Blade Sailing

[Abid]

Yeah...what he said. Making up new terms for an already difficult subject is not in the best interest of the sport.

Next knee-jerk reaction to Abid: Why present charts from your desk in Florida, aimed at a mainly USA audience forum, in KPH?

This forum has members from around the world.
Mike Goodrich mainly developed the GWS. He is a good friend and colleague and is in Europe. They use Km/hr. I can convert it easily. No big deal.
Mainly those logs would be recorded data looked at after an accident. Its not being looked at by regular pilots

 

[WaspAir]

I'm late to the party, but...I was trained by Chris Burgess starting in 2005, then Steve McGowan, Gary Neil, Jon Carleton, and Desmond Butts, to call it blade flap.

I have yet to personally discuss this thing with any CFI or seasoned gyro pilot who calls it anything else, and/or who does not recognize it by this name. They will further explain it using terms such as retreating blade stall, etc., and though "blade sailing" has been tossed in there as well on occasion, the common chatter is always "blade flap".

The "common chatter' may be part of what keeps gyros as a backwater, cottage industry, footnote in the rotorcraft world. Non-standard language such as "rotors" (plural) instead of "blades", and "flap" as a bad thing when every blade on everything necessarily flaps on every rotation, gives an impression of less than competent professionalism. With no insult intended toward Messrs. McGowan, Burgess, etc., I'm pretty sure that Messrs. Sikorsky, Bell/Young, Piasecki, Kaman, Hughes, and Hiller used (if not established) the conventional meaning of "flap". The fellows who do billions in the business have more sway over terminology than those who do thousands.

 

[Abid]

I'm late to the party, but...I was trained by Chris Burgess starting in 2005, then Steve McGowan, Gary Neil, Jon Carleton, and Desmond Butts, to call it blade flap.

I have yet to personally discuss this thing with any CFI or seasoned gyro pilot who calls it anything else, and/or who does not recognize it by this name. They will further explain it using terms such as retreating blade stall, etc., and though "blade sailing" has been tossed in there as well on occasion, the common chatter is always "blade flap".

I have been told by a CFI as recently as yesterday that ELA in Florida is cranking out new pilots on an assembly line without teaching ANYTHING related to what we all understand and know as blade flap.

This is horrifying to me.

TEACH YOUR STUDENTS ABOUT the damn BLADE FLAP before you sign them off to solo.

That's all I wanted to say.

We all know that. Blades are flapping all the time. The one is retreating blade stall and that is not normal flapping. You have to distinguish the two and its done right now by knowing context. But for new pilots or students that is extremely confusing. Its actually easier for new gyroplane pilots and transitioning pilots to separate the two. My $0.02
Where in Florida are they training for ELA?
I think every CFI can better train their students about retreating blade stall and aborting takeoff procedure. All of us can improve that. We better do that in fact. So far I have checked a few gyroplane pilots who flew with me and I asked them to show me abort takeoff. These were from multiple instructors. The results were not convincing

 

[ferranrosello]

I believe that all Gyrocopters pilots know that they need to take off with a minimum rotor rpm before apply full power. Is the very first thing that Instructors teach in takeoff.

In this chat you are calling the rotor instability because of too low rotor rpm blade sailing and retreating blade stall. However, it is not blade sailing and it is not retreating blade stall.

Blade sailing is the uncontrolled blade flapping caused by wind. It is of big concern when slowing down rotors in windy conditions, because the blade can hit the tail boom. Of course, this will be the result of trying to take off with too low rotor rpm, but the cause is not “blade sailing”.

Retreating blade stall happens because too fast IAS. It implies that the retreating blade has to work in a too high angle of attack and it stalls… The effect in the aircraft is a nose up or down depending on the rotor spinning side. However, what happens when trying to take off with too low rpm is a very bad and rough lateral flapping and a turn over.

The real reason of this undesirable phenomena is the lack of enough centrifugal force on the blades. A rotor, to be stable, needs the rigidity provided by centrifugal force, that is about 2500 kg (more than the doble in pounds) for an ELA rotor spinning at 360 rpm. Each blade is supporting about 230 kg of lift, and the ratio with centrifugal force give a small rotor conning angle and restrain the flapping movement.

The problem in takeoff is that we don´t have a direct control of rotor rpm. The easy solution is to prerotate the rotor to a safe level regime. In spite of rotor rpm decay in the first seconds of takeoff roll the centrifugal force will be big enough to provide rotor stability during all takeoff and continuous flight.

The problem arises when we request lift (by pulling the stick aft combinate with too great the air speed). The air speed can be provided by the pilot because of selected power or by the wind if it is strong enough. Then the ratio between lift/centrifugal force is too high. The advancing blade tries to climb to equalize lift in the two sides of the rotor. The retreating blade does the some by going down. But because of the lack of centrifugal force the flapping is too big, the rotor hits the stops and the bad flapping occurs. The rotor is unable to compensate de dissymmetry of lift and the gyrocopter turns over.

It is possible to take off safely from very low rotor rpm (60 rpm). In negligible wind condition we pull the stick full back and the without delay we apply little power, just to produce forward very slow movement (the speed of a walking man). The we wait until we can see that rotor rpm are increasing. Immediately you will feel that the forward speed is slowing down. Keep the speed by increasing power. Now two options, when you get the safe rotor rpm (210 rpm in the ELA) apply full power and perform a normal takeoff. Or continue maintaining low forward speed until the nose goes up: then full power.

 

[Abid]

I believe that all Gyrocopters pilots know that they need to take off with a minimum rotor rpm before apply full power. Is the very first thing that Instructors teach in takeoff.

In this chat you are calling the rotor instability because of too low rotor rpm blade sailing and retreating blade stall. However, it is not blade sailing and it is not retreating blade stall.

Blade sailing is the uncontrolled blade flapping caused by wind. It is of big concern when slowing down rotors in windy conditions, because the blade can hit the tail boom. Of course, this will be the result of trying to take off with too low rotor rpm, but the cause is not “blade sailing”.

Retreating blade stall happens because too fast IAS. It implies that the retreating blade has to work in a too high angle of attack and it stalls… The effect in the aircraft is a nose up or down depending on the rotor spinning side. However, what happens when trying to take off with too low rpm is a very bad and rough lateral flapping and a turn over.

The real reason of this undesirable phenomena is the lack of enough centrifugal force on the blades. A rotor, to be stable, needs the rigidity provided by centrifugal force, that is about 2500 kg (more than the doble in pounds) for an ELA rotor spinning at 360 rpm. Each blade is supporting about 230 kg of lift, and the ratio with centrifugal force give a small rotor conning angle and restrain the flapping movement.

The problem in takeoff is that we don´t have a direct control of rotor rpm. The easy solution is to prerotate the rotor to a safe level regime. In spite of rotor rpm decay in the first seconds of takeoff roll the centrifugal force will be big enough to provide rotor stability during all takeoff and continuous flight.

The problem arises when we request lift (by pulling the stick aft combinate with too great the air speed). The air speed can be provided by the pilot because of selected power or by the wind if it is strong enough. Then the ratio between lift/centrifugal force is too high. The advancing blade tries to climb to equalize lift in the two sides of the rotor. The retreating blade does the some by going down. But because of the lack of centrifugal force the flapping is too big, the rotor hits the stops and the bad flapping occurs. The rotor is unable to compensate de dissymmetry of lift and the gyrocopter turns over.

It is possible to take off safely from very low rotor rpm (60 rpm). In negligible wind condition we pull the stick full back and the without delay we apply little power, just to produce forward very slow movement (the speed of a walking man). The we wait until we can see that rotor rpm are increasing. Immediately you will feel that the forward speed is slowing down. Keep the speed by increasing power. Now two options, when you get the safe rotor rpm (210 rpm in the ELA) apply full power and perform a normal takeoff. Or continue maintaining low forward speed until the nose goes up: then full power.

I just want to note that there is some myth of losing way too much rotor RPM after pre-rotation while bringing stick back. I have tested, measured and even graphed and I think you can see in some of the videos with GWS I posted. The loss in rotor RPM from pre-rotating to bringing the stick back and applying power to move forward is as low as 4 to 6 rotor RPM if done quickly and correctly.

 

[ferranrosello]

Ok, I totally agree. However, in teaching to fly the drop is higher.

 

[Abid]

Ok, I totally agree. However, in teaching to fly the drop is higher.

You mean for student pilots learning. Yes of course but they don't know what they are doing yet so everything is slow going. It shouldn't be after they get their license though.

 

[Jean Claude]

The real reason of this undesirable phenomena is the lack of enough centrifugal force on the blades. A rotor, to be stable, needs the rigidity provided by centrifugal force, that is about 2500 kg (more than the doble in pounds) for an ELA rotor spinning at 360 rpm. Each blade is supporting about 230 kg of lift, and the ratio with centrifugal force give a small rotor conning angle and restrain the flapping movement.

I am afraid that I will see here the confusion sown by mathematical studies, which call the flapping angle the one of the blade in relation to the plane of the ball bearing.
If we distinguish it from the coning angle, the flapping angle is no longer restricted by the centrifugal force. It is only restricted by the aerodynamic balances, i.e. by the differences in angles of attack and airspeed between the advancing and retreating sides

The centrifugal force only change the coning, which are with no effect on the hit of flapping stops of our seesaw rotors

 

[Doug Riley]

Jean Claude is correct, of course. The centrifugal effect acts in the plane of the blades' rotation, even if that plane is not square to the rotor spindle. Centrifugal effect can't "lift" the low blade. It simply pulls spanwise on the blade.

The phenomenon we (informally) call "flapping" is retreating-blade stall in different circumstances than those experienced in helicopters. From the retreating blade's viewpoint, a stall is a stall, whether it occurs at high or low aircraft airspeed. The angle of attack of the blade exceeds its stalling AOA, and its lift decreases abruptly.

Our semi-rigid, teetering rotor system is marvelously simple, but it has its limits. By design, the teeter hinge always tries to increase the angle of attack of the blade having the lower airspeed, in order to equalize lift between the blades.

The retreating blade has a lower airspeed than the advancing blade. Increasing AOA only increases lift up to the stalling AOA, but the teeter hinge does not know this. It continues adding AOA to the retreating blade even past that blade's stalling AOA -- so the retreating blade stalls. Once this happens, the teeter hinge has failed in its mission; instead of equalizing lift between the two blades, it has created a large INequality of lift. The un-stalled blade rises wildly -- or sails, if you like. The stalled blade, being attached rigidly to the rising/sailing blade, drops hard and can strike parts of the gyro.

 

[Tyger]

Cogently stated, Doug.

 

[Abid]

I am afraid that I will see here the confusion sown by mathematical studies, which call the flapping angle the one of the blade in relation to the plane of the ball bearing.
If we distinguish it from the coning angle, the flapping angle is no longer restricted by the centrifugal force. It is only restricted by the aerodynamic balances, i.e. by the differences in angles of attack and airspeed between the advancing and retreating sides
View attachment 1155044
The centrifugal force only change the coning, which are with no effect on the hit of flapping stops of our seesaw rotors

Yup. Pretty common mix up about this I think. Thanks for your post

 

[Vance]

I am afraid that I will see here the confusion sown by mathematical studies, which call the flapping angle the one of the blade in relation to the plane of the ball bearing.
If we distinguish it from the coning angle, the flapping angle is no longer restricted by the centrifugal force. It is only restricted by the aerodynamic balances, i.e. by the differences in angles of attack and airspeed between the advancing and retreating sides
View attachment 1155044
The centrifugal force only change the coning, which are with no effect on the hit of flapping stops of our seesaw rotors

Thank you Jean Claude.

You bring a wonderful clarity to these discussions.

 

[XXavier]

Just one comment: the elasticity of the rotor blades of our gyros is usually quite high. In a blade-sailing episode, if the stops are hit at low rotor revs, the inertia of the blades will cause them to deform elastically, and the rudder and prop may be impacted by the blades. That won't happen if that blade-sailing episode takes place at higher rotor revs, since the elastic deformation of the blades will be much lower, because of the ‘extra rigidity’ added by the higher centrifugal force.

 

[Tyger]

the rudder and prop may be impacted by the blades. That won't happen if that blade-sailing episode takes place at higher rotor revs, since the elastic deformation of the blades will be much lower, because of the ‘extra rigidity’ added by the higher centrifugal force.

Are you implying that it's not possible for the blades to impact the tail or prop if you hit the stops at flight rrpms?

 

[XXavier]

Are you implying that it's not possible for the blades to impact the tail or prop if you hit the stops at flight rrpms?

It's not completely impossible, but –at normal flight RRPMs– the increased rigidity of the blades due to the centrifugal force makes that much more difficult. They behave –lengthwise– as if made of a stiffer material, tending to stay straight even if the roots hit the stops, bending much less.

 

[Vance]

It's not completely impossible, but –at normal flight RRPMs– the increased rigidity of the blades due to the centrifugal force makes that much more difficult. They behave –lengthwise– as if made of a stiffer material, tending to stay straight even if the roots hit the stops, bending much less.

It is my observation that in most power pushovers there is damage to the rudder from a rotor blade and often damage to the propeller.

I suspect the rotor is near flight rpm when it hits as the description of the event is often of parts departing the aircraft at the beginning.

 

[Jean Claude]

The increased rigidity of the blades due to the centrifugal force makes that much more difficult. They behave –lengthwise– as if made of a stiffer material, tending to stay straight even if the roots hit the stops, bending much less.

The stiffness of the blades increases due to the centrifugal tension, and this also increases the pressure on the flapping stops, when it hits (at 6 o'clock).

Also, the action of these collisions at 6 o'clock tilts the disc on the side (gyroscopic delay of 90 degrees) which unfortunately not reduces the longitudinal flapping: The hits continues...until the cause of the flapping divergence is removed.

 

[ferranrosello]

Jean Claude, thank you for your wise explanation. However… I can’t believe that centrifugal force is not playing a role in the rotor stability.

First of all, we use teetering rotors which don’t have the ability to get a correct conning angle because the two blades are rigidly joined between them. Only articulate rotor heads (and hinge less…) have this ability. The question, then, is what happens when a teetering rotor head with low rpm is forced to develop full lift.

What I have learned from incidents and accidents that has been shown in this forum is that both blades are bended “up” in gyrocopters with a good HS.

But what is going to happen in takeoff, when the air speed is low but to high for actual rotor rpm? The lift involved is not enough to bend the blades up, consequently the rotor hits the stops vigorously… I think that the cause is the lack of enough centrifugal force that would restrain the coning angle. But the teetering is not able to cone enough. Because of that the flapping is unstable and its range is increased.

We need to see that conning angle is different depending on rotor rpm and developed lift. Usually there is a mechanical conning angle made by the manufacturer. This conning angle is good for normal flying conditions, and blades flexibility works to adapt all normal situations. But this rotor is unable to cope with a rotor producing lift at too low rpm.

 

[Vance]

First of all, we use teetering rotors which don’t have the ability to get a correct conning angle because the two blades are rigidly joined between them. Only articulate rotor heads (and hinge less…) have this ability. The question, then, is what happens when a teetering rotor head with low rpm is forced to develop full lift.

What I have learned from incidents and accidents that has been shown in this forum is that both blades are bended “up” in gyrocopters with a good HS.

But what is going to happen in takeoff, when the air speed is low but to high for actual rotor rpm? The lift involved is not enough to bend the blades up, consequently the rotor hits the stops vigorously… I think that the cause is the lack of enough centrifugal force that would restrain the coning angle. But the teetering is not able to cone enough. Because of that the flapping is unstable and its range is increased.

We need to see that conning angle is different depending on rotor rpm and developed lift. Usually there is a mechanical conning angle made by the manufacturer. This conning angle is good for normal flying conditions, and blades flexibility works to adapt all normal situations. But this rotor is unable to cope with a rotor producing lift at too low rpm.

In my opinion the blades bend up in most gyroplane accidents because the bottom of the blade is what hits the ground first.

In my opinion with a semi rigid teetering rotor system; the pivot is there to manage dissymmetry of lift between the advancing and retreating blade and enable cyclic aerodynamic control of the rotor.

The coning angle appears to me to be a balance between the centripetal force of the rotor system and the lift generated.

In my opinion the retreating blade stalls when the critical angle of attack of the airfoil is exceeded.

The retreating blade has a higher angle of attack to manage dissymmetry of lift.

It is my observation that a retreating blade stall precedes most takeoff mishaps and at least a third of gyroplane accidents are takeoff mishaps.

The pilot’s operating handbooks I have read all have a procedure for avoiding retreating blade stall during takeoff.

None of the gyroplanes I have flown will go fast enough stall the retreating blade at flight rotor rpm.

I feel it is important for gyroplane pilots to understand rotor aerodynamics and rotor management to reduce the number of accidents caused by poor rotor management.

 

[Tyger]

The stiffness of the blades increases due to the centrifugal tension, and this also increases the pressure on the flapping stops, when it hits (at 6 o'clock).

However, the greatest tension will be at the attachment point, with the tension decreasing as one goes further out on the blade.