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By WhackityWhack
#403484
Ok, 1st, a disclaimer: I grew up in the Show-me-state so I don't automatically believe everything I'm told (or shown).

On several occasions, I've heard several very experienced pilots refer to a glider's "pitch curve". They are usually talking about the characteristics of a certain glider's design. I readily admit to them that I have no experience or knowledge of this aspect of HG aerodynamics so I have no idea what they are talking about. In other words, "save your breath until I figure out what you're talking about".

Now, I am familiar with Polar Curves and L/D curves and so forth. So, is a "Pitch Curve" a real thing or is it just pilot-speak?
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By red
#403485
WhackityWhack wrote:
Sun May 13, 2018 7:02 am
Ok, 1st, a disclaimer: I grew up in the Show-me-state so I don't automatically believe everything I'm told (or shown). On several occasions, I've heard several very experienced pilots refer to a glider's "pitch curve". They are usually talking about the characteristics of a certain glider's design. I readily admit to them that I have no experience or knowledge of this aspect of HG aerodynamics so I have no idea what they are talking about. In other words, "save your breath until I figure out what you're talking about".
Now, I am familiar with Polar Curves and L/D curves and so forth. So, is a "Pitch Curve" a real thing or is it just pilot-speak?
WhackityWhack,

Every airfoil has a pitch-curve. You can look them up on various aeronautical websites, but maybe you should use a wind tunnel to conduct your own evaluations, with pitch curves from your own tests. In general, the pitch curve of any airfoil would give an aircraft designer some idea of how large the tailplanes should be, or how far aft, on the proposed aircraft.

If you put a hang glider on a pitch-testing truck (or a wind tunnel, ideally), you can measure the force it takes to hold the glider at any certain Angle of Attack (with airspeed held constant). Plot the various AoAs on a graph, and the force needed at the control bar to hold it there, and you will have a pitch curve for that glider. It may be somewhat different, at various airspeeds (especially if the airframe deforms under load). The glider should pull up (recover from a dive) at any normal AoA. That would show as a positive pitch curve value. As the AoA goes negative, the glider should continue to pull up (still a positive value on the pitch curve). At some point, though, when the pitch angle (AoA) goes too far negative, the pitch force values will go negative, and the glider would dive itself, or even "tuck" violently in that case, if flying. The longer the pitch curve has positive values, as the AoA decreases, the better the glider will resist "tucking" in flight, and the more effort (from the pilot) will be needed to fly fast.

With some early HGs, the pitch curve was neither positive or negative (e.g. Cirrus-III, and the Standard Rogallo). Their pitch curves were just zero, at any normal AoA. The glider did nothing to pull out of a dive, from about 30 degrees nose up to 30 degrees nose down, on the test truck. Some early HGs on the test truck would rip the tail out of the tester's hands and tuck, even when at a positive AoA. These dangerous early hang gliders put the words "tuck" and "divergent" into our HG vocabulary.

Feel free to doubt anything said here, and verify those things for yourself.
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By TjW
#403488
It's a thing. Generally in aerodynamics it will be referred to as the pitch moment curve, or pitch moment coefficient, if it's for a single angle of attack.
The convention for airfoils is to express whether it wants to pitch up or down at a particular angle of attack as a dimensionless coefficient for the torque around the quarter-chord line.
You can also think of it as how the center of pressure on the wing shifts as you change the angle of attack.
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By lizzard
#403492
Red is spot on as usual,

I think the context of the conversation relates to bar pressure vs speed vs how fast you pull in and how far .

There are tuning configurations that if you pull in fast from stall , the par pressure remains light (only for hot shot pilots who are testing the limits ..dont go there ) Hang gliders are not suitable for aerobatics but if you want to practice, swing a bucket of water over your head becaus that is all a hanglider loop is .
You cant roll out at the top as far as i know to date ....the hotshots know all this but the new ones dont like the reference ....from an ex hotshot!
keep your wing overs 90 deg or less and you should be safe in smooth air (coastal).

Thermal conditions, esepcially at their peak are an entirely different scenario where all configurations are possible no matter what you do .
User avatar
By WhackityWhack
#403495
Excellent, guys! Thanks.

So, as I understand this, as it relates to Hang Gliders; Pitching Moment refers to Bar Pressure as far as the pilot is concerned. And that is generally affected by the wingtip and reflex design (or our "Tail")

Also, we are only concerned about the positive side of the curve because we are only flying with positive alpha (aoa).

And if the pitch curve were said to be flattened by a particular design improvement, we might see less bar pressure differential at different angles-of-attack? Or "similar bar pressures" through the envelope?

(Not only did I grow up in the Ozarks, I went to public school there! So go easy on me.)
By Lazypilot
#403502
I grew up in the Ozarks and went to public school there also.

About pitching moments and bar pressure: A glider having no dive recovery devices, such as reflex bridles ("luff lines" for us old timers) or sprogs ("washout struts") can have plenty of bar pressure as long as the sail is "inflated", but if a sudden downdraft "inverts" the sail the glider may dive straight down into the ground.

I just wanted to make sure you know this. Your glider may be plenty pitch-stable in normal flight, but older gliders often have sails that have shrunk over time. This shrinkage results in the luff lines being too long. Many gliders don't have any fixed washout via washout struts, they have only only the reflex bridles to ensure a positive moment when the AoA goes to zero or negative.

Some years ago, 15 or more, I bought a used glider and I could tell it had some sail shrinkage as the batten strings were extremely tight, I replaced those but didn't adjust the bridles.

I was flying along, minding my own business, when a rogue wind came up from below and behind me, my airspeed went to zero or close enough to it to qualify, and with no airspeed the glider began to sink quite rapidly. This resulted in the AoA going to a very high quantity, and my glider behaved as it should have, with the nose pitching down to where I was falling straight down vertical. I was about 300 feet over the ridge when this occurred, and I doubt that I had more than maybe 37 feet left to go when she pulled out of that dive. It was an interesting and very educational experience. I fixed those luff lines right away and for sure spread the word. Sail shrinkage is no joke, if your glider is an older one it pays to make sure that it has plenty of nose uppityness when the sail is unloaded.

I shudder when I think about the time I got sucked up into the clouds over Magazine Mt in Arkansas in 1977. My glider had no dive recovery devices, and I had no parachute, and I got tossed around quite significantly when I came out of the cloud in the rotor on the backside of the mountain. A wide spot in the highway showed up just in time for my "landing".

While I hiked back to launch to get my car someone stole my two longest battens, they were just fly rod blanks so I bought two cane poles at the local store, I think the glider flew better with them.
User avatar
By red
#403504
Campers,

Dive recovery system settings should not be guesswork. The HG manufacturer submitted those measurements (and how to make those measurements) to HGMA, as part of the glider certification process. Either the manufacturer or HGMA should be able to provide the information needed for you to check and set the dive recovery system of certified hang gliders.
User avatar
By red
#403505
red wrote:
Tue May 15, 2018 10:26 am
Campers,

Dive recovery system settings should not be guesswork. The HG manufacturer submitted those measurements (and how to make those measurements) to HGMA (no relation to USHPA), as part of the glider certification process. Either the manufacturer or HGMA should be able to provide the information needed for you to check and set the dive recovery system of certified hang gliders.

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