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Thursday, December 27, 2007

Angle of Incidence and Decalage in HLG

This is one of the real secrets of HLG flying. A stubborn glider that does not launch well can often be transformed into a fine performer by changing the angle of incidence of the stab or the wing. The photo (left) is DOGEAR, a 6 inch span hand launched glider that I designed and built. It did not launch well until I replaced the stab and now it's great. Its first ever timed flight was 38 seconds.

Why and how?

Angle of Incidence is the angle between the wing's surface and the fuselage axis. (Usually for polyhedral wing models, you can place a ruler flat with the underside of the wing near the root chord to get an idea of the incidence).

Decalage, geometrically and simply speaking, is the angle between the wing's surface and the stab's surface. If you were to use a ruler under the wing and another ruler under the stab as described above, then it is the angle between the rulers. See the diagram here.

0-0 (said "zero-zero") is when the two surfaces (or rulers in the example above) are parallel.

Some HLG modellers swear that 0-0 is the best setting. On the other hand, many others (including me) say that a HLG should have a small decalage. When I say small, I mean small. For example, if the stab has zero angle of incidence, then the wing should have just a few degrees positive, e.g. the LE of the wing is 1-2 mm higher than the TE. What I can say is that the vast majority of HLG pilots say that "large" decalage is bad. Now, bear in mind that it is not easy to measure AOI accurately even with rulers and "large" is really a referring to a small measurement.

How to recognise large decalage?

Usually, you see a loopy launch. Instead of the plane arrowing up in a straight-ish launch, it loops round and may even execute a full loop frighteningly close to the ground. In addition, such a model may balance well-forward of the ~2/3 rule: generally, for traditional HLGs the Centre of Gravity is at around 66% of the root chord (i.e. closer to the TE than the LE). To compensate for the large decalage, the CoG of such a model may be at 40% or less. Too far forward.

Your throwing energy is converted into a pretty but dangerous low level aerobatic manouvre, not into height!

How to recognise small decalage?

Straight up and straight down! It goes up like a rocket and down like an arrow. Sadly, there is little or no glide bit in between. Hopefully, it will not land with a sickening crunch. You may see quite a lot of "up" elevator bent into the stab to compensate.

If you have a HLG that does not launch well, try altering the decalage. Yes, that may mean removing the stab, sanding and/or planing the rear of the fuselage and fitting a new one, but it is well worth it. For a pod and boom model with tip up wing or tipping boom, you can insert a set screw or thin shim that for fine adjustments.

There is nothing like seeing your glider rocket up and flip over beautifully into a stable soaring pattern high in the air.

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