I've built a model glider and wanted to see the effect the weight of the fuselage had on its performance. It flew the further with more weight which seems counter intuitive. I can exert more force into the glider with more weight which results in greater airspeed, would this be the reason for the increased glide ratio? How does this compare to real life gliders? Thanks.
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1$\begingroup$ See here for the answer: Why would a glider have water ballast? If it is trying to stay aloft without an engine, wouldn't it be better to be as light as possible? $\endgroup$– Peter KämpfJun 2, 2018 at 11:14
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3 Answers
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Variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it.
The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio.
If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so.
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Real-life gliders require some minimum payload to have their C.G. within limits; without this they can't hit their performance numbers. For this reason it is common to ballast a 2-seat glider with a sandbag if it is operated by a single occupant: the glider pilots say that if underweight, the glider "penetrates" poorly i.e., it cannot be trimmed into a minimum sink rate state that also is comfortably above the stall. When underweight, the glider mushes along at low airspeed in a poorly-controllable state.
In my experience with model gliders, they too will fly poorly without ballast: hand-launching an underweight glider causes it to pitch up, then quickly lose airspeed and descend more like a parachute than like a glider.
answered Jun 2, 2018 at 6:16
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$\begingroup$ Thanks, if we had two identical life size gliders but one was heavier then the other, which would go further? $\endgroup$– VerpzJun 2, 2018 at 6:44
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$\begingroup$ The pitch up is caused by a too backward cg. location and the parachute-like descent means that it is fully stalled. You fly it with the wrong cg location! $\endgroup$ Jun 2, 2018 at 11:16
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$\begingroup$ yes, my point was that the hand glider, straight out of the box, required ballast to get the CG correct. $\endgroup$ Jun 2, 2018 at 12:10
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Even if your ballast doesn't change the CG location, there's a "sweet spot" airspeed at which the wing is most efficient (L/D ratio is highest), and this is always faster than minimum sink speed. It's usually possible to trim a glider to fly at best L/D without ballast, but this requires nosing down, which increases sink more than adding correctly located ballast.
