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FreeFlight Advice: TRIM: What is it, and what effect does wingloading or VG have?

Q: Recently, I bought a high performance glider from an experienced pilot. The previous owner of the glider weighs quite a bit less than I. I took the glider for a maiden flight a few days a go and one instructor told me that I am flying too slow. He added that the previous pilot may be a lighter pilot than I am and so, I should move my hang point forward.

I weigh more than the previous pilot and based on the higher wing loading I should be flying faster at the same hang point. I just could not understand why I was flying slower despite I weigh more. Could you please advice me in this subject and enlighten the physics behind this theory? Oh by the way, the VG setting of the topless was full loose on that day since I didn’t have the right bushings with a VG hole for my wheels. Therefore couldn’t tension the VG cord.  Another question I have is, if the trim speed of the glider is being affected by the VG settings?

A: This is an excellent question, and one that frequently confuses new and experienced pilots alike.  Thank you for asking- I know many people will benefit and appreciate this!

The very quick answer is that the instructor you spoke with is likely to be correct.  I of course can’t say if you were flying too slow or not (“too slow” italicized because that is sometimes a matter of opinion, depending how slow you were).  But I can confirm a heavier pilot often needs to adjust the glider’s hang point farther forward.

To understand why this is, let’s first clarify a big reason why this confuses people: the term “trim” is frequently used in two very different ways within hang gliding. Trim is the term used to depict the angle of attack the glider flies at without any input from the pilot. That angle of attack, in steady level flight, will produce a constant airspeed… and so people often use the term “trim” in that way too- to specify a particular airspeed.
Here’s the dealio with adjusting hang point due to wingloading- without moving the hang point at all, a much heavier person flying the same glider *WILL* fly faster than a light pilot did in almost all cases.  Greater wingloading = faster airspeed is exactly what intuition tells us, right?  BUT- what increases with wingloading? Stall speed. And what actually causes a stall (hint: it’s not airspeed)?

Where do we set trim on hang gliders? Min sink… which is basically just a mph or two above the first onset of stall. So a light pilot can set their trim to just a little faster than stall speed, and the glider will fly great hands off. But now a heavier pilot flies that same glider… the heavier pilot *IS* flying through the air at a faster airspeed, even though the hang point was not yet moved… BUT… this heavier pilot is likely to find that the glider now trims below their stall speed – too high an angle of attack.

All else being equal, stall will always occur at the same angle of attack for the same wing, regardless of wingloading. But with changes in wingloading, the corresponding airspeed at which that angle of attack is reached can be different.

So- the heavier pilot must move the hang point FORWARD- so that “trim” remains faster than the airspeed where stall occurs.

Truly curious souls will want to know why airspeed and stall speed don’t increase together, keeping the heavier pilot flying faster than stall as the lighter pilot was.  The reasons are many, but for one our wings are flexible- so greater wingloading will cause greater deformation of the airframe, and more washout (twist) in the sail.  While that might mean the wing doesn’t create lift quite as efficiently, it probably also shifts the center of lift.  Adding to that, the center of gravity for a flying hang glider (wing + pilot considered as a single unit) is not the same as the hook in connection point.  So changes in hook in weight shift the center of gravity of the aircraft, AND the center of lift may move as well.
The angle of attack that the wing naturally flies at with the pilot hands off- trim- is actuallymost accurately described as the equilibrium of forces.  Even small shifts in center of gravity and/or center of lift will result in equilibrium occurring at a different angle of attack.  It’s like a balanced see-saw… changing the weight on one side, or moving the fulcrum, will unbalance it.
You asked an excellent follow-up question about VG effecting trim!  Some people don’t think of that, let alone understand it- so I’m super glad you asked that too!
Taking what we just learned above about trim, let’s think about what effect the VG might have.  What does pulling the VG do?  It pulls the cross-bars back, spreading the wings farther, and tightening the sail; the trailing edge especially.
Flying with VG tight, bar pressure is reduced- on some glider models, it’s reduced significantly.  This comes from a combination of two things; spreading the wings wider means the twist at the tips isn’t as far aft of the CG, so there’s less leverage for creating a nose-up force.  At the same time, spreading the wings wider tightens the sail and pulls tension along the trailing edge, which reduces how much twist is in the wing.  Reduced twist obviously also means less nose-up force.  With what we talked about earlier, altering the planform of the wing and the twist, the center of lift probably shifts.
Pulling the VG does all of this by pulling the cross-bars back (excluding the rare glider that employs a cam-VG system).  Cross bars have weight, obviously, and so pulling them back can shift the total aircraft CG.
So- if center of lift probably moves, and center of gravity probably moves- trim (equilibrium) is likely different too.
On most modern gliders, I find they trim faster with more VG on.  I find this more pronounced as you go up in performance class of wing- probably not surprising since higher performance gliders are designed to get tighter for gliding efficiency, but still get loose for handling.
On topless wings it can be so pronounced it’s actually got a name: “Transient Trim”.
Most, if not all, manufactures generally recommend 1/4 VG for launching and landing, and so that is the most likely VG setting to check if a wing is trimmed correctly for you.  If you flew VG completely off, it’s not entirely surprising the glider was flying slow without any input from you.
A separate-but-related note I can’t help but include, is that I’d strongly urge you to work on airspeed recognition skills.  Whether this glider was trimmed too slowly for you, at your higher wingloading and with the VG off, that’s not much of an excuse for flying around “too slow” (per the observant instructor).  If it’s too slow, it’s really important you recognize that and pull in some!
Not knowing or understanding some of these complexities in weight-shift flex-wing hang gliders is understandable… they’re super dynamic, and frankly NO ONE 100% knows all there is to know!  I sure as hell don’t!  But I’ve had a lot of opportunity to study and learn, and so here I’m stoked to help and share 🙂
Thank you for writing in with these excellent questions, opening this topic for all to learn from.  Please keep learning, and I will too!  Please share the things you learn, and I’ll do the same… and we can help improve skill and safety among pilots everywhere!