RAF 2000 crash

[Doug Riley]

Bob:

For flight tests, go to the Magni USA Web site and look at the test procedures that Greg Gremminger has posted there.

If you want to do some on-the-ground tests and numbers, let me know and I'll post some procedures elsewhere.

 

[Al_Hammer]

I agree with Doug, that the Hstab should be of sufficient size and incidence to counteract a high thrust line. However, I do take issue with the idea that once the stab stalls, "it's all over."
Without drag there would be two ideal angles. One is 45 degrees and one is about 15 degrees.
These angles produce an almost equal coefficient of lift for an NACA 0015 (symetrical) airfoil.
15 degrees = CL 1.1 45 degrees = CL 1.05

The 15 degree CL is based on the way lift is normally made by an aircraft wing, a combination of Newton's Third Law and Bernoulli's theorem which says air is accelerated over the wing and decelerated under it, causing a low pressure area above the wing and turning the airflow to produce a downwash.
The 45 degree CL is based on 'Flat Plate' lift where air is just deflected off the bottom surface.

But drag does exist and matter: The coefficient of drag for 15 degrees is 0.1; for 45 degrees, 1.1(more than ten times greater)

Lift and drag act 90 degrees to each other. Right after the maximum coefficient of lift is achieved (in the example airfoil, 16 degrees). In this case the coefficient of lift rapidly goes down to 0.6 and the drag increases.

In a plane, stall of the wing is a bad thing. Lift is reduced( but not eliminated) and drag continues to increase. The aircraft loses speed as a result of the increased drag. Loss of airspeed causes further reduction of lift.

On a gyro Hstab, we are not concerned with lift of the airfoil so much as the moment of force that it can produce. These are different things, because lift is defined to be perpendicular to the relative flow and that means lift acts vertically when the aircraft travels horizontally.
However, at high angles of attack drag actually acts as a beneficial agent in that it is trying to restore the gyro to level by acting on the moment arm of the stab.
At 90 degrees(nose of the gyro pointing straight down) the airfoil is a pure flat plate at 90 degrees to the wind and it is making no lift, but lots of drag. All the drag is acting to lift the nose. The coefficient of drag of a flat plate is 1.28 https://exploration.grc.nasa.gov/education/rocket/shaped.html
This says that the stab actually produces more force when it is making drag than when it is making lift. We don't care which it is making, as long as the force is in the desired direction to tend to restore the gyro to level.

An airfoil shape has less drag at normal angles of attack compared to a flat plate, but both will make lift. The difference isn't so much in the lift making properties as in the drag. As long as the leading edge is rounded and not blunt, a flat plate stab is probably just fine.
At low aspect ratios, the lift vs AoA curve of a flat plate is very good. (Damping is also an important feature of any stab and that is more a function of moment arm than of airfoil shape.)

To use a sailing analogy, you might change your headsail from a genoa to a spinnaker when reaching and running(downwind sailing.)
The spinnaker catches wind like a parachute and uses drag to propel the boat downwind. The stab shifts from becoming a "genoa" at low angles of attack to a "spinnaker" at high angles of attack.
Stall is usually misunderstood to mean a sudden lost of lift, when actually the CL beyond stall AoA may be significant. Its just that drag rises exponentially with AoA and this is bad for a wing, but maybe not an issue for a stab.

I'm not much of a sailor or an aeronautical engineer, so I look forward to being shot down, but so far, no one seems to have picked up on this idea that stall of a stab may be not something to get too worked up about.

 

[Udi]

Bob - go to Greg Gremminger's web page https://www.magnigyro.com/USA/features.htm and scroll down to the "Articles" section. Read Part 1 thru Part 6. This will give you the background and detailed procedures for testing your gyro for static stability.

Udi

 

[gyro]

Hmmmm......loss for words, I just met him this year at Mentone. I am sadden and angered to hear this. These RAF's have really taken there toll...What gets me is that good veteran pilots are continually flying them knowing there are pitch sensitive. Me and Louis Hartzler watched Duane hunn this year at Mentone doing high speed thrill rides and three times we noticed his machine start to oscillate...Why try advanced maneuvers with a pitch sensitive machine is beyond me.

on this video from the butterfly web page you can see someone do what is called a double flare....scary stuff hearing them rotors unload and then plunk in hard. the video is call "flying the manarch" at 2 min. on the counter clock he does the double flare. I can see a RAF going over easily after a move like that! https://www.thebutterflyllc.com/video/videos.htm video is about 3/4 way down on left side of page.

 

[Mayfield]

Hi Bob

Greg has expanded quite a bit on the test protocols we discussed a few years ago and you should, as Doug and Udi say, read them.

To get you started I've copied below the longitudinal static stability test I sent to Greg.

Airspeed stability (longitudinal static stability):

1. Climb to a safe altitude and trim the aircraft for level flight at MPRS (minimum power required speed).

2. Measure the distance from some point on the stick to the instrument panel.

3. Add power as necessary to increase airspeed 10 MPH.

4. Stabilize the airspeed and trim for level flight at the new condition.

5. Measure the distance from the stick reference point to the same point on the instrument panel.

6. If the stick is farther forward at the new speed the aircraft is exhibiting airspeed stability in the speed range tested.

Repeat this test series in 5 or 10 MPH increments from minimum level flight speed to Vh or even Vne. (In my opinion you should use extreme caution below 50 MPH indicated and above 75 MPH indicated)

Slower speeds should require increasingly aft stick position. Faster speeds the opposite. Conduct these tests in very calm conditions. If you find, at any point, that you need to move the stick aft with increasing speed or forward with decreasing speed, you have reached a speed range where the aircraft is exhibiting a lack of airspeed stability.

 

[raghu]

However, I do take issue with the idea that once the stab stalls, "it's all over."

Al, you are absolutely right. We need to resolve the resultant of lift and drag perpendicular to the keel. This is the moment that balances the engine thrust. I have looked at lift drag curves through 90 degrees ( I will try an see if I can find them and post) from what I seem to recall (I could be wrong) that the peek moment is still at about the stalling angle. It then decreases by about 50% and then comes back to peak at 90 degrees. I looked at this when I posted my HS size formula as I wanted to make sure gusts would be adequetly taken care of.

I agree with Doug, that the Hstab should be of sufficient size and incidence to counteract a high thrust line.

Al, HS incidence has nothing to do with whether an HS can prevent a PPO or not- size and moment arm are the only thing that matter. Though, incidence does have an effect on stability, but it is not the only way to achieve it.

 

[Doug Riley]

Al, these gyros are crashing despite having H-stabs. Whether acting as drag device or lifting device, these stabs clearly have insufficient power when and where it's needed..

At the AOA where the lift starts to decline (say 12-14 deg), the drag is still only a fraction of the lift (NACA gives CD for a 0012 foil as .018 when the CL is 0.8, for example). Also, drag pulls straight back, which means that only a vector portion of it is stabilizing, while the rest just stretches the tail tube.

Mostly, though, I suspect angular inertia. If the HS can't snub the process very early, the gyro becomes a giant flywheel, with continually building kinetic energy of rotation. As a result, it takes more and more HS force to stop the pitching, the longer the HS "waits" to do it. What's more, the rapid pitching of the airframe will cause precession stall with uncontrolled rotor flapping and tail or prop contact. The drag effects may not get a bite soon enough to prevent this. Stopping the rotation at 90 deg nose-down, for instance, will be far too late to help.

I agree, though, that a drag device could exert the appropriate force to stop a PPO. Even a little parachute (reverse spinnaker?) could do it in theory.

(Ideally, we wouldn't be talking about "arresting" PPO's anyway. We should make them impossible to get going. It is far preferable to set up the aircraft so that none of this gets going, however. That means, in the case of a HTL gyro, a HS down-loaded enough so that the rotor is not holding the nose up in the first place.)

 

[WHY]

In several of the posts the phrase"we may never know the cause " has been used and this is most certainly true, BUT WE SURE KNOW THE MAKE AND MODEL.

I wonder how the powers at RAF will put a spin on this, something like "Horisontal Stab caused it by overriding the design stability of the magic Mast"

Tony

 

[Udi]

Doug and Jim - there is a saying that goes - the devil is in the details. Although it would be pretty hard to botch-up the AAI SH kit badly enough to make it PPO, it is possible (install the HS with a positive AOA, for example). It would be much easier to make a case for how the Bruty/Boyer mod may, or may not, eliminate the risk of PPO. If Bob McGuire comes back and reports that HIS modified RAF passes all of the stability tests with flying colors does that mean that all the RAFs that were modified with the Bruty/Boyer modification are stable? No -- they are all different.

What is my point? You can’t make the assumption a gyro would be safe/unsafe because it looks in a certain way, has a certain size stab, or went thru a given mod. It is the responsibility of the builder/owner to make sure their gyro is safe. If you don't have a good understanding of gyroplane aerodynamics, AND you fly an unproven and untested design (or worse - a proven unsafe design), AND you push the envelope of your gyro -- watch out below. When you push the envelope of an amateur and untested design, you should expect to reach a point of departure from controlled flight. I am afraid this RAF pilot has just found this point for his gyro...

I know I am preaching to the choir, but I thought I would say it one more time for the benefit of those who are willing to listen.

Udi

 

[Al_Hammer]

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.

 

[Chuck_Ellsworth]

" I wonder how the powers at RAF will put a spin on this, something like "Horisontal Stab caused it by overriding the design stability of the magic Mast"

Tony, seldom if ever will RAF post on these forums to market their product.

But hang in for a while and the usual small group of RAF supporters will spin this latest fatal accident to deny there is anything wrong with the RAF 2000 and the problem is pilot error.

Which of course is partly true for several reasons one of the most disturbing reason being the fact that RAF and their supporters are still in denial that the root cause is failure of RAF to change their design so it can not PPO.

So pilot error could be a contributing cause as evident when one bunts.

Chuck E.

 

[Al_Hammer]

Doug, I mainly wanted to point out that stall is not like dropping off a cliff. Lift declines, to maybe 1/2 while drag continues its exponential rise. You're right, that drag coefficient is still relatively low at angles near the stall. That's why I'd like to see Raghu's plots to better visualize the sum of forces at all angles.
Granted, you don't want to wait till 90 degrees to have your stab become fully effective.
Re: rotor precession stall, I really don't have any idea how close the rotor might be to having this happen without working out some numbers. I'd think the pitch rate would have to be phenomenal to stall the rotor, given that fast roll rates are often seen in gyros and helicopters and you don't see much rotor stall in that case. I'm not saying it doesn't happen, but it sure would help to see some numbers.

 

[automan1223]

How many ?

How many ?

Flying is dangerous, yes sir. So is motoring down the highway. Wonder how many people got killed on the highway this past weekend ? Should I stay indoors ? I have learned life can sometimes come under the chapter of stuff happens.

This accident is sad but preventable.
I am sorry to hear about it.

I talk about that "one time" that conditions will be just right and then your toast in a HTL machine. I know converting your machine is a pain but there is no other safe alternative.



Jonathan

 

[Al_Hammer]

Mostly, though, I suspect angular inertia. If the HS can't snub the process very early, the gyro becomes a giant flywheel, with continually building kinetic energy of rotation. As a result, it takes more and more HS force to stop the pitching, the longer the HS "waits" to do it.

I need to think about this, Doug. If you add mass to a pendulum, does it change the period, or the amplitude? I believe not. Only the length of the string matters and the acceleration of gravity.
While it is true that an object is harder stop once it starts moving, it is also true that it won't be moving as fast if it's more massive. Acceration is proportional to force and inversely proportional to mass.
A gyro with high moment of inertia(MOI) will rotate more slowly, giving the stab more time to resist it movement. The fact that it builds momentum is thus self cancelling, it would seem. Stability is a function of damping and the magnitude and direction of restoring forces. I think MOI comes into it in terms of pilot reaction. More MOI usually means more delay in response to pilot input which leads to PIO.

 

[Doug Riley]

"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. In practice, this means that a HS of a given size mounted above the CM is somewhat better at preventing PPO than one low on the keel -- ignoring propwash effects. The "high" HS's drag is more helpful, sooner.

The point I think everybody's converging on is that the drag effect doesn't help much early in the PPO event. "Later" would appear to be too late.

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.

 

[WHY]

Hi Chuck

Your right on about no Post from RAFM, but about the pilot error contributing to Bunt over, I do think that a well designed machine might " reduce" the effect of pilot error greatly giving the pilot a second chance to "learn and live"

Tony

 

[Doug Riley]

PPO simply will not happen in a properly designed machine. It's not a matter of stable machines' being more forgiving of pilot error. In stable machines, the particular "error" in question can't happen.

The "pilot error" in a PPO crash consists of momentarily letting down your guard with respect to the machine's continuous desire to tumble. Stable machines don't have that desire, so no vigilance is necessary and that type of "error" isn't an issue.

 

[WHY]

Thanks Doug, "perfectly said", my post was an effort to point out that what is often labeled "pilot error" if more likely "design error"

Tony

 

[Chuck_Ellsworth]

Tony, my comment that pilot error may be a contributing factor in a PPO bunt over was meant to point out that for a gyro to PPO two factors must be at work together.

First the machine has to be configured with a high thrustline which can result in a power pushover if it is being flown in the danger area for a power pushover.

Second the pilot has to actually allow or accidently put the machine in the position and power setting that will result in the power pushover.

So therefore to a certain extent it could be considered pilot error.

 

[StanFoster]

My thoughts and prayers go out to his family. What another shame.


I sold my RAF today....and also sold the new owner a AAI conversion kit for it. I gave him the commission on the modification kit. He will have a safer gyro...but maybe this would not have made a difference in this situation?


Stan