accidents.. areas to avoid your advice

Doug Riley

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G'day, Jordan -- you nailed it in Post #51. "Zero G" is one of those colorful and inaccurate expressions that we use so often in aviation.

Sure, the condition FEELS in your butt as if gravity has just been switched off. We know it hasn't.

The condition we nickname "zero G" is more usefully identified as "zero-lift disk AOA." Either a downdraft or a forward stick movement reduces disk AOA to the point where there's essentially no lift coming out of the rotor. In terms of what the rotorblades "see," it's identical to slamming down the collective in a helicopter. The blades experience a drastic reduction in their individual angles of attack -- actually below zero degrees if the blades are cambered.

The pilot may have a perfectly good reason to WANT zero lift for a moment (say to level out of a zoom climb). There is an unwanted byproduct of reducing disk AOA, though.

Uniquely in a gyro, positive disk AOA is necessary to keep the "air drive" that turns our rotors engaged. At zero AOA, we not only turn off the lift, but we also, in effect, release the "air clutch" that indirectly transmits power from the engine to the rotor. RRPM drops because there's no air-power going to the rotor anymore. That's the ultimate danger of zero G.

However, nearly all so-called "zero G" accidents are most likely NOT mainly the result of loss of RRPM. Losing RRPM to the point of no return typically takes longer than most of the aircraft "flips" that we see.

Instead, the accident is the result of the interruption of rotor thrust in an aircraft that NEEDS rotor thrust to stay upright. Don't build your gyro so that it needs rotor thrust to stay upright and you will eliminate this type of accident.

This means designing the frame to have no net pitching moment (or better yet, a net positive pitching moment), taking into account the drag of the frame itself and the effect of thrustline placement. It also means building in some form of torque compensation to prevent torque-over. IOW, don't rely on rotor thrust either to hold the nose up against engine thrust or to resist the torque reaction from the prop.

A gyro with uncompensated HTL can certainly experience a PPO that begins with mere fractional G, not zero G. That's because such a gyro drops it nose any time disk AOA is reduced slightly while throttle remains high. The nose-drop (if the pilot holds the stick firmly) further reduces disk AOA, causing more drop, and so on in a vicious cycle. That's exactly what a PPO is.

Vance and Fara -- yes, instances of zero and even negative G are inevitable in thermally summer air. And, yes (if the gyro doesn't instantly flip out of the air because of lousy design and naive piloting), these natural zero/neg G events will be of short duration because either (1) the gyro will fly out of the "down air" or (2) the gyro will begin sinking in response to the loss of lift, thereby "catching up" with the sinking air and restoring a normal disk AOA.

Imagine a gyro flying through a large patch of really rough stuff, though. It's possible (on paper anyway) that the gyro could fly into a large downdraft that is more intense at its center than at its edges. The gyro would be continually falling, trying to restore its own disk AOA, but flying into stronger and stronger down air. It might not be able to catch up, resulting in lower and lower RRPM until the loss of RRPM became critical.

I don't know enough meteorology to say whether this kind of large "natural sustained low G" could happen. Certain design features would make this scenario less likely; the ultimate might be Dick DeGraw's gyrodyne drive.

A pilot-induced sustained low G would take the form of an entry to an outside loop.

Again, however, our real-world problem has not been storm-chasing or outside looping. Most in-air flips have resulted from airframes with perverse moments, flown by pilots who didn't know how to compensate for these design errors.
 

WaspAir

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I think it would be instructive if each participant here could actually fly with a proper and accurate g-meter in the aircraft through turbulence, and watch the meter indications. Even better would be to reset the max/min needles, cover it, fly through some bumps, uncover it, and see how far off your seat of the pants estimate was.

Most people will think they are weightless at low positive g, and will mis-identify the condition. 0.25g simply feels odd to critters who evolved in a reliable 1g world, and it seems that it "must" be zero. It's a common human response; many people not accustomed to acrobatic flight will swear they were inverted it taken quickly and briefly to a bank angle of over 80 degrees (even though never passing 90) because it so different from their ordinary environment.

I have flown with a g-meter in my sailplane through standing lee mountain wave systems, including the worst of the rotor flow (I'm talking about the weather phenomenon, not a rotary wing), and I have seen a SuperCub towplane land with seven degrees more dihedral in the left wing than when it took off (and five more than in the right wing) after towing gliders through that same rotor. I have hit vertical shear so sudden and abrupt that it sucked my dive brakes open and extended my landing gear, and I watch a friend that same day get swatted down like a fly into a barbed wire fence. I know what the atmosphere can do.

But I really suspect that rapid onset of a different g-force fools people, and that they have no innate calibration in their senses for any value other than +1g.

It is entirely possible to fly a zoom climb, feel a little light at the top, and come back down again, having been in positive g the entire time. I am not suggesting the gyro pilots should feel safe doing so; I am instead suggesting that your butt is not a very reliable g-meter unless you have worked long and hard (and recently) to calibrate it.
 

Vance

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I will try to answer you question Brian

I will try to answer you question Brian

Ok...so I thought zero-g was required. Are you saying a gyro can flip even when in normal flight maneuvers, if the prop thrust is pushing above the center of mass of the fuselage?
In my opinion if a gyroplane has a thrust line well above the center of gravity it is using something in forward flight to prevent the thrust from rotating from the misaligned forces. Some high thustline gyroplanes rely on the drag of the rotor to balance the torque.

I feel when this rotor drag is reduced in a low g event particularly if at high power it may be possible for the gyroplane to experience a power push over. In my opinion it only takes a reduction in rotor drag to upset the balance. I feel more thrust, more offset or less rotor drag work together to upset the balance of forces.

Because not all low g events are commanded this can create a PPO hazard in windy conditions.

I was taught to reduce power when confronted with a low g event in a high thrust line gyroplane.

Some people feel this high thrust line torque can be balanced with a lot of horizontal stabilizer volume in which case they feel the drag of the rotor becomes less important.

Thank you, Vance
 

brett s

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Exactly - feeling light in the seat doesn't mean zero or negative g's.

Most probably overestimate the positive side too - most helicopters would have a hard time getting past 3 g's, the rotor stalls.

Off topic, but if you want a real eye-opener trailer an aircraft with a g meter installed & see how high it can get. It should convince folks that leaving the blades on is a really bad idea if nothing else...
 

Rusty Russell

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Getting behind the power curve... anywhere?
Rotor management before and after flight ( from one who knows!
 

birdy

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Man Doug, you are an encyclopedia and I learn so much from guys like you and Birdy! Thanks.
Never put me in this class.

Has anyone here ever been caught in and flown a gyroplane through similar server turbulence for any length of time and if so what was it like?
Nuthn special John, but i got caught in a washn machine a few years ago.

Theres a gap in the ranges between ere n town that i often fly through, mainly coz its too high [ for me ] to fly over.
This day it was blown 40-50 mph and across this gap, 1/2 mile wide x 3 mile long and 2000' deep.
Well f*** me, iv never before or since been so petrified in any situation.
Roll turb off the top ment i couldnt climb out and the constantly changn wind directions had me pinned in the gap. Sumtimes itd try to bash me into one side of the gap so id be full soot tryn to stay off the rocks, then itd change and id be full soot pointn the other way. Calmest air seemed to be closest to the ground, but i didnt want to get any closer n 50' in these conditions.
Trees standn still out one side, and gettn their limbs riped off out the other side, was either on WOT or idle, dependn on wot i hit.
20 mins of this n i finaly broke out of it [ 20 mins to cover 3 miles.]
Oh, yeh, plenty of belt tension testn in that 20 mins too. ;)
 

birdy

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I am instead suggesting that your butt is not a very reliable g-meter unless you have worked long and hard (and recently) to calibrate it.

If the shoulder strap [ if you have one] bruises your shoulder, it was -g.
If you didnt tighten the strap, it was between 1 and 0.
 

Doug Riley

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Vance: My opinion of your opinion in Post 64 is that I agree EXCEPT for one thing.

It is misleading to think of rotor DRAG as the force that holds the nose up in a HTL craft. The rotor produces 4-5 times as much lift as drag; the two together are really a single force, namely rotor thrust. The rotor's thrust line is not horizontally aft (as the drag component is, by definition). The rotor's thrustline pulls up-and aft about ten-12 degrees aft of vertical. In a Bensen, it's roughly parallel to the mast.

In a HTL gyro without a compensating H-stab, this rotor thrust line comes down in front of the aircraft's CG, pulling up and back. This total force, or thrust, is what holds the nose up agianst the pushover tendency. When the thrust is reduced, the nose begins to push over unless throttle is cut.
 

Vance

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I stand corrected Doug.

I stand corrected Doug.

I did not mean to be misleading.

I was trying to address what I feel is a misconception that it requires zero Gs to have a power push over.

I was trying to keep the concepts simple to avoid further misunderstandings or a potential debate.

I do not possess your level of understanding or communication skills.

Thank you for the correction, Vance
 

ferranrosello

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Thank you very much for your answers.

However there are two no answered questions:

1st. Can you imagine different ways to command a 0 g situation in a gyro than pushing the stick hardly forward?

2nd. Doug, I know that is unlikely to have a commanded 0g situation lasting enough to produce a critical rrpm loss. I appreciate very much your comment about manufacturers providing this information. I think that this is a shortfall in all gyro certification regulations. And I believe that this is a cause of accidents in stable gyros.

The related question is How is going to develop a sustained 0g situation in a stable gyro (a gyro not prone to PPO).

Ferran
 

greeny

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Non castoring nose wheels?

Non castoring nose wheels?

most classic accidents seem to fall into one of this list ..
- non castoring nose wheels
Since most European built gyros have non castoring nose wheels and I am yet to learn that the rate of incidences with them is higher, I wonder whether this point should be in this (otherwise perfectly useful) list.

I'd say with castoring vs. non castoring nose wheels it's somehow like taildragger planes compared to airplanes with tricycle gear.

Both ways to build do make sense - but the planes/gyros behave differently in some situations.

A gyro with non castoring nose wheels is nicer to taxy, one with castoring nose wheels is easier to land in crosswind situations.

Neither is accident prone if handled correctly.
 

ckurz7000

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1st. Can you imagine different ways to command a 0 g situation in a gyro than pushing the stick hardly forward?
The only other intentional pilot input that would unload the rotor (which I can think of at the moment) is a steep bank followed by rudder input in the direction of the bank. You'll end up with your nose pointing down steeply. I can easily imagine a panic yank back on the stick to get the nose up, which leads to a cyclic stall and chops off the tail. A maneuver like this might even have been the cause of some accidents attributed to side slips.

2nd. Doug, I know that is unlikely to have a commanded 0g situation lasting enough to produce a critical rrpm loss. I appreciate very much your comment about manufacturers providing this information. I think that this is a shortfall in all gyro certification regulations. And I believe that this is a cause of accidents in stable gyros.
Personally, I don't think this information would be of much practical use.

I would also think that loss of rotor rpm is more a cause of accidents on or near the ground during take-off than in flight.

-- Chris.
 

ferranrosello

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I would like to do another comment. There are several answers to this link that implies that a pilot doing a 0g maneuver in a gyro is stupid because it is avoidable. I´m pretty sure that all pilots could do such a maneuver if you encounter some circumstances... It is unlikely to happen, but it happens...

Some years ago I put a superpuma flying over the sea in a true 0 g situation for a couple of seconds (subjective time appreciation). I was flying and I saw a seagull getting bigger and bigger in a frontal flight interception path against my head. I pushed the cyclic and lowered the collective. I didn´t hit the seagull, but a couple of seconds later, in the recovery; I felt how some protective equipment landed in the helicopter’s ground... I have always believed I was very lucky to fly a superpuma and not be flying a teetering rotor helicopter.

I´m sure that 0g maneuver can be done instinctively easily in order to avoid a midair collision. And my feeling is that we only know about PPO´s… And that we need to know more.

It is amazing and very worrying the accident rates we are supporting in gyrocopters, which are called to be the “safest aircrafts”. Sorry, but if we have a look to accident statistics this is not true. There has to be something causing these accidents, and we cannot be happy thinking that they happened because their pilots are stupid… The pilots are people, and people make mistakes: this is the mankind nature. We are able to learn from these mistakes, and this is what I´m trying to do here.

The two accidents of which I´ve talked about in my other posts (a MT in Germany and an Ela in Spain) are very similar. Both imply a sideslip what ended in control loss… The strange question is that in both accidents the rotor blades were bended upwards… This bending is not coherent with the ground impact. In the Ela case one blade was found nearly no damaged and separated about 20 feet from the crash site…

What do you think has bended the blades (not the rotor head) upwards?

Ferran
 

ferranrosello

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Chris, to unload a rotor is needed to put the rotor disk in a 0 lift situation that means 0 deg or negative AOA...

This can only be made flying with some airspeed. The airspeed can be got flying forward (you can unload the rotor pushing the stick) or side slipping. Is no need to have a total lateral sideslip, if you pushes the stick in the direction of relative wind, then you are unloading the rotor.

Furthermore, if you are side slipping hard, then you have a lot of rudder. The rudder acts as roll brake: you have the stick located far away from neutral to the sideslip side. If you release the rudder and keep the stick you will be commanding a roll that can end in a 0g situation. But if when you release the rudder you keep the lateral stick location and at the same time push the stick then you can be commanding directly a 0 g situation.

I cannot understand these two accidents without a severe drop in rrpm. When the rotor reloads itself there are not enough rrpm and the lift bends the rotor blades…

So I think that side slip has a lot to do with these two accidents.

I´m much more concerned about these accidents, because they are fatal, rather than a flapping situation on takeoff.

Ferran
 

ckurz7000

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I am not sure that loss of rotor rpm caused by side slipping was the problem. It seems more plausible the a sudden and unexpected change in attitude with the nose suddenly pointing steeply down followed by a violent yank back on the stick, which caused a cyclic stall. There may be some decline in rrpm but I just don't see the cause for catastrophic and unrecoverable loss of rotor speed in a sleep.

-- Chris.
 

birdy

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Can you imagine different ways to command a 0 g situation in a gyro than pushing the stick hardly forward?
Plenty of ways with oversped rotors.
Even long duration.[relitive for gyros]
Stick in any position and machine in any attude.
 

XXavier

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There's a point in Devulsky's post above that is, IMHO, wrong. If you're flying @ 25 KIAS, and then some observer on the surface says he's measuring a wind of any magnitude and direction, you will still be flying at 25 KIAS... The flight performance of the aircraft is not affected by the measurements of that ground-based observer...
 
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Penguin

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Nice work, Devulsky.
A taxonomy for poor gyro ADM.
 

SandL

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now we have some form of list of danger areas... by catagorising accidents
it would be interesting to see the frequencies, how often the accidents occour in each category

and more importantly which are the fatal categories

eg forign object strikes are probably quite common, but I guess rarely fatal, just costly
where as first flight of a new build by a new guy may show up as extreemly risky.
we may even see that PIO is no longer a risk in any gyro with either CLT or long reach large tail stab
we may find that weather, though scary, is rarely fatal in a gyro
if a chart produced 20 first flight fatalities then maybe the PRA would consider sending a few experienced people on a gyro test pilot course

it just needs someone to trawl through many international accident reports.
 
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