Something else to argue about (AGAIN)-- Wuffos and HR

[mcelrah]

I have had launch assistance from rated pilots which was inadequate at best.
You have to indoctrinate pilots just as carefully as wuffos. - Hugh

[Danny Brotto]

Good point Hugh. Here are a couple of wire crew suggestions for “best practices” that will improve wire crew safety especially at precipitous cliffs like HR. They all have to do with side wire crew hand placement.

I think it’s important to grasp the wire with both hands especially in gusty conditions. The second hand acts as a backup in case the wire gets away from the crew (I’ve see it happen on more that one occasion.) Also with one hand it’s easier for a sudden gust to pull the side crew off balance (seen that too.) Two hands allows for a better stance and helps prevent an off balance condition from occurring.

Another suggestion is to grasp side wires from above with fingers pointing down. A glider getting away from the crew will do so forward and up. Wires grasped from below can more easily be yanked up and forward. Grasped from above, with both hands of course, the wire is much less likely to get away from the crew.

The other suggestion is to always grasp the wire (with both hands and from above.) Many times, I’ve seen crew with open hand(s) in the vicinity of the wire (usually during light conditions) looking at and telling the pilot “Your glider”. (Of course it’s his glider, he paid for it LOL.) Seriously though an errant gust could allow the wire to totally escape the crew’s grasp. Even if the crew is astute and notices the wire getting away, trying to grab a jumpy, 3/32” grey cable at the edge of a cliff could become hazardous. In light conditions, it’s better to keep a light hold of the wire but maintain positive tactile connection incase that wire begins to dance. This can be done very lightly in weak conditions so as to not interfere with the pilot’s perspective of the glider’s feel prior his launch.

I’ve been out of the HG flying scene for a few months as my “frozen shoulder” mends. Last time I flew (Ridgley) I had to effect an acrobatic maneuver in the control frame on final to reach up for my right down tube; not cool. Things seem well now and my PT has released me to go forth and dangle.

Danny Brotto

[mcelrah]

Welcome back, Danny! Good detail on wirecrew best practice. - Hugh

[Flying Lobster]

The flip side of all this is the interesting aspect that as a waivered pilot you basically give up your rights when you decide to participate in a wirecrew. I recently witnessed an out-of-control rigid launch attempt where the pilot was giving wrong commands and the glider nearly tossed the (HG pilot) wireman overboard into the abyss at Hensons.

marc

[mcelrah]

So how DO you wirecrew for a rigid anyway? Are there handholds at hardpoints on the wing to grab? I don't imagine you can use the sidewires since they are actuating control surfaces... - Hugh

[Flying Lobster]

Smart rigid pilots (that footlaunch alot) put little wooden t-bars that attach to retractable bungees that are out on the wing. The Aeros Phantom comes with built-in hand-hold slots. The incident I mentioned was a new VR that had no handholds--so the wire guy, who simply held on to the LE, was lifted off the deck and the wing had started yawing forward as it went up. I rushed in and grabbed the control wire you mentioned--there's no doubt in my mind the wing would have completely flipped/ground looped off the side of the ramp (and the wire guy ejected into space) had I not done so--the outboard wing had already started dragging on the rocks below and to the side of the ramp.

marc

[Flying Lobster]

I just realized that ALL of our launches down here are cliff launches. I still think of HR as one of the best anywhere. But I also think there is a pressing need for regular annual CL and AWCL clinics that should be conducted on-site by the club's observers or advanced instructors (not just at HR, but cliff launches anywhere). Cliff launches seem to fairly regularly stuff pilots of all experience levels.

marc

[Dan T]

I think Danny Brottos description of how to wire crew is right on the mark. Both hands on the wires, palms down, alert and paying attention to what you are doing. I've seen enough near disasters with one hand, palms up, screwing around, I'm too macho to hold on etc. The Tennesse Tree Toppers contest wire crew instructions are virtually identical to Danny's description. I'd add just one thing I learned from Kevin Carter. If the winds are strong enough to be significant get that nose down so the glider is at equalibrium. This makes it much easier to handle and to launch in firm winds. Try it the next time the winds are bouncing you around a little on the Pulpit or High Rock launch. I think you will find it much easier as will your wire crew.

Dan T

[mikel]

Maybe I'm offbase with this one , but, Aren't wirecrew at the rock supposed to wear fall protection per protocall ??

Mike

[mcelrah]

Don't know if it's written down, but it is certainly recommended to provide ropes and have crew tie themselves in... - Hugh

[Matthew]

As to Mike Lee's comments. If it's windy you should have tie down ropes for your wire crew. Everyone who flies at High Rock should own a pair of tie down ropes or straps for their wire crew-- we have straps with waist harnesses.

As to Dan's comments--

If the winds are strong enough to be significant get that nose down so the glider is at equalibrium. This makes it much easier to handle and to launch in firm winds. D

Dan T

-- I beg to differ-- somehwat. The glider is much more difficult to handle with the nose down too far at High Rock in any significant wind. The nose angle should be set at the proper position. If it's too far nose down you start to experience glider suck and the wing bounces around. Here's a trick I leard from Richard Hays eons ago. When moving forward on the block or picking up the glider, raise the nose a bit so that it's not pulled foward. Then stand STRAIGHT UP with your sholders down and back and ease the glider's nose down. If you start to feel like the wing is being pulled forward then you've put the nose down too far. The glider should feel locked in place and balanced when you have the nose angle set properly. Usually (but not always) at High Rock, the nose angle is set properly if the keel is flat and parallel to the ground.

Also, don't go jumping off the block when you hear two neutrals for a split second if it's been squirrely at launch. Make sure that the wire crew is in agreement and that the wing FEELS balanced before launching. I've seen too many pilots go careening off of launch to the left or right because they jumped as soon as they heard two neutrals.

Matthew

[dbodner]

If you start to feel like the wing is being pulled forward then you've put the nose down too far.

Matthew, doesn't that go against BVH's explanation of ramp suck? And I seem to remember someone blowing a launch at HR because he lifted his nose to dispel ramp suck (and because he was too far back).

[Matthew]

There are lot of explanations about ramp suck. But if the glider feels like it's being pulled forward, it isn't balanced. Simple as that. And as I noted, you don't lauch with the nose high. You lift it a little so as to get the wing squarely and easily on your shoulders and then you pull the nose down to the proper angle.

Matthew

[brianvh]

My theory, if it's correct, predicts maximum forward 'suck' when the keel is horizontal. As the nose comes down (closer to the wind direction) the percentage of force in the forward direction increases, but the total aerodynamic force decreases. Maximum forward force thus occurs at the horizontal angle. Once the nose is perfectly into the wind all forces should vanish since you have zero angle of attack.

But forward and lifting forces aren't all there is. I think part of what people are experiencing as ramp suck is the tendency of the wing to rotate. If you just stand on flat ground with no breeze and rotate the nose so that it's pointing downwards, it can't possibly be balanced on your shoulders and will start to go over in the forward direction. There may be aerodynamic forces that accentuate this, but I don't know what's going on in that case.

I personally have a hard time holding the nose downwards into the wind because of the grip I use. Plus I'd rather not dive off.

[Dan T]

I didn't mean to imply that one should hold the nose below a zero angle of attack. However if you are getting "heavy up, heavy up," or even "medium up" whatever that is, it's pretty clear that all the forces aren't neutralized. A little more down on the nose in those conditions could calm things down a lot. Afterall that's the very definition of near zero angle of attack isn't it?

I also didn't mean to imply that you should dive off, but rather that a smooth exit in firm but not excessive winds is much lower stress with a little lower nose angle resulting in a wing that isn't trying to lift you and your wire crew off the ramp.

Key point here is "a little lower." Like most things in hanggliding changes should be made in small steps.

[mcelrah]

One thing about ramp suck is that the keel man is there less to control pitch than to haul back on the back wires to prevent the glider getting sucked off the block. Another argument for wirecrew to wear gloves: enables you to really hold on if needed (plus in winter makes it more comfortable if it takes awhile for the pilot to get off safely). - Hugh

[Danny Brotto]

The center of the glider stalls (or stops generating lift) before the tips do. That’s what allows the nose to drop straight ahead during an in-flight stall maneuver. (If the tips stalled sooner or even at the same time as the root, we’d all be spinning out of the sky.)

“Suck” comes from the center of the planform not flying (stalled, not generating lift) but the tips flying (generating lift.) Since the tips are behind the CP (center of pressure), there is a feeling of rotation. That’s what you feel as ramp suck; the glider is effecting a stall recovery.

This is especially prevalent at sties where the wing tips are more exposed to the airstream than the root such as like at HR.

Danny Brotto

[brianvh]

That is a very likely explanation of some of the rotation effect. But stall recovery rotation should increase as the nose goes higher: instead the 'ramp suck' seems to increase as the nose goes lower. Maybe at a cliff launch the AOA is so high the root AND tips are stalled at high nose angle, then begin to fly before the root as the nose is lowered?

My theory is lacking too: it says the forward force should increase until the wing is level to the horizon, then decrease. I'd like to attribute the increase people seem to feel as the nose goes below horizontal to a rotational force (which can be mistaken for a forward force), but classic stall recovery doesn't seem to match the symptoms. We need to throw in some extremes like what I said above or Danny's idea of the root being literally blocked from the wind. Is the ramp suck at Lookout mountain weaker than what we have? The ramp there is as wide as the glider.

[Flying Lobster]

What Danny said.

Wing wants to fly.

marc

[brianvh]

Since ramp suck is weaker at Lookout Mountain, Danny's theory wins on emperical evidence.

I still think my explanation explains the feeling of being pulled straight off, even if it can't explain the nose rotation effect.

[XCanytime]

Ramp suck can be explained by simple vector geometry and using a helicopter takeoff as an analogy. The helicopter initially rises vertically from the lift generated by the rotor blades. The blades have a certain AOA that generates a completely vertical lift force vector. Then the AOA of the rotor blades is decreased to tilt the lift vector forward, giving the helicopter forward groundspeed. The lift vector can be drawn as a vertical lift vector and a horizontal thrust vector. A glider on the cube who has its nose down a bit is the same as the helo leaning over and starting to move forward. Whether the lift under the glider is provided by airflow over the root or by rotor lifting the tail, the effect is the same.

The increase in rotational force when lowering the nose can be explained as well. I wrote an article for the Capitol club newsletter back in 1995 detailing why this happens, since I had a heart-in-the-mouth dive off of High Rock earlier that year and wanted to figure out why. The leading edge has twist/washout, a varying AOA from nose to wingtip. The AOA decreases from the nose to each wingtip. This enables the stall to start at the root of the glider, then proceed to the wingtips. At a certain AOA, in theory, only the wingtips are flying, and the wingtips are aft of the CG. If you stepped off of the cube at this time, the nose would have a certain rotational force downward. Back on the cube, if the AOA is decreased some, more of the wing towards the nose is flying, but most of this is still aft of the CG. If you stepped off of the cube at this time, the rotational force downward would be greater since there is more lifting force aft of the CG. Using a see-saw as an analogy, the more lifting force behind the fulcrum (CG), the faster the see-saw pitches down.

Bacil

[Danny Brotto]

Bacil

I believe the rotational force is along the center-of-pressure (CP) not the center-of-gravity (CG). The CG is the gravimetric equilibrium point while the CP is the aerodynamic equilibrium point.

Danny Brotto

[brianvh]

Bacil - your first paragraph is more or less my theory, the second is more or less Danny's. If we all obsess over this stuff I would *hope* we come to the same conclusions. I think your helicopter imagery makes the concept easier to accept intuitively than anything I've come up with.

I'd like to see that original article.

[brianvh]

When solving physics problems the axis of rotation is a matter of preference. if you choose the cg, then the motion consists of rotation around the cg plus translational motion of the cg. This is the best choice for free moving objects. For fixed objects that are not yet translating (still on launch) you can place the axis as you wish to either clarify the problem or eliminate a force as a torque. Using the cg means gravity does not cause rotation around that point and you only have to analyze the aerodynamic forces; using the cp means aerodynamic forces do not cause rotation about that point and you just have to look at how the glider is balanced. But since the glider translates somewhat about the hang point when the pilot is standing on launch, I think it's best to use that and ignore the pilot's mass and weight.

[XCanytime]

Danny,
Thanks for bringing up the CP. Didn't say that the rotation was around the CG. Just said that the lift is aft of it. The CG is in front of the CP. Gravity pulls down at the CG, lift pushes up at the CP. Hence the see-saw analogy. The CP moves as the AOA changes, but is always aft of the CG.

Bacil