NTSB and Loss of RRPM??

[bmoore2156]

I have been reading alot of the NTSB reports. I did this before I bought my gyro and now that I am flying it, I decided to reread them in hopes I can avoid putting my name on the list and learn from others mistakes.
One thing I noticed in ALOT of the accidents are... "Pilot failed to maintain proper RRPM."
These are not helicopters and it's not like we can lower collective to "maintain RRPM"
Are they refering to keeping the rotors loaded, and maintaining proper airspeed during an emergency landing? How else could you manange your RRPM while in flight?
Thanks,
Brad

[Minnesota_Mike]

Maintaining flight speed within the power curve.

One of my friends most humbling moments while learning in his Bensen was when making one of his 180* turns, outside of the power curve.
Within seconds he was rapidly descending to the ground in a forced flair out from about 125.'
The machine vertically descended- impacted, bounced 15', pitched forward while impacting again, rolled and roto-tilled sideways into the soybean field.
He escaped the reaper and walked away sore and scratched.
Nothing salvageable remained but the Aluminum.
To say he was lucky is an understatement.

[ferranrosello]

I agree entirely, Brad. Has no sense to talk about maintaining rotor rpm in a gyro without collective. Of course, the rotor rpm can change with some maneouvers, but in a limited range compared with an helicopter. The thing we need to maintain is an airspeed.

Ferràn

[C. Beaty]

NTSB reports are a good resource as to number, date, time, weather conditions, etc. and with judicious reading, at least an indication of probable cause. In most cases, the investigator has never before seen a gyro, much less with any understanding of how they work.

“Failure to maintain rotor rpm” means, in general, that the rotor chopped off the tail as the machine tumbled forward. The chopping typically begins before there has been a significant reduction of rotor rpm.

“Pilot failed to maintain control” has about the same meaning.

[bmoore2156]

So, to sum this up, something else happend to casue a "bunt over" or PPO (Power push over) which is mainly seen in high thrust line and unstable gyro's.
Thanks,
Brad

[PTKay]

On another thread we are discussing electric in-hub prerotators with
partially powered rotor capability.

I mentioned there, as one of the possible advantages of such design
a possibility to build in a "safety function" if form of kind of "rotor governor".

When the rotor rpm (RRPM) drop below certain value the controller
should automatically apply power to the in hub motor to regain RRPM.
It will, of course create some yaw reaction, but it's probably less
dangerous than loosing the RRPM and lift.

Low G manoeuvres unloading the rotor and killing the RRPM
are one the most serious causes of accidents IMHO.

What do you think about it?

Any comments?

:argue:

[Vance]

I find value in reading the NTSB reports for all aircraft.

Low rotor rpm may be indicated by the rotor blades folding upward in flight or hitting something.

In my opinion, an effect of rapid pitch excursions may be intermittent low G events and blade stall; either of which may cause low rotor rpm.

I feel that inverting the gyroplane may also cause the rotor rpms to decay.

The pilot in command does have options in avoiding things that will slow the rotor.

It is also possible that the person writing the report did not understand how gyroplanes work and their causal analysis is in error.

Thank you, Vance

[Ross Wheeler]

Vance, what will cause the rotor blades to fold upward as you mentioned? Would this a result of PIO or can the rotor blades be loaded to much and just snap upward?

Thanks, Ross

[ferranrosello]

A decay in rotor rpm implies a lack of centrifugal force acting on the rotor blade. But the rotor blade is still generating lift... so it folds upward.

Ferràn

[scott heger]

NTSB explanations regarding low rotor RPM gyro accidents are very little help. Sometimes in bunt overs this is mentioned, but the real damage has already been done in the tumble.

However there are several techniques available for gyro pilots to use to avoid lower than normal RRPM near the ground(mushing in normally caused by too low airspeed). Preloading the rotors, normally using a series of sharp turns, to yield 20-30% or more than normal additional RRPms. This is an advanced pilot method of getting lots of lift out of your gyro near the ground in a tight area. I believe Birdy and and Ron A have videos showing several ways this is done. The results are amassing, and a gyro can be set down in a 180 degree opposite direction from flight if necessary from low level and very low ending airspeed. Timing is extremely critical, and it not something that should be tried without a lot of flight hours and a high performance machine. Don't try this at home folks!!!!!!



Scott Heger, Laguna Niguel,Ca N86SH

[Vance]

Vance, what will cause the rotor blades to fold upward as you mentioned? Would this a result of PIO or can the rotor blades be loaded to much and just snap upward?

Thanks, Ross

Sorry Ross, I missed your question in my haste to write about my limited progress on the Predator and get cleaned up from the day’s endeavors.

I feel that Ferran described it correctly.

It is my understanding that rotor blades get their strength from centripetal force and if the rotor rpm decays enough there is not enough strength to keep the lift generated from bending the blades upward.

One of the wonderful things about a gyroplane is it takes care of managing the rotor rpm simply by providing lift, more lift more rotor rpm. The rotor pitch is fixed.

In my opinion an extended low G event or a blade stall are the most common ways to achieve low rotor rpm in flight with a gyroplane. Blade flapping can be part of this event.

In a helicopter rotor rpm is controlled by the pilot applying engine power and/or adjusting the pitch of the blades. These are not options in a gyroplane.

Thank you, Vance

[C. Beaty]

Vance, do you not find it strange that no stable gyro has ever bored a smoking hole as a result of “pilot failed to maintain rotor rpm”?

[Vance]

Hello Chuck,

I don’t know enough about gyroplane stability to draw that sort of conclusion.

The reports aren’t really clear on the configuration of the accident aircraft.

The last time I posted about stability it was not fun.

I don’t want to debate anyone.

I still don’t understand about my friends Terry and Doc.

Some people tell me the Predator is an unstable configuration.

You tell me she is too heavy.

You tell me I would be safer if I trained without a CFI.

Thank you, Vance

[scott heger]

Vance,you have a very nice unique gyro. I have seen it, on the ground and flying.

Chuck, has alot of mostly good opinions, but I don't think he has ever seen your gyro, in person or fly. He once told me by seeing the picture of my SportCopter that it was high thrustline unstable. Now you have watched my SportCopter fly for hours doing pretty much every maneuver short of a loop. Did it ever act or look unstable? From the pilots seat it sure is not. So take what Chuck says with the weight of an opinion made from someone who has never flown your gyro.

A battleship class gyro like yours has the advantages of taking a passenger with a certified engine and being a more cross "county"(notice I didn't say cross country) touring machine.

You are missing some great fun of what single place machines do best, crank and bank.

Just like motorcycles, all the different models do different things, Harley's do one thing, Honda CBR's do another. Short of having two gyros, I think you made a great choice for the type of flying you want to do. I have a 70 mile fuel tank limit. Mine is never going to be flown too far. Your leisure flying time schedule far exceeds what I can do. In a few years you will have more gyro hours than I have in 10 years. Your commitment to this sport is substantial , and just keep it safe. Hope to see you soon.

Scott Heger, Laguna Niguel,Ca N86SH

[Hognose]

“Failure to maintain rotor rpm” means, in general, that the rotor chopped off the tail as the machine tumbled forward. The chopping typically begins before there has been a significant reduction of rotor rpm.

There are some mishaps in which the rotor did not strike aircraft structure, but instead folded upwards. This is due to lost RRPM due to low G in a teetering rotor gyro that has near center-line thrust and is not producing a lot of forward thrust.

Without an offset thrustline to rotate the unloaded machine around the lateral axis, or torque to rotate the machine around the longitudinal axis, nothing rotates the machine at all and it is ballistic from that point onward. Rotor rotation may stop entirely and the blades, if they depend on centrifugal force for their strength when loaded, will fold upwards as the rotor slows down.

Unloading a teetering-rotor rotorcraft (gyro or helicopter) is always unwise. In a helicopter it is one major cause of mast bumping. Unloading a gyro with a high thrustline can instantaneously cause power push over as Chuck describes. It can also, in a little more time, lead to torque roll (in any gyro) or given enough rotor rpm reduction, the fold and collapse I described above.

This folding failure is also seen if the pilot loses rotor RPM in a helicopter in autorotation, particularly a Robbies. Because of this, it may be more familiar to NTSB investigators than other modes of gyroplane failure.

These accidents are not generally survivable.

cheers

-=K=-

[C. Beaty]

I‘m not convinced there has ever been a rotor folding accident in a gyroplane; or if it has occurred, it was too late anyhow.

With translational velocity, a slowed rotor will “flap” and flail around long before it can fold. By “flap”, I mean the sort of thing that sometimes happens when the rotor is crowded during rotor startup rather than normal cyclic flapping.

The photographs I’ve seen of “folded” rotors could as easily have been the result of ground impact.

Most of the “failure to maintain rotor rpm” reports are from interviews of non-expert witnesses who perhaps did observe a slowed rotor after the stall tumble.

A HTL gyro behaves exactly like a rotary lawn sprinkler when the rotor is unloaded. The high propeller thrust line begins a very high rate of nose down pitch acceleration about the CG, the rate of rotation being faster than the rotor’s ability to follow. The rotor precesses through the generation of differential lift between one side and the other of the rotor disc; if its capacity for differential lift is exceeded, the rotor stalls and the airframe runs into it.

At zero airspeed, a gyro is a parachute and the pilot can do nothing to unload the rotor.

[Jazzenjohn]

What would happen if a high powered 2 place gyro was flown by one lightweight person and was hanging on the prop for an extended period of time? Wouldn't the blades slow to a dangerous speed, possibly low enough to have the hub bar bend, or is that not possible? What would the pilot feel through the stick before it got dangerously bad?

[MichaelBurton]

What would happen if a high powered 2 place gyro was flown by one lightweight person and was hanging on the prop for an extended period of time? Wouldn't the blades slow to a dangerous speed, possibly low enough to have the hub bar bend, or is that not possible? What would the pilot feel through the stick before it got dangerously bad?

Airspeed does not govern rotor rpm lift does. An extended zero forward airspeed will not cause a dangerously low rotor rpm.

[C. Beaty]

The vertical component of prop thrust does reduce the load on the rotor.

The vertical component of prop thrust varies as the sin of the angle; at 30º the vertical component is 0.5, at 90º it is 1.

If you had 800 lbs of prop thrust on an 800 lb machine, it should be possible to stand it vertically with zero load on the rotor.

Most people that have a gyro with good power to weight ratio will notice a significant reduction of rotor rpm during a steep climb at WOT.

But I think torque roll will get you before the rotor folds up.

[PTKay]

Chuck,

I would like to repeat my theoretical question put before.
With an electric in-hub motor prerotator, as already presented in Germany
could we have a safety feture like this:

When the rotor rpm (RRPM) drop below certain value the controller
should automatically apply power to the in hub motor to regain RRPM.
It will, of course create some yaw reaction, but it's probably less
dangerous than loosing the RRPM and lift.

Thanks in advance for your comment.

Paul

[C. Beaty]

Gyros don’t crash as a result of low rotor rpm, Paul. Low rotor rpm is a side effect of something more serious just as a headache may be a side effect of brain tumor. Aspirin may relieve the headache but it won’t cure the tumor.

Stable gyros don’t crash. Unstable gyros crash when a gust or sometimes the pilot unloads the rotor and forces beyond his control take over.

If someone must fly an unstable gyro, the tumble could be prevented if he could instantly chop power. Power off, there is no difference between one gyro and the next.

[Jean Claude]

We wonder about accidents in the sky, not because of hardware failure.
The HTL can be responsible? Yes:
I calculated the angle of flapping of a rotor (24 foot blade 20 lbs 350 RPM) when subjected to a pitch rate. I get 9 ° (stop flapping blades) for a pitch rate of 100 ° per second. Such a roll rate is possible?
If the pilot suddenly removes the rotor thrust (g = 0), this rate of dangerous pitch of 100 ° / swill be reached in just 1 second (calculated HTL = 5 inches, engine thrust = 160 lbs, pitch inertia = 1200 lb. ft square)
After that, the stick is pushed by the rotor to the rear stop (0.25 seconds) while the pitch rate is maintained.
I think the pilot did not have time to understand the need to stop the engine.
In 1 seconds from 0 g, the rotor loses only 20 RPM (no significant).
Jean Claude

[PTKay]

I think the pilot did not have time to understand the need to stop the engine.
In 1 seconds from 0 g, the rotor loses only 20 RPM (not significant).
Jean Claude

Chuck, Jean Claude,

very well explained, "loss of RRPM" is not an issue in gyroplanes,
hence no need for extra devices.

Thanks.

Paul

[C. Beaty]

A HTL gyro becoming a yo-yo or rotary lawn sprinkler when equilibrium is disturbed is only part of the story.

In steady trimmed flight, the rotor thrust vector must pass forward of the CG to balance the propeller thrust offset and to maintain equilibrium, making a tail heavy machine and all that entails.

Tail heavy aircraft, whether fixed wing or gyroplane, are unstable vs. angle of attack. An upward gust tends to pitch such a machine nose up, magnifying the effect of the gust.

Such aircraft can be flown by most pilots in most circumstances with sufficient training and experience, leading to the mistaken belief that training is the solution to the awful accident rate of gyroplanes.

[Jean Claude]

I agree with you, Chuck, but a bicycle is unstable, yet it can be used. So it seemed necessary to quantify. At what time an unstable aircraft becomes dangerous? Yes, a dramatic reduction of "g" by the turbulence, with this aircraft HTL, produces the same effect. To undergo g = 0, simply enter abruptly in -6 m / s (-7 ° rotor) After it is likely that the pitch "nose down" perpetuates this situation several seconds to become deadly.The best pilot can do nothing.
Jean Claude

[Brent Drake]

Jean.
Most people fall off a bicycle many times before getting it right. Flying in a gyro, your considered lucky, if you make the fall the first time and survive. I don't believe a gyro compares at all to a bicycle. Yes I understand your point. But I do not believe that is a good comparison.

[ferranrosello]

I agree that in normal flight the loss of rotor rpm is much more a side effect than the main cause of accidents. But, in my opinion, it doesn't mean this is not a factor in gyroplane accidents.

Firstly, "bad flapping" in take off is because of a too low rotor rpm, and this thing has caused some fatal accidents.

Secondly, if the rotor is unloaded in flight in a stable gyro (the unstable one will push-over) the pilot will be not more in command... A lot of bad things could happen still. The last one could be a bad flapping while reloading the rotor.

Ferràn

[Jean Claude]

That's so what I want to know. Ferràn. For this, I prepared a table Excel. it tells me the flapping angle for differents aerodynamic pitch, diameter, angle of attack of the rotor, flight speed, RPM. It shows always a maximum at 45 ° rotor. Only if RPM < 0.8 RPM normal, then the flapping blade exceed 9 ° (at crusing speed)... but this assumes negligible a main phenomenon! The Blade stall. (I'm sure the torque of rotation persists well beyond the stall of the blades, but I am not sure of the result of the flapping angle)
Jean Claude

[Resasi]

A question if I may regarding loss of control at the other end of the spectrum where one begins to go faster and faster and the rotors load up.

My understanding is that the controls with become more and more sensitive. If during this phase an oscillation begins and the power is not reduced and the gyro begins to nose over steeply, what sequence of actions follow?

How soon will it be before the gyro inverts?

And at that stage will the rotors fold, or, because they are still rotating at some speed maintain rigidity but contact the airframe and breakage of airframe, rotors, or both, occur?

Has loss of control like this been observed and described?

[ferranrosello]

First of all, more airspeed does not mean more rotors load. The load depens on the lift, not the airspeed.

Secondly. because of the rotors inherent stability, it is not possible to go faster and faster. Rotor's blowback due to airspeed and flapping hinges puts a natural limit to airspeed.

The only possibility to an uncontrolled pitch down tendency are bad blades, with negative pitching moment. If this was the case there is no solution...

Ferràn

[Resasi]

Thank you for your response Ferran. I acknowledge my misconstruction on rotors loading up as speed increases.

Not looking for the solution. Simply, if possible, any other eyewitness report on subsequent behaviour when a gyro does, for whatever reason, begin a steep pitch down descent that end in catastrophe.

Attempting if possible to interpret what was seen recently from a distance with no observable detail.