Research report on slips in a gyro

ckurz7000

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Slips in a gyro have been the topic of many heated discussions. The various opinions about them ranging from dangerous, via superfluous, to indispensible. Particularly with some notable accidents over the past couple of years in which slips have come up as a possible exascerbating circumstance, it has become clear that not enough hard facts are known about this maneuver.

As a reaction to this, slips have been edited out of the German training syllabus and the largest gyro manufacturer has added lots of cautionary verbiage about slips to their handbooks to the point where if you do slips and something happens to you, you're on your own.

So what's the scoop about slips? Wouldn't it be nice to have a clear-cut guideline specifying when slips are safe to perform and under what circumstances they can bite you?

The DLR (German Center for Aviation and Astronautics) picked up the ball and started to investigate this issue. They followed a two-pronged approach: First the developed a mathematical model of a gyroplane and built a pretty sophisticated simulator. Then they did extensive flight testing in a heavily instrumented MTOSport and used the data to tweak the model so that it would agree with the experiment. The benefit of this approach is that they now have first hand experimental data as well as a mathematical model which can be used to extrapolate real life flight experience to a point where no pilot would want to sit in the gyro anymore.

The test pilot is a friend of mine with 6000 hours in gyros and a firm scientific background. We talked at length about slips and found that our individual experiences match very closely. What's also nice is that our explanations about what happens aerodynamically in a slip are in good alignment. There is one important thing that I learned which I had no clue about, and it seems to be important in slips. I will tell you more about it in a bit.

The instrumentation they carried in their test gyro is pretty impressive. They measured rotor head angle, rotor blade angle, rotor rpm, stick position, rudder position, flight attitude in roll, pitch and yaw and stick force (in addition to all the usual flight parameters). They also had threads taped all over the horizontal and vertical stabilizer and rudder to measure the airflow across these surfaces. The maneuvers they performed were slips in all variations, to both sides with varying deegrees of bank and yaw with all imaginable power settings including engine off.

(Just as an aside to Bryan Cobb: they did measure flapping angle and nobody was surprised to find out that the rotor disk rides highest in the front.)

The results of this research are being written up in a Master's Thesis and will be available publicly once the thesis is published. There was a brief superficial report published in the German UL quaterly (DULV Info) but it was too watered down for public consumption so that you couldn't really grasp the important implications. Everything I write in this thread is from private communications with the test pilot. Just wanted to be clear about this.

Slips in gyros differ from slips performed in fixed wing aircraft in several important aspects. First, the required control inputs are much smaller in gyros, making them, in efffect, more sensitive to entering and exiting from a slip. This point alone can lead to precarious situations for pilots transitioning to gyros from their Cessnas and the likes.

The second point of difference is that the aerodynamics of slip performed in a gyro are more complicated than in a fixed wing. This requires the gyro pilot to be aware of, and often manage properly, a large number of variables which his fixed wing colleague wouldn't even have to know about. Among these parameters are engine torque, airspeed, direction of roll, direction in which the rotor turns, wind gusts, etc. Let me discuss some of the more important ones in turn.

A gyro depends on the horizontal and vertical stabilizer surfaces to enhance stability around the pitch and yaw axes. This is true whether the gyro is CLT or HTL. In an aggressive side slip, these stabilizing surfaces receive airflow from the side and beome less efficient with increasing degree of yaw. This was demonstrated by observing the airflow across the tail surfaces using an array of wool threads taped to its surface. The effect of this is that sensitivity to cyclic pitch input (stick for-aft) increases. Also, yaw and roll stability both decrease. Thus, the gyro depends more on the pilot to provide accurate cyclic input to maintain the desired flight attitude.

The gyro used for flight testing was an MTOSport powered by a Rotax 912. I mention this as the direction in which the propeller turns (counter clockwise, as seen from behind) as well as the sense of rotor rotation (clockwise as seen from below) are important.

Engine torque exerts a roll moment to the right. Therefore a slip with the nose pointing left and a bank to the right will be less stable due to the engine torque working to increase the bank angle. For the same reason it is important to enter a slip with a retarded throttle and not change the throttle setting during the slip. A common mistake is to enter a slip with the power well back, then realize that airspeed is too low or the descent angle too steep and shove the throttle forward. This will suddenly increase engine torque and lead to an increased roll moment to the right. If you're already slipping in this direction (i.e., nose left, right bank) you'll find yourself confronted with a suddenly increasing bank which you may not have the wits, skills or control authority to counteract.

It is a common mistake to judge airspeed in a slip by the indication of the ASI. The ASI will read low in a slip due to airflow hitting the pitot tube at an angle instead of right on. Students who forget to take this into account will instinctively add throttle in the attemptto regain the apprently lost airspeed. This reaction is a prime setup for increasing engine torque which -- in slips with banks to the right -- is a destabilizing condition.

Another factor which tends to steepen the bank has to do with the center of aerodynamic pressure of the cabin. In many gyros it is located below the center of gravity. Therefore, as the relative wind impinges on the cabin from the side, it will push it to the opposite side and increase the bank. This condition becomes a factor if slips are entered at too high an airspeed so that drag forces are significant.

The last factor is something I didn't know about before and learned during the dicussion with Jörg. Apparently it is a well known effect in helicopters, though apparently nobody thought it would matter in gyros. During test flights it became clear that the rotor disk behaves different in slips to the right vs. to the left. The stability margin is measurably less in slips banked to the right. The mathematical model was not able to reproduce this until the effect of airflow around the cabin interfering with the downwash of the rotor was added. Apparently there is a significant effect on the rotor dynamics which comes from how the airflow around the cabin interacts with the downwash of the rotor. Banks to the right decrease the stability margin and banks to the left increase it. Once the model was changed to incorporate this effect, agreement between theory and exeriment became extremely good.

To sum up, here is the recepe for a safe slip in a gyro:

1) Enter the slip with engine at or near idle and airspeeds not in excess of about 80 km/h.

2) When entering the slip, lead with rudder and apply lateral cyclic as needed to maintain your flight path.

3) Use smooth and gentle control inputs to establish the slip and minute corrections throughout the slip to maintain a constant bank and yaw attitude. Don't let the gyro fly itself, stay on the controls all the time.

4) Disregard the ASI and judge airspeed by flight attitude, wind noise and other outside clues. Keep the slip attitude constant.

3) Do not touch the throttle while slipping, leave it at or near idle.

5) Upon exiting the slip, reduce lateral cyclic input and rudder to neutral, leading with the stick. Keep the pitch attitude constant. Use smooth and gentle control input on stick and rudder.

6) Once again in a level attitude, check your airspeed and use throttle as required.

The upshot is that slips are an advanced maneuver which can be performed safely if those points are heeded. There are situations where slips offer an advantage over other maneuvers as e.g., in an emergency landing situation into a confined space. If you want to add slips to your repertoir, train them with an experienced instructor first.

Greetings, -- Chris.
 
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PW_Plack

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Chris, thanks for the excellent information and the detailed report. One question...

...There are situations where slips offer an advantage over other maneuvers as e.g., in an emergency landing situation into a confined space...

I've seen slips, sometimes extreme, performed for fun, but how would they help landing in a confined area as opposed to a vertical descent? This seems like an misapplication of airplane technique which risks a crabbed touchdown if not executed with precision.
 

PTKay

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...
I've seen slips, sometimes extreme, performed for fun, but how would they help landing in a confined area as opposed to a vertical descent? This seems like an misapplication of airplane technique which risks a crabbed touchdown if not executed with precision.
Paul, I fully agree with you.
This is the main advantage of a gyroplane over the fixed wing, to be able to descent vertically, if needed!
 

Texasautogyro

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Chris this is almost exactly what I have found and explain to students. The part about not doing them at high speed and false airspeed is dead on. I normally do them for keeping the nose straight for landing.
 

ckurz7000

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A vertical descent is not the best maneuver in that situation. I don't think that either you, Pavel, nor Paul have had the chance to become proficient in both and use them in real life ina true "situation".

If you abort a vertical descent a smidgeon too early you are too fast and float a long way. Do it too late and you wreck at least the gyro. While descending at low airspeeds you lose control authority and have less than optimal control over your flight path.

With a slip you can come down extremely steeply while maintaining airspeed and full control over your flight path. What's wrong with that? Just because many gyro pilots are not proficient to do one, have never been in a situation where a slip saved the day and only regurgitate what they hear other peoe say doesn't make a slip a bad maneuver.

-- Chris.
 

ckurz7000

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Chris this is almost exactly what I have found and explain to students. The part about not doing them at high speed and false airspeed is dead on. I normally do them for keeping the nose straight for landing.

Good to hear, Desmon. We should go flying some time.

-- Chris.
 

Flyguy450

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Chris thank you for all the info on slips in a gyro compared to a FW .I have used the slip a lot in my Diamond DA 20 but only a couple of time (with Desmon) in my MTO and like most maneuvers it does feel quite different than a FW.
So if I understand you correctly it really is better to do slips with the nose to the right and left cyclic?
One small point and I might have read it wrong but my MTOs rotor spins counter clockwise.
Mark.
 

EI-GYRO

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Chris, is the information considered sufficient to explain the three approach accidents over the past couple of years?
 

ckurz7000

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Chris, is the information considered sufficient to explain the three approach accidents over the past couple of years?

If your question is whether an inappropriately done slip can explain the three accidents, then my answer would be that this is a definite possibility.

As you know, there really is no way to know for sure what caused these accidents. But a slip gone wrong maybe paired with other factors (partially unloaded rotor or gusty winds, for example) would be a likely explanation.

Certainly, we have come quite a ways in understanding better what happens in slips and what could potentially lead to problems. The value of this reasearch lies in a better understanding in how to do the correctly and safely.

-- Chris.
 

fara

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Chris:
Good summary of effects of these maneuvers.
I have a couple of questions from the top of my head.
In airplanes there is an asymmetrical loading in slips that is put on wings - generally 70% on one wing and 100% on other causing shear loads in the wing/fuselage carry-thru. Although gyroplanes do not have such a fuselage structure, the mast hub point would take some kind of shear loading in this condition? or not? Does the hinge take this asymmetrical load out of the equation somehow? How does the gyroplane mathematical model predict this load near the hub.

Are there any special torsional loads due to slips that blades experience themselves along different spanwise stations? Does the mathematical model suggest any prediction for that.

Was there any conservative test estimate suggested for side loading of the tail surfaces to account for limit and ultimate loads that would be expected in service with such maneuvers?

Thanks in advance.
 

kolibri282

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Chris,

thanks for this great writeup! One thing I currently don't get sorted out is how the well known damping in roll affects a slip. Could you briefly elaborate for a non pilot?

Thanks,

Juergen
 

reelmule

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Chris,
This is one of the most informative, best post backed by instrumented flight data to ever appear on this. Forum! Training is the key word. We are indebted to you..
 

Resasi

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Great article, thanks for the information Chris.
 

BEN S

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This seems to be directed solely at dreadnought class enclosed stupid money gyros, does the info hold true for light single seat open frames I wonder?
I have tried doing these maneuvers both left and right and have not noticed much of a difference other than my rudder goes to the left more than the right due to off set. As for the throttle, maybe my 503 just doesn't have enough get up and go to torque over my heavy rig and extra heavy butt.
I would be hard pressed to look at my ASI to determine how fast I was going,rather than just feeling the increase in air pressure.
Very nice post and awesome info Chris, thanks for compiling it!
 

Gyro_Kai

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Ben, the non enclosed, storck-legged HTL gyros, like the Dominator, would be less effected, as the aerodynamic resistance below the CoG does not increase as much when turned sideways. However, there are some basic findings which still apply:
The gyroplane's fusalage is balanced with the overall, mostly induced drag of the rotor in level flight. When turned sideways, it increases, while the rotor drag does not, so there will be a banking effect, even though less than with a half enclosure below.
Also the prop-torque as well as the newly identified rotor torque still apply, so leave the throttle alone and turn away from the prop-torque.

Kai.
 

ckurz7000

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...In airplanes there is an asymmetrical loading in slips that is put on wings - generally 70% on one wing and 100% on other causing shear loads in the wing/fuselage carry-thru. Although gyroplanes do not have such a fuselage structure, the mast hub point would take some kind of shear loading in this condition?

The big difference between fixed wings and gyros with respect to this topic is that in fixed wings the orientation of the fuselage and the orientation of the lifting surfaces (i.e., wings) are rigidly coupled. This is not the case in gyros. Your fuselage can point one way and the rotor wouldn't know. The universal joint between gyro fuselage/mast and the rotor pretty much ensures that in flight there are no torques transmitted from one to the other.

Are there any special torsional loads due to slips that blades experience themselves along different spanwise stations? Does the mathematical model suggest any prediction for that.

I can't speak about details of the model because I am not really familiar with it beyond a couple of conversations. But to a reasonable degree of accuracy you can assume ignorance of the rotor about the orientation of the fuselage.

But there is a caveat for everybody who by rote just thinks of a gyro as a pendulum mass suspended below a rotor. As has been demonstrated in this research, airflow around the rotor is influenced by airflow around the cabin with, in some circumstances, notable effect.

Was there any conservative test estimate suggested for side loading of the tail surfaces to account for limit and ultimate loads that would be expected in service with such maneuvers?

The loads which any part of the gyro has to be able to withstand are spelled out in detail in the BCAR Section T (the MTOSport is certified under that regulation). Side load margins are ample so that aerodynamic loads because of a slip are no worry.

-- Chris.
 

ckurz7000

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Chris,

thanks for this great writeup! One thing I currently don't get sorted out is how the well known damping in roll affects a slip. Could you briefly elaborate for a non pilot?

Thanks,

Juergen

Moin Jürgen,

When talking to Jörg, he mentioned the name "Jürgen" for somebody instrumental in writing the code for the model. If I remember correctly, the model is programmed in Mathematica. So I assumed that it's you who's involved in this program. I guess it's just somebody with the same first name.

There is a difference in the flow pattern between helis (power driven rotor) and gyros (autorotating rotor) which leads me to be cautious about translating experiences literally between the two classes of aircraft. But apparently the stability curve around the longitudinal axis has a different shape depending on the slip angle with respect to the direction in which the rotor turns.

If you are interested, I can put you in touch with Jörg who can answer detailed modelling questions or even refer you to the other Jürgen who can. Just let me know.

Greetings, -- Chris.
 

ckurz7000

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Ben, the non enclosed, storck-legged HTL gyros, like the Dominator, would be less effected, as the aerodynamic resistance below the CoG does not increase as much when turned sideways. However, there are some basic findings which still apply:
The gyroplane's fusalage is balanced with the overall, mostly induced drag of the rotor in level flight. When turned sideways, it increases, while the rotor drag does not, so there will be a banking effect, even though less than with a half enclosure below.

Thanks for chiming in, Kai. That's my view of things, too.

Also the prop-torque as well as the newly identified rotor torque still apply, so leave the throttle alone and turn away from the prop-torque.

Just to be absolutely certain: it is not the torque of the rotor which is responsible here. It is the interference between the aerodynamic flow around the rotor and the flow aroud the cabin. Lean the cabin one side and the flow will be more affected on the retreating side of the rotor; lean it to the other side and you will affect primarly the advancing side.

-- Chris.
 

birdy

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The only slips iv ever used are horisontal ones, at min AS and WOT.
If i want to decend virticaly, its either a virtical decent or spiral.
 
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