Blade Sailing

[zachmeyer99]

I am an instrument rated pilot with a commercial rotorcraft license. I recently switched to a new school to pursue my CFI and CFII. They gave me a checklist of topics that my lesson plans should cover, and one of those listed is something called "blade sailing". I have never heard this term in my two years of flight school, and neither has the Helicopter Flying Handbook. Google searches have led me to plenty of academic studies on the phenomenon, but nothing from a pilot or training source. I understand the basic idea, which is that at low RPM and in high winds, rotor blades can flap excessively and harm either the aircraft or ground personnel. Am I missing anything here, or would this be sufficient information to give a potential student? Thank you for any help you can give!

[helonorth]

That pretty much sums it up. In the GOM, the starter was located on the cyclic in the 206's so you could "fly" the blades on start up. Had a very anxious moment in a 407 one time. The blades looked like they dropped several feet over the nose. I never heard of a Bell striking the tail but I do remember it happening to an Astar.

Edited April 20, 2020 by helonorth

[zachmeyer99]

Thank you for the help! I'm glad you have a real world example to give; those usually help me to wrap my head around dangerous/expensive events.

[adam32]

There's a few videos on youtube of guys holding onto 205/214 blades on startup to prevent it from happening...

[helonorth]

21 minutes ago, adam32 said:

There's a few videos on youtube of guys holding onto 205/214 blades on startup to prevent it from happening...

There were a few different techniques they played with in the days of the bigger, two bladed Bells. One was to use a broom to keep them drooping too much. I also heard of leaving the rotor brake on until the N1 got up a ways. 

[Eric Hunt]

Most aircraft will have a limit on winds for startup. It is generally best to start with the wind fairly close to the nose - and turbines don't like it up the backside either.

In the rotor head are devices to protect the mast from inadvertent strikes from the bottom part of the hub. On the B206 you have the spring-loaded flap restraints, but once the blades are turning, the weights pull the stops out of play. If a sudden gust comes along, the blade will respond by flapping up as it comes into the wind, and the out-of-wind blade will flap down. Without the stops in place, the whole hub can bump the mast on the downside, causing damage.

Hueys, on start-up, would usually have the crewman holding the blade as the engine winds up, then letting the engine pull it out of his hand to accelerate faster than if it was loose. Doesn't actually stop the problem, but reduces it.

When shut down, the teetering blades are tied down at the back with the droop stops, underneath the hub, in contact with the mast, and secured. Stops the wind bouncing it around and causing damage.

static stop.tiff flap restraint.tiff

[Wally]

Yes, 206s had/have blade sailing issues in the GoM as did any helicopter that wasn't equipped with some variation of the rigid rotor system.   It's been 25 years since I flew the GoM, but I did tens of thousands of starts in 206Bs, Long Rangers, TwinStars and 412s. I've started them in winds up to 50 knots. 

Yes, you 'fly the blades' as they accelerate, but one is actually trying to keep the whole disk at the neutral. By the time a blade starts sailing, there's nothing you can do to stop it bouncing other than try to get the disk back to neutral.  If you start trying to damp the sailing blade with cyclic, you destabilize other blades.

I don't recall ever having a blade sailing issue 'on the beach'- that is, on a clear, flat surface, like a a pad on a base heliport, a ramp or an runway, except in very gusty conditions.  Even then it was much easier to control than on an offshore pad.

The blade sailing issue is most serious in the lee of with turbulent flow, but especially challenging if you're near the edge of the pad on a structure with marked vertical surfaces up to the pad and minimal pad overhang.  Being at the upwind or downwind edge was very difficult, you were somewhat better off on either side. If one could relocate the bird towards the center of the pad, the turbulent flow becomes a little more predictable, but one could not always do that. Whatever, you have to have the wind on the nose or it becomes very difficult to decide which way to move the cyclic- the individual blades aren't strongly affected by gyroscopic precession until they get some rotational speed.

I have tried the rotor brake trick, but you have to be very careful with that and not allow the engine to develop much torque through the drive train. I almost never ever used the brake in that way. Never did the broom, don't want anybody under the disk if I am not assured of control.

My suggestions for dealing with blade sailing issues is nose into the wind, cyclic centered, and away from the lee turbulence. If you have any question at all about starting in the lee, move the helo out of it.  Using the ground handling wheels is far less embarrassing than a tailboom strike.

 

Anecdotes on offshore structural turbulence-

I was offshore one morning on my base pad with some of the station crew helping me untie blades, aircraft tie downs and then skid it into the wind. Yep, them Twinstar blades [same-same Astar] are flexible and were bouncing. I heard 'whack', looked to right front and one of the station crew was standing there, said the blade above had just struck his hard hat a good lick. I've seen those blades bounce down almost to waist level in in front.  I've seen 206 blades flex down to 4-5 feet of vertical after hitting the hub stop and rocking the transmission pretty good.

Another time

I was trying to put a crew on a structure that had a rooftop pad, no overhanging edges, atop a single level 60'x60' quarters structure. It was one of those cold, gray, drizzly winter days with almost 40 knots blowing but steady. The seabirds don't like those days anymore than people do, so there was a bunch of'em on the pad. One comes in very slow and as far downwind as possible so the flying sea rats don't go though your disk. The last bird went to the upwind edge, launched and got a couple feet straight up, blew back across the pad, and was then blown down onto the pad.  The embarrassed gull scrambled back to the upwind edge and tried three more takeoffs and crashes before he went to a corner and successfully launched. Gulls are very strong flyers, very agile, although you mostly see them soaring.

Edited April 22, 2020 by Wally