Some thoughts on cross wind takeoffs.

[Vance]

I feel there is value in re-examining my takeoff procedures flying The Predator from time to time.

Whatever gyroplane you fly; please follow the procedures in the pilot’s operating handbook.

I had a particularly challenging client who asked a lot of good questions about the how and why of the instrument scan during the takeoff roll.

I want him on the centerline so I don’t want him focused on the instruments.

It is a busy time and I may not pay enough attention to my rotor tachometer focusing too much on how things look and feel.

I often takeoff with a strong cross wind and guess at how much I need to tilt my disk into the wind to have an elegant takeoff.

My prerotator is an automobile starter and can be engaged at any angle.

Without wind I might see a hundred rotor rpm on my thirty foot diameter blades.

I begin my prerotation behind the hold short line and start rolling immediately. I bring the cyclic half back at a hundred rotor rpm because the rotor blades are coning up and I began to have aerodynamic control of the rotor and I bring the cyclic full back at a hundred twenty rotor rpm at which point I release the prerotator. This is generally about the time I reach the center line of the 150 foot wind runway.

One way to lose control of the blades (commonly called flapping or sailing the blades) with a two blade semi rigid teetering rotor on takeoff is to have too much indicated air speed for the rotor rpm. The retreating blade stalls first because it is at a higher angle of attack and the advancing blade sails forcing the stalled retreating blade to collide with the vertical stabilizer, rudder or propeller blades.

This is a very common gyroplane takeoff mishap and generally results in considerable damage to the gyroplane.

I leave the cyclic full back until around a hundred eighty to two hundred rotor rpm and no more than fifteen of ground speed at which point I smoothly advance the throttle and soon the nose comes up and I began to move the cyclic forward keeping the nose tire near the ground.

She waddles into the air on her own around forty five knots indicated air speed and around 300 rotor rpm.

She lingers near the ground until I see fifty knots and around three hundred thirty rotor rpm where she begins to climb at something over five hundred feet per minute depending on the takeoff weight and density altitude.

I typically fly from an eight thousand foot runway so there is no need to be so precise with the rotor rpm to shorten the takeoff distance.

Using the rotor tachometer gives me a basis for teaching rotor management.

I recently paid more attention to how tilting the disk into the cross wind affects my rotor acceleration and found there is substantial difference in how fast the rotor accelerates.

Because of the reduced drag of the rotor The Predator also accelerates faster with the disk tilted into the wind for a cross wind takeoff.

My conclusion is to be more conscious of the difference a cross wind makes; start the takeoff roll with the disk flat and tilt it into the wind just before liftoff.

In other words I need to take more time with a crosswind takeoff to allow the rotor to accelerate and perhaps add a little less power to keep from too much indicated air speed for the rotor rpm.

I fly so often in winds it is easy to become overconfident and become careless with the numbers.

Even in a gyroplane that prerotates to over two hundred rotor rpm; rotor management is important and the relative wind is still responsible for accelerating the rotor to flight rpm. Tilting the rotor into the wind will still slow the rotor acceleration and increase the speed the aircraft will accelerate for a given power setting.

 

[GyrOZprey]

Good discussion openers Vance! On my first flight review in 2016 .... my CFI showed be a technique on how to utilize the crosswind to help the rotor accelerate in such conditions ... at little disconcerting as it is hardwired to keep the rotor disc well into the wind ...to NOT get flipped over by a sudden gust ... I expect it is much experience & a good ADM judgement call ...whether conditions ( with steady crosswind NOT erratically gusting winds) warrant using this technique!
I have been unchallenged in having to deal with full crosswind for most of my flying time has been out of airports with crosswind runways available! Wide runways also allow flexibility to angle ones TO run ...to reduce the cross component!

 

[Vance]

That is the compromise exactly Christine.

I find it is difficult to teach people the effect of left or right disk angle on the takeoff roll particularly with linked nose wheels because the pedals control the direction.

The surprise comes when they lift off and they have to correct the disk angle to maintain the desired track and the position of the rudder may not be appropriate.

A rotor creating lift has the potential to upset a gyroplane if it is not at an appropriate angle for the conditions.

Santa Maria has a cross wind runway and if we do not encounter cross winds on runway 12/30 we will switch to 2/20 to get some cross wind experience.

I feel rotor management is a critical part of flying a gyroplane.

I agree that gusts are a bigger challenge than a steady wind.

 

[Tyger]

Interesting topic.
I routinely did takeoffs and landings with a LOT of crosswind during my primary training in the flatlands of MO, and I don't remember any specific discussion about the technique, tbh; I guess now it's just muscle memory for me. In Magnis, the pedals are linked to the nosewheel, so I just keep that straight during the takeoff roll. Once lifted off, I keep the machine flying runway heading with just the stick, not worrying so much about rudder, and thus letting the nose naturally crab into the wind.
Of course, on landings (or if doing crow hops) I have to be on the rudder pedals the whole time, keeping the nose pointed straight down the line till that nosewheel is back on the ground. At that point, the caster of the nosewheel takes over and it's important to have a light touch on the pedals when that occurs.

 

[Jean Claude]

I often takeoff with a strong cross wind and guess at how much I need to tilt my disk into the wind to have an elegant takeoff.

During the run, why do you think you need to tilt disc against the crosswind?
Since the rotor vector thrust is almost in line with the bearing regardless of the relative wind direction, isn't it better to keep it in the gyro's plane of symmetry?

If α is the rear longitudinal inclination, β the transverse inclination of the disk, and ψ the direction of the relative wind relatively to the nose, then the angle of attack of the disk is α* Cos(ψ) + β * Sin(ψ)
So, if the stick is held on the Rearstop (α = 20 degres), with no transversal tilt (β = 0 degree), then a forward speed equal to crosswind (giving ψ = 45 degrees) gives a A.oA = 20*Cos(45°) - 0*Sin(45°) = 14 degrees, while the Rrpm acceleration keep the same, because the airflow is greater of 40%
But now with only half the transversal travel of the stick against the wind (β = -5 degres) gives a A.o.A of : 20*Cos(45°) - 5*Sin(45°) = 10 degree. Thus, the Rrpm acceleration is clearly lower.

 

[Vance]

During the run, why do you think you need to tilt disc against the crosswind?
Since the rotor vector thrust is almost in line with the bearing regardless of the relative wind direction, isn't it better to keep it in the gyro's plane of symmetry?

If α is the rear longitudinal inclination, β the transverse inclination of the disk, and ψ the direction of the relative wind relatively to the nose, then the angle of attack of the disk is α* Cos(ψ) + β * Sin(ψ)
So, if the stick is held on the Rearstop (α = 20 degres), with no transversal tilt (β = 0 degree), then a forward speed equal to crosswind (giving ψ = 45 degrees) gives a A.oA = 20*Cos(45°) - 0*Sin(45°) = 14 degrees, while the Rrpm acceleration keep the same, because the airflow is greater of 40%
But now with only half the transversal travel of the stick against the wind (β = -5 degres) gives a A.o.A of : 20*Cos(45°) - 5*Sin(45°) = 10 degree. Thus, the Rrpm acceleration is clearly lower.

Thank you for your input Jean Claude.

It appears to me you have put my intended communication in more technical terms.

I am advocating for keeping the disk flat until just before liftoff and then tilting the disk into the wind so the lift off is in a straight line.

I have flown with some flight instructors that advocate for tilting the disk into the wind as airspeed increases and I feel this is not best practice.

If I lift off with the disk flat (cyclic centered) in a wind with from the left I will move right across the runway at very low altitude until I tilt the disk left.

If for some reason I touch back down as I am moving across the runway it will at the least cause a perturbation and at worst a tip over.

The way I know I guessed at the left tilt correctly is she smoothly lifts off over the centerline with no left or right cyclic input.

Additionally I encourage keeping gyroplane aligned with the ground track with the rudder (uncoordinated flight) until a positive rate of climb is established so if I touch back down it is not problematic.

It is a compromise because most gyroplanes don’t climb as well in uncoordinated flight so if I just let her turn into the wind she will get away from the ground sooner.

From the Rotorcraft Flying Handbook on how to do cross wind takeoffs:

“Begin the maneuver by aligning the gyroplane into the wind as much as possible. At airports with wide runways, you might be able to angle your takeoff roll down the runway to take advantage of as much headwind as you can. As airspeed increases, gradually tilt the rotor into the wind and use rudder pressure to maintain runway heading. In most cases, you should accelerate to a speed slightly faster than normal liftoff speed. As you reach takeoff speed, the downwind wheel lifts off the ground first, followed by the upwind wheel. Once airborne, remove the cross-control inputs and establish a crab, if runway heading is to be maintained. Due to the maneuverability of the gyroplane, an immediate turn into the wind after lift off can be safely executed, if this does not cause a conflict with existing traffic.”

I feel my opinion is in conflict with the Federal Aviation Administration guidelines and as a Certificated Flight Instructor it is my job to teach to the standards.

This may be why I am in conflict with some other flight instructors on crosswind takeoff procedures.

I teach and demonstrate the FAA procedure so my clients can pass the FAA practical test and then demonstrate what I prefer and explain the reasons for my preference.

I feel there have been some mishaps from tilting the disk into the wind prematurely.

 

[Mike G]

I seem to remember we've had this discussion before but I can't find the thread.

Mike G

 

[JEFF TIPTON]

A couple of previous threads.

https://www.rotaryforum.com/threads/some-thoughts-on-cross-wind-takeoffs.1146560/

Takeoff in strong crosswind.

Recently in France a gyrocopter Calidus lay on its side because of a divergence of the flapping of the blades. Fortunately the pilot was not injured. He said, "I sure putting the stick completely on the side where the wind was coming in. Maybe not totally backward: I do not remember." My opinion...

www.rotaryforum.com

 

[Mike G]

Jeff
Thank you, I couldn't find it with this new forum.
I think my post 48 explains what JC is telling us.
I can confirm what he says, when testing the GWS with the stick back and hard into a cross wind with "normal" pre rotation Rrpm the GWS gives me a flapping risk warning due to the slow rotor acceleration that it doesn't do when the stick is hard back and centralized.
Mike G

 

[Abid]

On crosswind I simply start with a slight tilt into the wind to start pre-rotation but as I go past 120 rotor RPM I centralize the stick and continue to prerotate till 180 to 200 and then begin my ground roll with stick centered and pulled back. As gyro lifts off the ground it tries to establish crab into the wind. I can counter that with a little cross control of stick into wind and opposite rudder till I get to an altitude I am comfortable with where I coordinate the controls (usually 50 to 100 feet) and let the gyro fly a crabbed heading over the runway but coordinated to get best rate of climb.

 

[Jean Claude]

Vance,
My bad English let me understand that you were tilting the disc against the wind during the run until the take-off. But I had misunderstood.
Thanks for your clarification

 

[Vance]

Vance,
My bad English let me understand that you were tilting the disc against the wind during the run until the take-off. But I had misunderstood.
Thanks for your clarification

I am grateful for what you bring to the Rotary Wing Forum Jean Claude.

I feel you do remarkably well with English.

I learn from every one of your posts and I am often reminded how careless I am with terminology.

I strive to be more precise.

 

[Tyger]

I can confirm what he says, when testing the GWS with the stick back and hard into a cross wind with "normal" pre rotation Rrpm the GWS gives me a flapping risk warning due to the slow rotor acceleration that it doesn't do when the stick is hard back and centralized.

That only makes sense. Tilting the rotor toward the (relative) wind, whether forward, to the side, or some combination, is going to mean the rotor will not spin up to flying speed as quickly.

 

[All_In]

On crosswind I simply start with a slight tilt into the wind to start pre-rotation but as I go past 120 rotor RPM I centralize the stick and continue to prerotate till 180 to 200 and then begin my ground roll with stick centered and pulled back. As gyro lifts off the ground it tries to establish crab into the wind. I can counter that with a little cross control of stick into wind and opposite rudder till I get to an altitude I am comfortable with where I coordinate the controls (usually 50 to 100 feet) and let the gyro fly a crabbed heading over the runway but coordinated to get best rate of climb.

Thank you for teaching the actual maneuver with RRPM.

 

[dabkb2]

The part that gets fun is knowing how far to tilt the rotor into the wind. It is nice when you get it right.

I know this is a little off topic, but when the wind is blowing 20+ I was taught to start prerotation with the gyro facing 90* to the wind and tilt the rotor as far into the wind as possible, after prerotating turn the gyro into the wind and slowly bring the stick back.

There is no reason to worry about a crosswind takeoff in these conditions because there is usually less than a 5' roll to takeoff.

 

[Mike G]

Fara
What advantage do you get by tilting the rotor slightly into wind at the beginning of pre rotation?
Mike G

 

[Resasi]

Reduction of the likelihood of a gust getting under the rotor disc before the rotor has reached a more stable situation, ie higher rotor RPM and less flexible state, and therefore less susceptible to the possible onset of blade sail/flap occurring?

 

[Abid]

Fara
What advantage do you get by tilting the rotor slightly into wind at the beginning of pre rotation?
Mike G

Its to prevent possible gusts from trying to surprise me on an attempted tipping over or sailing but there is no advantage otherwise that I know off. If anything there could be a slight disadvantage that if I tipped the controls slightly the other way, I would get some headwind that could help in pre-rotation being faster but at the risk of being caught by a gust. I am talking about the phase where I am still stationary just doing initial pre-rotation

 

[Tyger]

I was taught to prerotate with a flat disk to about 100rpm at the hold short line, then move onto the runway and tilt back away from the wind till I get to about 220, then commence the takeoff roll. I have never had any issue with a gust surprising me using this technique, despite lots of takeoffs (and landings) in very blustery conditions, often with a lot of crosswind.

 

[Jean Claude]

...There is no reason to worry about a crosswind takeoff in these conditions because there is usually less than a 5' roll to takeoff.

With ELA 07 100 hp at 450 kg, pre-launched at 220 rpm, my calculation gives a run of 250 feets with a 20 Kts headwind.