RAF 2000 crash

[Aussie_Paul]

Unless RAF is modified to this geometry, it is not as safe as can be made.

I have a slightly simpler mod that achieves the same geometry without all the cutting and plate canping of the keel. Emai me for help and pics etc at [email protected]

Aussie Paul.

 

[gyrogreg]

This problem is avoidable!

This problem is avoidable!

My Condolences too! We need to fix this problem!

I believe that buntovers (including PPO) can be designed out of gyros! The gyro needs to be flight tested to see if it is truly statically stable AND buntover proof.

Please conduct the static stability flight tests on your gyro. Know where it is safe to operate and where it is not safe. The THREE static stability flight tests have been published in Rotorcraft magazine and are available at:
https://www.magnigyro.com/USA/features.htm. The three tests are outlined on Parts 3, 4 and 5. (But, please get familiar with the techniques and aerodynamics in all 6 parts!)

There is much talk about static stability. The flight tests describe three different static stability tests. They all are important. But G-Load static stability is the important issue with buntovers (including PPO).

No one can tell you from just looking at it, or even measuring things on it, if your particular gyro configuration is buntover proof. But, the G-Load static stability flight test can tell you in a few minutes! Test it - it is safe and easy and very quick! Then you will know. You will also find out what flight envelope (power / airspeed/ loading) is safe, and what conditions you should avoid!

G-Load static Stability: If an aircraft is G-Load stable at a specific flight condition (power / airspeed / load), it WILL NOT be capable of a buntover (including PPO). In the flight conditions where an aircraft is statically G-Load stable, it cannot buntover (or PPO) because for any G-Load disturbance the aircraft will automatically pitch appropriately in the right direction to correct for the G-Load disturbance. When any aircraft is G-Load unstable, that means that a G-Load disturbance from 1G will automatically become worse - like the yardstick balanced on your finger as Udi describes.

So, all you really have to do is test you gryo to be G-Load stable - over the full range and combination of airspeed, power, and loading.

How to test: Simply set the trim for an airsped in s & L flight. Then roll into about a 30 degree bank and verify that you need to pull aft on the stick in order to maintain the original "trimmed" S & L airspeed. That proves that when the aircraft is presented with a G-Load disturbance, as in the banked turn, the aircraft is trying automatically trying to pitch in order to restore to 1 G.

Now, you do need to test this over the full range of combinations of airspeed, power and loading. This is because your gyro may be G-Load stable under the condition of speed or propwash on the HS, but the HS may not be adequate at some other combination of airspeed and power. Doing this full test at one load over the full combination range of airspeed and power should take about 10 minutes! Is 10 minutes of your time worth knowing for sure? Find out for sure, then you will know.

If you don't test for static stability at a certain combination of airspeed and power, then simply consider that combination outside you safe envelope until you do test it.

In the case of this accident - likely a low speed, high power combination - this can be safely tested as described above at low airspeed and high power!

The other static stability tests - Power and Airspeed - are also important things to test - test them! This might take another 10 minutes! They will give you insight as to whether your HS is adequate to "balance" your propeller HTL offset - or not! This might indicate your gyro might not be G-Laod stable at higher power levels or airspeeds. Traditionally, these two static stability criteria also make the aircraft easier to fly and learn to fly - airspeed is fairly constant with power levels, and airspeed returns to "trimmed" airspeed upon a disturbance.

But, G-Load static stability is the key to a gyro configuration that will be immune from buntovers (including PPO) - as long as you verify G-Load static stability under the full range of flight conditions:

High Power / High Airspeed
Low Power / High Airspeed
High Power / Low Airspeed
Low Power / Low Airspeed.

Then check this full range at minimum, maximum and middle loading configurations! Sorry, that may take another few flights!

- Thanks, Greg

 

[raghu]

Raghu, thanks. I wasn't thinking hard enough when I made the comment about incidence. It would be interesting to see whatever curves you have regarding Cd and Cl.

I will try and look for it later tonight. I also have some curves for the VPM 16 measured in a windtunnel upto about 40 degrees. I will try and track these down too.

 

[gyrogreg]

On a different thread, I had suggested that an RAF might be able to be made statically stable - including G-Load stable - by appropriate application of an adequate HS.

I still believe this is marginally possible - but it does take a large and efficient HS on a long moment arm. The mod I am suggesting is similar to what Pat McNear has done (Pat says he has run all the Static Stability tests and had verified that his gyro is Power, Airspeed and G-Load stable. I can't vouch for this, or that that is G-Load stable at all combinations of power, airspeed and Loading - but, I think it is possible.

On my original post on this, I had assumed an 8 inch HTL prop offset for the RAF. I also assumed a 500 Lb thrust of the prop. I understand that some suggest the RAF HTL offset is as much as 12 inches (1 ft). With 8 inches HTL offset, I thought the numbers were reasonable. With 1 ft offset, the job for the HS becomes tougher, and looks less practical without improving the HTL offset. So, here are the numbers again with the 1 ft offset:

First - Prop HTL moment:
Assuming HTL offset of 8 inches = 1.0 ft
Assuming prop thrust = 500 Lbs
Nose-down moment = 330 Ft-Lb
Assuming HS moment arm aft of CG = 9 ft (extended tail)
The HS down force required to balance the HTL moment of 330 Ft-Lb = 55.5 Lb (Down-Load)

Second - windscreen - destabilizing moment:
Alan Laughrey performed some RAF enclosure wind tunnel (trailer) tests in New Zealand a couple of years ago. If I recall correctly, he found a windscreen induced download at normal flight speeds (70 mph?) in the order of 120 Lbs. I do not believe this included any nose-down moment from offset drag - just download from the enclosure. This is extra load on the rotor, but also a destabilizing nose-down moment that needs to be balanced by the HS:

Assuming windscreen download = 120 Lbs
Assuming windscreen moment (from CG) = 2 ft
Nose-Down moment = 240 Ft-Lb
Assuming HS moment arm aft of CG = 9 ft (extended tail)
The HS down force required to balance the enclosure download moment of 240 Ft-Lb = 26.7 Lb (Down-Load)

"Wing Loading" for an 8 sq ft area HS and 36 Lbs is about 6.9 Lbs / sq ft! this tough for 70 mph relative wind. The HS on this example is the new AC "Tripple Tail" with it cocated on a longer tail boom for more leverage from the CG.(For instance - an Ultralight wing loading might be in the range of 3-4 Lbs per Sq ft. at similar airspeeds). This also suggests that the HS may require some "immersion" in the propwash (as Paul B. has suggested) - the HS's biggest job is to balance the HTL prop offset!

You can see my original post at: https://www.rotaryforum.com/forum/showthread.php?t=10144&page=3

- Thanks, Greg

 

[C. Beaty]

Raghu, thanks. I wasn't thinking hard enough when I made the comment about incidence. It would be interesting to see whatever curves you have regarding Cd and Cl.

Is this what you had in mind, Al? Working the stalled stab like the rag tail on a kite?

 

[Timchick]

I am very sad to hear this news. My prayers are with his family. I met Mr. Monard and his wife when he was taking flying lessons in June 2005. He'd been flying for 2 weeks without the stab and decided the day I was there to put it on to try it even though his instructor told him it wouldn't help. I helped him install it and when he came back from his first flight with it on he couldn't believe how much of a difference it made. He said it was staying on and he continued his lessons with it on.

The cord on his stab was not very large but it had a pretty good angle to it that probably provided good downforce with airspeed. I agree with some of the people who have said this type stab wouldn't provide enough pitch dampening to help prevent a ppo.

I met Mr Monard and his wife again at Bensen Days 2006 where he received an award for his gyro. His wife was the one learning to ride a motorcycle. She kept driving back and forth on the airport entry road. I sat across from them at the dinner and awards ceremony.

RAF's spin on this accident will be that he had a stab and not a stabilator.

 

[dragonflyerthom]

Tim I believe he had a stabilator also. The pics showed something that looked like a bracket.

 

[paulp]

Howdy All,

For those of you that have an RAF I have my working drawings(and the parts list) for the fabrication of a stepped keel for the RAF. It utilizes the Parham(Aircommand) stab with the vertical winglets. When mounted on the modified keel, it will be immersed in the prop wash. which will produce an approximate 60-65 Kt airflow over the stab. I also have some photos which may prove to be beneficial. In addition, the keel is extended 18" and the rudder is elevated by 1".

If you will pm me I will forward same.

My condolences to the faimly for their loss.

 

[C. Beaty]

Another thing to keep in mind is that even with CLT, the danger of torqueover is still present whenever the rotor is unloaded.

The only remedy for torqueover is either differential tailplanes in the propeller slipstream or a tall tail. Another advantage to the AAI mod kit.

Torqueover, fortunately, isn’t divergent or self-sustaining like a forward tumble.

An RAF-2000 has 250-300 ft-lb of torque applied to the propeller and with the roll axis moment of inertia being substantially less than the pitch axis MOI, roll acceleration in a direction opposite to propeller rotation can be nearly as fast as pitchover.

I expect the accident report of Monard crash, if witnesses’ statements are included, will state the aircraft pitched nose down while rolling to the right.

 

[Chuck_Ellsworth]

How can this be?

" He'd been flying for 2 weeks without the stab and decided the day I was there to put it on to try it even though his instructor told him it wouldn't help. I helped him install it and when he came back from his first flight with it on he couldn't believe how much of a difference it made. He said it was staying on and he continued his lessons with it on. "

*******************************************************

In nature we have birds, these animals were either designed by a creator or evolved into very manouravable flying creatures.

The wing on a bird is capable of changing shape and to some extent will have some control on the pitch attitude and pitch rate of the bird as it flys.

However as far as I can remember all birds have horizontal stabalizers in the form of a tail .

One day my Cat jumped a Robin on the deck of our house and ended up with the Robins tail in its claws and the Robin managed to escape and fly away.

I was really suprised and the erratic flight path it flew with very noticable up and down pitching due to the loss of its stabilizer.

Much like the RAF Stabalator the wings were unable to prevent pitch diversions caused by the loss of the tail feathers.

There can be no arguing that proper training would have allowed the Robin to fly without its tail, if that argument were to have validity nature would have seen to it that the Robin could compensate for no H.S.

Chuck E.

 

[Greg Mitchell]

Very Sad News

Very Sad News

I recall asking this Chap if I could take some pics of his gyro at 2005 Mentone. I have excellent video footage of this gyro and features in a DVD I made of Mentone 2005. I am deeply distressed by this tragic loss of life.
Here are two pics of the gentlemans HS setup. The chord did appear narrow.


My thoughts are now with his family and friends.

Mitch.

 

[Chuck_Ellsworth]

Greg:

Just looking at thse pictures of the beautiful work that was put into his pride and joy brings unbearable grief to all of us who are trying to deal with another loss.

Chuck E.

 

[Greg Mitchell]

Very Sad Indeed Chuck

 

[raghu]

Is this what you had in mind, Al? Working the stalled stab like the rag tail on a kite?

Al here are the graphs I have. The drag one looks similar to what CB posted. My notes say I picked these graphs from aerospace.org. I am not sure if it is still there.

 

[Al_Hammer]

Raghu, Chuck, thanks for posting those graphs.
It appears that the airfoil does become effective again, certainly near 45 degrees and beyond, for what its worth. Too late to stop a PPO, no doubt.

Chuck, good point about the torque-over. That problem is often neglected in these discussions.

 

[Screw]

s

s

Screw-In


First my condolences to the family and friends. This is very sad us all of us.

Second....Does anyone know his attitude upon impact? Did anyone witness the gyro flip unpside down? Or roll over?

Everyone is talkig about PPO/PIO, but no one has stated that the gyrp "Flipped."

I've noticed on steep powered climbs a serious reduction in rotor RPM, however, I was told to always throttle back to regain level, not push over.

I'm wondering if his altitude was low, and if he simply got the rotors too slow and induced a percession stall. If so, I think his gyro would have rolled over to the left and impacted the ground.

Not enough info yet to figure out what happened.

Screw-Out

 

[raghu]

"We need to resolve the resultant of lift and drag perpendicular to the keel."

To be annoyingly picky... the resultant we care about is one that's perpendicular to a radius sticking out from the center of mass of the gyro and passing through the aerodynamic center of the HS.

I agree to both (being annoyingly picky and resolving with the perpendicular to a radius from the CG) .


AL: I wasn't trying to compare craft of varying MOI. My point was simply that it takes less force (OK, impulse) to arrest a PPO early in the event than later. If all that drag were there at the outset, the PPO might not get going.[/QUOTE]

Doug, your earlier comment on the giant flywheel effect and this further clarification of less force to stub ppo early are incorrect. Say there is a certain HS AOA at which it generates enough force to balance the PPO torque. Let us say this is -5 degrees. If we ignore damping for a moment, then the HS will oscillate between 0 degrees and -10 degrees when the rotor thrust is taken out. No PPO will occur just this oscillation (ignoring all other degrees of motion). It does not matter what the AOA required is the force to stub the PPO is the same- the thrust offset

Of course there will be damping, and depending on how much the response will be somewhere between deadbeat and the above case.

 

[Student]

It's been discussed that reducing throttle in time in the event of a zero-gravity maneuver can help combat a PPO. In actual practice, how effective is this technique? It makes sense that without the offset thrust vector, there would be no desire to push over, however the thrust vector is not removed from the system at the point of throttle reduction. The vector's magnitude would only be reduced, correct?

To make a long story short, I was wondering if connecting a zero-gravity sensor to the trottle would be of any use? At the time a zero-gravity (or perhaps near zero-gravity) situation, the throttle would be overridden by this sensor and its power would be automatically be reduced or extinguished.

At the risk of asking a stupid question.... would this work?


EDIT: As a side note, I realize this would not help with unloaded rotor flapping and possible prop/tail strikes.

 

[Doug Riley]

Raghu, as a lawyer I'm happy to take refuge in the vagueness of language.

By "snub" I mean something imprecise, like "stop really fast."

The ultimate snub, so to speak, is the case of the HS whose down-load makes a moment that exactly counters the PPO moment in level flight. The PPO will never even start at that rate.

If the HS were a constant-thrust retro-rocket that achieved, and continued to produce, exactly the counter-moment to the engine's PPO moment, but at some time after the start of a pitch-down, then, at that moment, the gyro would stop "accelerating angularly." It would continue to pitch down at the angular rate it had achieved when the rocket lit off, however, as result of its own inertia.

However, as a better approximation, the HS's moment builds from some starting value (maybe zero) and increases sort of linearly as the AOA. This is the same setup as the torsional spring/balance wheel in a mechanical watch. As long as the spring isn't so weak that it breaks, the device will overshoot, snap back and oscillate until it damps out.

In the case of a gyro in a PPO, the overshoot is apt to be enough to allow the rotor to suffer precession stall or at least enough cyclic lag that it smacks the tail or prop. Such a system doesn't "snub well enough" for our purposes.

I would guess that a stabbed RAF that PPOs is more analogous to the spring simply breaking. The HS's restoring moment never gets to a level even equal to the PPO moment.

There are a few complications that even us liberal-artsers can see. One is the fact (discussed earlier) that only a component of the aerodynamic force on the HS acts in a useful direction. The other component pulls uselessly along the tail tube lengthwise. Another is that the rotary motion of the airframe itself adds to the AOA of the HS.

Trying to model those things in an equation would take me the rest of my life (or more). You probably could do it on a coffee break!

 

[C. Beaty]

The concept of some sort of “G” switch to either kill the engine or shut the throttle has been a topic of discussion for years, Justin.

These discussions bubbled to the surface during the Bensen era where the cause of forward tumbling was not at all clear.

The answer is; no one knows. The problem arises from not knowing at what point the forward tumble becomes irreversible.

Perhaps even better would be a sensor that detected nosedown pitch acceleration. The model airplane rate gyros that employ a quartz resonator would make nifty sensors.

Now that the mechanism of tumbling is more fully understood, it is so easy to avoid designing a tumbler that patches and Band-Aids shouldn’t be necessary.

On the other hand, the world will never run out of shade tree designers, slick salesmen and gullible customers.