am reminded of the "there are lots of old pilots and lots of bold pilots, but no old bold pilots". VNE pushing is a very touchy thing to do in a craft that, once it stops flying, is a rock forever (well til it stops at the bottom anyway). It appears that most of these stories about pushing the machine past its (often unknown) limits result in very abrupt and heart stopping activity....careful.

As someone just getting back into the gyro game, I certainly appreciate these discussions. When I felw before there wasn't one within 1500 miles that I knew about. Although I am a little hesitant to admit it, I taught myself, and while lucky and successful in that endeavour, I was very cautious. Perhaps I'm not much of an "inventor" or test pilot, but I wouldn't push anything even close to its limits unless I had a lot of hours in different conditions on that particular machine....and then only after filing a will.

I does seem strange to me though that thosae with the mathematical and engineering smarts (especially those manufacturing these great machines, haven't figured this VNE thing out yet. Gyros after all lare not "new". Do Gyros not take any notice of natural physical aerodynamic laws under certain conditions?....so many questions, but I'm sure the answers are out there.

I like the cautious notes to new flyers in this forum. Scary to even talk about pushing limits if some "young" pilot will get cocky at a paltry 30 hours. Most aircraft are easy to actually fly...it is what we do to avoid needing our experience or how (well and quickly) we respond to the unexpected potential calamity that will determine our longevity of experience.

Cheers

Gerry

[scott heger]

I thought that gyros have two major speed limiting factors. One is the basic stability of the machine. This includes aerodynamics and stabilizer properties as well as forward stick limitations of the rotorhead. The second being retreating blade stall with the rotor.

In my particular gyro, my biggest limiting factor for speed is lack of horsepower. The Rotax 582, at 65 HP is never going to be a speed demon. I can induce a full throttle 120MPH dive in no wind conditions without running out of forward stick, or feeling any kind of instability. The airframe actually gets more responsive as speed increases. The RRPM increases from about 330 to 340 as its goes over 110MPH and then seems not to increase. This condition may be caused by the bigger heavier blades I use (25 X 8 SportRotors, 54 pounds). I have been told that the factory SportCopters have hit 130+ in a straight down full power dive, but really don't think I will try that. I have verified my airspeed indicator with a two way gps pass, and it is less than 3 percent off, so I know the numbers I report are good. If I had to list a VNE for my machine, I would go with the 120MPH no wind, and probably 80MPH with winds exceeding 10MPH or more. I understand the upper speed is far out of the range that most of you are willing to fly at in a open machine. That includes me 99.9 percent of the time.The wind blast is quite extreme, and if a bug hits you , it really hurts. Wind conditions have to be perfect for me to fly at higher speeds. The reality is that once the wind speed increases, my flying speed is going down even further than I listed to about 60MPH for safety reasons. I don't mind flying in very heavy winds if it is smooth, but will be looking to land ASAP if it gets gusty. Newer pilots should not attempt higher speeds until you have many hours of experience.

Scott Heger, Laguna Niguel, Ca N86SH

[birdy]

One reason why gyro's ' seem' to not feel the bumps, besides the high ' wing' loading, is coz they don't fly fast.
Every time we hit a ' bump', we do so relativly slowly, so any transition from level flight to a sink/ rise is steady. Hit the same bump in a faster FW and not only do you feel it more coz of the lower wing loading, but you 'hit' it faster also. The trasition time is reduced as AS increases.
IOW, the faster you fly, the rougher it 'seems' to get.
Same as driv'n on a rough dirt road, the faster you go, the rougher it seems to be.
The main difference is, air has compressability, BUT this difference is not so noticable on a lightly loaded fly'n surface, like a big H stab.
The rotors, be'n higher loaded than the stab, will react much less and later than the stab.[ at a high AS, the stab could be on its way back down, when the rotors have only just started to pitch down from the previous updraft, or worse still, vise versa.]
The faster you fly your gyro, the greater the discrepency between the reaction time/ effect of the rotors and stab.

This dangerous tendency can and should be designed out of any machine.

It can and should be Doug, but to wot speed limit.
If you use the teeter stops as a AS limiter guide , then only fly in calm air.
If your fly'n with the bar just off the stoppers, wot stresses are you going to put on the hole systm if you hit turbulance.

All the 'bench mark' VNEs mentioned so far don't allow enough buffer for turbulance.

A proper stable machine only pitches in relation to its plane of travel, not to the relative airflow, no??? It may be track'n with the airflow, but can it handle the stress of the increased rate of pitching when the AS is alot higher??
So, at wot rate of pitching is it starting to get dangerous?, see'n as pitching 'rate' will increase as AS increases.
And rotor turbulance off moutains/below tree lines can have some pretty savage changes in airflow directions, and can change many times in a short space o time.

Before anyone grabs me by the throat, i'm not lamblastn stabs, stability, CLT or anything, only try'n to figure wot happens at excessive speed, in any machine.

[CLS447]

Chuck B., If I increase the pitch on my SportCopter blades, will that make it climb better? What will that do to RRPM's ?

[C. Beaty]

Chuck B., If I increase the pitch on my SportCopter blades, will that make it climb better? What will that do to RRPM's ?
The amount of power the rotor eats in dragging the blades through the air varies as the cube of tip speed; double the tip speed and the profile drag power increases 8x.

So the answer is yes. Increasing the pitch reduces rotor rpm and leaves more power for climb. It also limits top speed as discussed previously.

As a general rule, rotor tip speed should be set for 420 ft/sec if 100 mph is fast enough. There will be considerable variation between different makes of blades and blade loading; the rotor limited top speed will be higher for correctly twisted blades or conversely, twisted blades can be operated at higher pitch setting and lower tip speed for a given top speed.

The NACA tested a full size Pitcarin PCA-2 Autogiro in a wind tunnel with various pitch settings. Best lift/drag, ~7:1, always occurred when air speed was ~35% of rotor tip speed in which case the stick will be on or near the forward stop.

http://naca.larc.nasa.gov/reports/1936/naca-report-515/

The NACA also tested 10’ model rotors with various airfoils and pitch settings and got about the same results.

http://naca.larc.nasa.gov/reports/1937/naca-report-552/

[Doug Riley]

Ray, I don't know what to say about Todd's experience. If he had a front pod, that might catch wind and "tuck," as Bensen's Gyroboat did and as many other pods do. That would be more likely to happen in a low-throttle dive, when the HS would be getting less prop wash.

McCutchens are fluky, too, although generally the "fluke" involves uncommanded ballooning -- as I'm sure Steve McGowan related.

I never ran into any sudden departure in my un-podded Air Command lowrider. It just rode more and more nose-low and got touchier. It never got to the point of being unstable; that is, it continued to require more and more forward stick pressure to speed up.

It did feel, however, as if there was a point beyond which the nose would "catch" and pull it over; I guess this feeling derived from the stability margin getting skinny (it took less and less ADDITIONAL forward pressure to add X mph as you speeded up). This was a HTL machine with no HS, however. As the nose rode lower, the lower (fixed) end of the trim spring came up and partly defeated the purpose of the spring.

Birdy, the evil tendency that I meant ought to be designed out is this bit about the nose riding lower and lower with increased airspeed. With a proper thrust line and H-stab, the deck angle won't change as you speed up. You're stuck with the harder ride in turbulence -- that's just the physics of faster accelerations; no different from driving too fast on a bumpy road. Turbulence in a 500 mph airliner feels like you're hitting rocks.

[RayPierce]

Ray,

Did he have McCutchen Skywheels on his machine?

I remember some discussion about the Skywheels doing this because the were not mass balanced on the quarter-chord point.

Or something to that effect.
No
They were Dragon Wings....that's why Todd asked ERNIE.

[gyromike]

No
They were Dragon Wings....that's why Todd asked ERNIE.

Gotcha.
Just askin'.

[GyroRon]

No
They were Dragon Wings....that's why Todd asked ERNIE.

I am pretty sure he had a set of 8 inch chord Dragon wings - EIGHT INCH CHORD!

Ernie only made a few sets of the 8 inchers and found there was problems with those blades and one way or the other asked that those blades be no longer flown. I know that at ROC a few years ago, Todd traded in those blades for a new set of 7 inch dragons and I think the problem went away.

[birdy]

Hmmmmm. looks like Paul's thread has had a premature death.

Still a few unanswered questions.
Is that coz no one knows the answers or no ones go'n to put his head on the block with 'theoretical' answers.
I think an understanding of WHY gyros have a VNE and why you shouldn't exceed it is a pretty important part of fly'n, coz all you need to push this envelope is excess power, minimal brains and maximum balls.[ recon theres a few on this forum thatd fit that catagory.]

A proper stable machine only pitches in relation to its plane of travel, not to the relative airflow, no??? It may be track'n with the airflow, but can it handle the stress of the increased rate of pitching when the AS is alot higher??
So, at wot rate of pitching is it starting to get dangerous?, see'n as pitching 'rate' will increase as AS increases.
And rotor turbulance off moutains/below tree lines can have some pretty savage changes in airflow directions, and can change many times in a short space o time.
If these questions have been answered, just tell me to shaddap.

[Doug Riley]

Birdy, I don't believe there's a "one-size-fits-all" answer.

All aircraft certified in the U.S. must have a "maneuvering speed" prescribed in the manual. This is a slowish flight speed that minimizes the airframe stress in rough air. I think that's the stress you're worried about, right?

In a gyro, I'd think that it would depend on the design whether you'd run into unacceptable frame stresses or unacceptable amounts of retreating-blade stall first, as you sped up. If you built a stout enough frame, the retreating-blade stall would determine VNE.

In general, high "G" numbers are much less likely in a rotorcraft than in a FW. To get 5 or 10 times the normal lift from a wing (IOW, to make 5 or 10 G's), all you have to do is overspeed it and cram on enough angle of attack. Dive and pull.

A rotor resists this process. The rotor's "airspeed" is dependent on its RPM, not on the forward speed of the frame. So adding diving for speed doesn't work to magnify lift the same way it does with FW.

You can't pile on RRPM that much, either. Tip drag acts like a "wall" to block large increases in RRPM. The power needed to bust through this wall is far more than you can expect to extract from the air in our windmill machines -- so you're not going to get huge lift gain by greatly overspeeding the rotor.

What's left? Increasing the rotor AOA. That, too, is a self-limiting process. As you add AOA by pulling back on the stick, you get more and more retreating-blade stall. That sucks power and gives little lift in return.

Jukka Tervamaki (ATE and Magni gyro designer) stated years ago that a gyro rotor can only achieve about 2.5 G's. Jim "Loopmaster" Vanek reported that his G-meter recorded momentary G's during loop entries of maybe 3.5 or 4 -- it was discussed on the old forum.

Bottom line? G's are made by extracting extra lift from your lifting device. It's easy to get multiples of normal lift from a FW. Rotors resist this process. Structural damage is less likely to be the speed limiter in a gyro than in a FW. You don't know for sure without in-flight strain guaging and/or G-metering, though.

Disclaimer: This ramble is based on general principles, not any personal experiment by me. I've never flown over 100 mph in a gyro and have no interest in doing so. 60-70 is more my speed.

[Aussie_Paul]

Doug, in Oz there have been a few people with G meters pull 5 Gs at the bottom of a spiral dive.

I have pulled a little over 3 Gs in my side by side A/C by speeding up the 70 mph, initiating a turn and heaving back on the stick. As with Jims loops this is only momentary. The energy disipates very quickly.

Question? If you have a gyro that is capable of conducting a 360 degree turn with a 60 degree angle of bank, will that show 2 Gs on the G meter?

Aussie Paul. :)

[Doug Riley]

Paul:

Utility category aircraft in the U.S. have to be able to stand something like 4.4 G's, plus a safety factor.

Yes, your G-meter will show 2 G in a LEVEL, stabilized, coordinated 60-deg banked turn if the meter's rigged correctly.

5 G's would have to be momentary. You'd be running the rotor at what is basically a "mush" angle of attack, but at high airspeed. Such an AOA would quickly slow you down to.. well, a mush. You couldn't get the RRPM up high enough to make a continuous 5 G's. That would require more than doubling RRPM, and I really doubt you can get enough power to the rotor to make it do that. I think I heard that Ernie Boyette took DW's up to some 600 RRPM by shaft-driving them with a big V-8... a far cry from what you can get to them through autorotation.

[Harry_S.]

Components may depart the aircraft and possibly...the Aviator.;)


Cheers :)

[Chopper Reid]

To answer the question of what happens if VNE is exceeded, would it be fair to say that the gyro would crash due to inflight break up or rotor strike ??