Low G and Rrpm decay rate

Mike G

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Doug Riley asked on a previous thread, that I cannot find, about rotor rpm decay rate as a function of low G.
Here is an actual recording using my GWS system during recent bunting trials.
G is read on the left axis and rate of change of Rrpm on the right. You can see that at a sustained 0.6 G the Rrpm decay rate was -15 rpm/second. The 1 sec lag between g and Rrpm rate is probably due to the treatment of the digital signal.
1634998089932.png

Mike G
 

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fara

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Thanks Mike for this measurement. I know its risky to do.
It seems to me at this rate at least on an AR-1 lightly loaded (and likely many other alike gyroplanes), you will need to be at 0.6 G for 3 to 4 seconds continuously before you get in the territory of being in danger of not recovering of rotor RPM. Fortunately that does not happen in normal weather phenomenon
 

fara

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How do you get into low g condition? Climb then nose down?
Yup even levelling out will do it but abrupt/quicker and unsmooth control inputs. A pull up and nose down or level with quick abrupt movements will do it. Cut the power on nosing down or it may not be good
 

Vance

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How do you get into low g condition? Climb then nose down?

A few of the things that I have found to cause a momentary low G event:

Pilot induced oscillations

Wind shear

Zoom climb (push over not required)

Push over

I suspect there are more.

I have not seen less than .6 Gs in The Predator no matter what I do.
 

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martin-av8r

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Merci beaucoup Mike pour ton travail , tes tests et ton partage de données , aussi pour le travail de Jean Claude en arrière - plan

thank you very much Mike for your work , testing and sharing data , also for Jean Claude work in the background
 

MAK

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Has anyone seen rotor decay in a power hover. Seems logical to me that it could happen, always keep my eye on RRPM when doing them, but never seen much of a decay.
 

Vance

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Has anyone seen rotor decay in a power hover. Seems logical to me that it could happen, always keep my eye on RRPM when doing them, but never seen much of a decay.
I don’t know what a “power hover” with a gyroplane is.

The gyroplane I fly will descend at around 20kts indicated air speed at wide open throttle.

If I have the nose up in order to minimize the descent at zero indicated air speed and wide open throttle the rotor rpm will decay.

In a power off zero airspeed descent the rotor rpm will not decay significantly.
 

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MAK

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If I have the nose up in order to minimize the descent at zero indicated air speed and wide open throttle the rotor rpm will decay.
Hi Vance, that is what I call a power hover. You say the "rotor rpm will decay", my feeling as well, but in real life I don't see it.
Have you seen it happen?
 

Vance

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Hi Vance, that is what I call a power hover. You say the "rotor rpm will decay", my feeling as well, but in real life I don't see it.
Have you seen it happen?
Yes, if I have her nose up at full power I may see the rotor rpm decay from three hundred thirty rotor rpm to perhaps three hundred depending on conditions. I have seen more on occasion.

I feel anything over two hundred seventy RPM is acceptable for my rotors on The Predator based on information from Sport Copter.
 

Tyger

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How do you get into low g condition? Climb then nose down?
Ya, notice how the Gs go up to nearly 2 first? I'm guessing a dive then pull up, then push the nose over just a bit.
The good thing about getting the positive Gs before trying this is the rrpm will begin the <1G portion higher than usual.
I would be nice to see actual rrpm graphed in addition to the delta.
 

Mike G

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Just for you Tyger, the G value is read off the left hand axis as 100 times g, so 2 g = 200 etc.
1635149731377.png
The aim of these trials was to simulate a"typical" bunt where the pilot zoom climbs and pushes over as seen in Japanese video on YouTube.
We're using the Rrpm decay rate as one of the parameters for predicting a bunting accident and giving the pilot advance warning.

Abid
Don't use this as an indication of how your gyro will behave, this was a relatively light gyro with little fuel a light pilot and a high inertia, large diameter rotor so not very representative. When I finally get to Florida we'll be doing this with your AR1 to set the GWS limits for you.

MAK just for you, a vertical zero airspeed (the green line) descent with a "behind the curve alarms". This was with the throttle back (yellow line Erpm/1000 read off the right hand axis), not at max power I'm afraid so doesn't really answer your question. The rpm falls from 310 to 290 and stabilizes there which seems to agree with Vance's observation
1635151214412.png

Martin thanks for your kind words, although the French forum wouldn't agree with you. Yes Jean Claude is quite involved I feed him my data and he uses it to compare with his calculations. Then he often questions my numbers if they don't agree. We then have to find out if the GWS or my instruments need correcting or his program needs to include a factor that he hadn't considered before. It's a win win, it allows me to know better where the limits are and to approach them with more confidence and he gets to improve his program which is already very powerful and remarkably accurate.

Mike G
 

Doug Riley

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The old accident reports from the Bensen era suggest that a common trigger of low-G crashes was simply levelling off after a turn.

An inexperienced pilot likely makes a tight turn, rolls out and feels the "ballooning" that occurs from the higher RRPM that develops during the turn. He/she then pushes the stick forward, which starts a low-G sequence. Therefore, the first curve, which starts with over 1G of RRPM, simulates this chain of events. In a gyro with PPO tendencies, such an event can become self-energizing.

A number of the Bensen crashes took place when the gyro was partway around the pattern, and was just rolling out of a pattern turn.

Experienced gyro pilots normally ignore the high-RRPM ballooning coming out of a tight turn, since it's momentary. In "trick" flying, you can exaggerate the ballooning by pulling back on the stick, intentionally trading airspeed for additional climb -- again, momentarily.

IMHO, it would be ideal if the gyro's airframe were configured so that the frame nosed up when rotor thrust was reduced. This tendency can be induced by using either a slight low prop thrustline (LTL), a downloaded H-stab, or both.

In my own experiments, I found that a substantial H-stab with a few degrees of negative incidence, if immersed even partly in the propwash, produced the same effect as a low thrustline -- even if the prop thrustline actually was slight ABOVE the CG.

It would be interesting to rig a gyro in this way and use Mike G's equipment to see if a self-correcting configuration is possible (or if some gyros already are self-correcting out of low G). IMHO, a potentially deadly scenario that can be reached inadvertently during an absolutely routine maneuver is not ideal.

As discussed in the other thread that Mike mentions, slightly outboard flap hinges reduce the loss of control power that occurs at low G, at some cost in rotorhead complexity and weight.
 

fara

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The old accident reports from the Bensen era suggest that a common trigger of low-G crashes was simply levelling off after a turn.

An inexperienced pilot likely makes a tight turn, rolls out and feels the "ballooning" that occurs from the higher RRPM that develops during the turn. He/she then pushes the stick forward, which starts a low-G sequence. Therefore, the first curve, which starts with over 1G of RRPM, simulates this chain of events. In a gyro with PPO tendencies, such an event can become self-energizing.

A number of the Bensen crashes took place when the gyro was partway around the pattern, and was just rolling out of a pattern turn.

Experienced gyro pilots normally ignore the high-RRPM ballooning coming out of a tight turn, since it's momentary. In "trick" flying, you can exaggerate the ballooning by pulling back on the stick, intentionally trading airspeed for additional climb -- again, momentarily.

IMHO, it would be ideal if the gyro's airframe were configured so that the frame nosed up when rotor thrust was reduced. This tendency can be induced by using either a slight low prop thrustline (LTL), a downloaded H-stab, or both.

In my own experiments, I found that a substantial H-stab with a few degrees of negative incidence, if immersed even partly in the propwash, produced the same effect as a low thrustline -- even if the prop thrustline actually was slight ABOVE the CG.

It would be interesting to rig a gyro in this way and use Mike G's equipment to see if a self-correcting configuration is possible (or if some gyros already are self-correcting out of low G). IMHO, a potentially deadly scenario that can be reached inadvertently during an absolutely routine maneuver is not ideal.

As discussed in the other thread that Mike mentions, slightly outboard flap hinges reduce the loss of control power that occurs at low G, at some cost in rotorhead complexity and weight.

I have yet to see any bunt over accident videos from older days that shows a bunt over from just coming out of a turn but perhaps the accident reports do show that. All the ones I know were climbing and either levelling out or climbing and then pushing down. Are there any accident reports where coming out of the turn accidents are listed. I imagine, the slight ballooning up would be corrected by an airplane pilot by pushing the nose down and that creates the bunt in absence of any Horizontal stabilizer to dampen and starting to correct an unbalanced vertical moment (due to power) causing a rotation around the CG if the power is not taken out of the equation

The following video shows a list of several accidents that all seem to be from inexperienced low time pilots or student pilots and the reasons listed are almost always, pull up and then ... None seem to show coming out of a turn or anything indicating similar thread


The outboard flapping hinge or offset flapping hinge seems to be not allowed by regulation in Europe or in the US for Sport pilot. Offset flapping hinge should also reduce 2/rev
 
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Tyger

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I may be misreading the graph, but shouldn't the rrpm (blue) continue to go up while the delta (yellow) is above zero (i.e. prior to time of 160)?
 

Vance

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I have yet to see any bunt over accident videos from older days that shows a bunt over from just coming out of a turn but perhaps the accident reports do show that. All the ones I know were climbing and either levelling out or climbing and then pushing down. Are there any accident reports where coming out of the turn accidents are listed. I imagine, the slight ballooning up would be corrected by an airplane pilot by pushing the nose down and that creates the bunt in absence of any Horizontal stabilizer to dampen and starting to correct an unbalanced vertical moment (due to power) causing a rotation around the CG if the power is not taken out of the equation

The following video shows a list of several accidents that all seem to be from inexperienced low time pilots or student pilots and the reasons listed are almost always, pull up and then ... None seem to show coming out of a turn or anything indicating similar thread


The outboard flapping hinge or offset flapping hinge seems to be not allowed by regulation in Europe or in the US for Sport pilot. Offset flapping hinge should also reduce 2/rev
I feel there are not more videos of the accidents that Doug is describing because there is not much reason to video a flight of someone just flying the pattern.

An airshow demonstration is more likely to have a video of it.

It appears to me the video shows a bunt over at the top of a zoom climb in a high thrust line gyroplane.

It appears to me as the gyroplane nears to top of the zoom climb the rotor slows down and the pilot no longer has control over it.

In my opinion the rotor is no longer providing thrust to resist the overturning moment from the high thrust line wide open throttle.

I feel this causes the gyroplane to nose over without significant pilot input.

Perhaps if the pilot had reduced the throttle he might have reduced the overturning moment in time to break the accident chain.
 

Doug Riley

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Fara, one crash-out-of-a-turn incident in the UK specifically comes to mind, but there were others. Old PRA magazines and chapter newsletters are the best (OK, least poor) sources.

Accident reporting in, say, the 1960's, 70's and 80's was hit-or-miss. Photographic accounts (video or still) are almost unheard-of. Many U.S. crashes did not make it into the FAA records. We're left with hearsay verbal accounts from within our gyro community, or the very rare movie record, such as the Pee Wee Judge-Wallis crash at Farnborough around 1970.

P.S. Vance is right on, in my opinion. If you look very closely at the rotor and the pilot's stick hand, you'll see that he pulled back as the nose started to drop, and got no response whatsoever from the airframe. Chilling. Besides closing the throttle, another common tactic to prevent the type of accident shown in the video is to bank into a turn after initiating a zoom climb.
 
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fara

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Fara, one crash-out-of-a-turn incident in the UK specifically comes to mind, but there were others. Old PRA magazines and chapter newsletters are the best (OK, least poor) sources.

Accident reporting in, say, the 1960's, 70's and 80's was hit-or-miss. Photographic accounts (video or still) are almost unheard-of. Many U.S. crashes did not make it into the FAA records. We're left with hearsay verbal accounts from within our gyro community, or the very rare movie record, such as the Pee Wee Judge-Wallis crash at Farnborough around 1970.

P.S. Vance is right on, in my opinion. If you look very closely at the rotor and the pilot's stick hand, you'll see that he pulled back as the nose started to drop, and got no response whatsoever from the airframe. Chilling. Besides closing the throttle, another common tactic to prevent the type of accident shown in the video is to bank into a turn after initiating a zoom climb.

That is very likely for the 60's, 70's and 80's accidents. Thanks for clarifying.
I think what I see in the video watching it on a big screen is that after the low pass he pulled stick smoothly back to initiate a climb and as he got to the top, he did a quick jab at first forward which started rotation forward. That quick jab did him in. After that yes he did try pulling stick back but the rotation was already set in and in the absence of a dampening surface there was nothing slowing it down to allow pilot to do anything afterwards. Very quick and very sad.
Certainly rolling into a turn instead would have kept the G's to allow control. These quick little jabs forward is not a rotorcraft thing. This is not even good technique in airplanes or trikes. I see some trike and airplane pilots using this and I consider it sloppy pilot technique but there is aerodynamic control in airplanes with elevator with dampening from a horizontal tail and there is usually plenty of washout and delta shape of the wing with billow on each wing allows the tips on the trike that are behind the CG to act as a horizontal stabilizer and there is enough dampening due to that to allow corrective action in trikes till a certain point. A high thrust line with no dampening or balancing surface in earlier gyroplanes leaves them with an unbalanced moment that when poked simply quickly turns into unrecoverable rotation. Very quickly. Giving no time to have corrective action applied.
 
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Vance

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That is very likely for the 60's, 70's and 80's accidents. Thanks for clarifying.
I think what I see in the video watching it on a big screen is that after the low pass he pulled stick smoothly back to initiate a climb and as he got to the top, he did a quick jab at first forward which started rotation forward. That quick jab did him in. After that yes he did try pulling stick back but the rotation was already set in and in the absence of a dampening surface there was nothing slowing it down to allow pilot to do anything afterwards. Very quick and very sad.
Certainly rolling into a turn instead would have kept the G's to allow control. These quick little jabs forward is not a rotorcraft thing. This is not even good technique in airplanes or trikes. I see some trike and airplane pilots using this and I consider it sloppy pilot technique but there is aerodynamic control in airplanes with elevator with dampening from a horizontal tail and there is usually plenty of washout and delta shape of the wing with billow on each wing allows the tips on the trike that are behind the CG to act as a horizontal stabilizer and there is enough dampening due to that to allow corrective action in trikes till a certain point. A high thrust line with no dampening or balancing surface in earlier gyroplanes leaves them with an unbalanced moment that when poked simply quickly turns into unrecoverable rotation. Very quickly. Giving no time to have corrective action applied.
In my opinion based on my gyroplane flying experience a quick jab of the cyclic won't initiate anything.

In my opinion if he had three hundred pounds of thrust a foot above the center of gravity there are three hundred foot pounds of torque trying to rotate the aircraft around its center of gravity.

When the opposing rotor thrust is removed the gyroplane rotates forward without pilot input.

I feel this is an important distinction.

At one time I imagined if I kept the stick back a forward tumble was impossible.

I discovered this was not the case at the Cable Air Show flying a Cavalon performing my typical routine that I fly in the near centerline thrust Predator.

Near the top of a zoom climb I discovered I had no control of the rotor with the cyclic.

I was not able initiate a turn.

I closed the throttle and as she began to sink rotor control was restored.

I found this experience disquieting and suspect the delay of the rotation from the horizontal stabilizer is what gave me enough time to explore my options.

Upon reflection I realized the horizontal stabilizer is not very effective at low indicated air speed.
 

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Doug Riley

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It's very important to untangle the multiple effects of a fore-aft tilting of the rotor spindle. The results of such a change in spindle angle are not precisely like the results of similar inputs in any other aircraft. Longitudinal control in a gyro is a unique animal.

In chronological order, here's what happens when you change the spindle tilt relative to the oncoming airflow (OK, Vance, here's what happens in my opinion):

(1) Cyclic pitch change: the spindle-and-teeter hinge assembly works exactly like a swashplate system, causing a cyclic pitch change having maximum at 3 o'clock and 9 o'clock. This effect is purely mechanical and happens in no more than 1/4 of a rev.

(2) the rotor precesses (changes the alignment of its rotational axis in space) in response to the newly-unbalanced lift between one side of the disk and the other. Depending on the lift curve of the airfoil, the RRPM and the mass of the rotor, this movement may take between a rev and a few revs. Until this movement is underway, the airframe experiences no effect from the change in spindle tilt (hence our sense that our controls have a bit of lag).

(3) rotor thrust angle and disk AOA change simultaneously. Breaking it down:

(a) the rotor disk tilts either more nearly level or more steeply aft. If there is rotor thrust, the changed disk angle alters the angle of the rotor's pull on the frame; this in turn will rotate the airframe either nose-up or nose-down. If there's no rotor thrust, the disk will still precess, but it will not affect the stance of the airframe at all.

(b) the rotor blades EACH experience a change in their angles of attack, on top of the earlier cyclic change. E.g., if the disk tilts more aft relative to the airflow, each blade will "see" an increase in its "personal" angle of attack, all the way around its orbit. From each blade's viewpoint, this effect is quite similar to a change in collective pitch in a helo. It's at this stage that this whole process results in a change in the magnitude of the rotor's thrust: An aft tilt increases this thrust, while a forward tilt reduces it. Note, especially, that this change in thrust PRECEDES any change in RRPM. IOW, the change in rotor thrust happens within a couple of rotor revolutions and is initially angle-of-attack related, not RRPM related.

(4) RRPM changes. The change in each blade's AOA in #3(b) changes the amount of autorotational drive that that blade makes. An increase in blade AOA creates more drive and speeds up the rotor; a decrease in blade AOA reduces the drive and allows the rotor to slow down. This is the longest-term of the three effects that a fore-aft change in spindle angle creates. It's also the hardest to un-do, because the rotor has mass and can't be instantly sped up or slowed down.

The pilot in the Japanese PPO video probably was unlucky enough that his rotor's angle to the airflow stayed at zero even when he pulled the stick back. This is possible because the nose had already dipped from the HTL effect, while the gyro continued on a ballistic trajectory, still travelling level while the nose pointed downward.

Vance, I imagine that a forward pulse on the stick actually could set off a PPO sequence in a vulnerable gyro -- one with a high thrustline and inadequate H-stab. A gyro with this kind of layout is a loaded mousetrap, waiting to spring.

A low-mounted H-stab, out of the propwash and with no incidence, will slow or even stop the PPO process -- even if the gyro has a small amount of HTL.

However, IMHO, it's even better to set the H-stab and/or prop thrustline so that the nose rises (stick fixed) when rotor thrust goes to zero. By being immersed partially or completely in the propwash, the H-stab will do this work even at zero airspeed -- at least it will when power is up, which is when it matters.

In terms of Mike G's curves, such a gyro would show a recovery of RRPM and G-load with the stick locked.
 
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