Effect of Weight On Speed & Glide, Small Wings
Heavier pilots go faster, how much faster?
We all know that more weight means more speed but how much? My wing
reviews are obviously done with my 150 pound self so I'm unable to see
what happens at real high wing loadings since they don't make wings that
small. The Plasma 22 and Pluto 21 were the smallest production wings
I've tried lately and my Spice 22 is too big for me. To get a high wing
loading I have to fly a 17 and nobody makes those in enough quantity to
call them production.
So how much faster would a heavier pilot go on a Spice 22?
Thankfully, some brainiacs (Dana Hague and Carlos Curti)
show us how. The formula is:
SQRT (New Weight / Original Weight) * Original Speed
(either airspeed or sink rate).
For example, lets say a 300 pilot+wing+motor wants to know what his
speed will be on a wing flown by a 250 pound pilot+wing+motor. He knows
how fast the 250 pounder flies on that wing.
1. Calculate Weight Ratio: New Weight / Original weight. Lets say New
Weight = 300 and Original Weight = 250. Weight Ratio is 1.095
2. SQRT (Square Root) ratio to get Speed Ratio. That's 1.2 here.
3. Multiply Speed Ratio (1.2) * the speed or sink rate to find out
the new speed or sink rate.
If the original (250 pounder) reported speed of 25 mph you can see
that HIS speed on that same wing will be 27.4
mph. And if the original sink rate was 400 fpm than the new sink rate will
be 1.2 * 400 or 480 fpm.
Risk of Small Wings
I love small wings and it's tempting to keep going smaller but know the
risks and tradeoffs. I've flown
an 11 meter wing which was, not surprisingly, remarkably responsive.
1. What happens after a motor failure? If it happens
just after takeoff, you'll be faced with a dramatic surge and dive at very high
speed. If you're a foot
off the ground, an immediate pull of brakes should keep the bones intact.
If you're 10 feet
off then a brief pause to allow slight surging to a mostly level attitude
must be followed by nearly immediate flaring. If
you're above 100 feet than all is well as you would have enough time to
let the wing surge with some dampening (to prevent front tuck), establish a glide and do a normal flare. If you have
not rehearsed fine pitch control already, the instinctive responses
required cannot be thought out.
2. You'll be running really fast, especially in light wind and/or higher
elevations. Tripping on surface perturbations will have graver
consequences.
3. Another tradeoff is high fuel flow. Just like with reflex wings
trimmed fast, it takes a lot of power to overcome our enormous drag.
4. Handling will be extremely sporty especially on models already
known for sporty handling. I can only imagine what a 16 meter Spice
would be like (admittedly I'd love to find out). A common cause of
crashes on high end racingtype wings is an oscillation just after
takeoff.
5. Lastly, although speed is great, if you do suffer a collapse the
recovery will be "dramatic" at best.
So indulge in small wings carefully, understanding that with speed,
inflation ease, and light weight, comes sharp teeth that will bite
unwary bravehearts who don't pay appropriate respect.
Glide Ratio And Weight
Glide ratio can be expressed either as the lift/drag ratio or, more
commonly, the distance you'll travel forward for the distance dropped,
and its the same at any weight. Every glider has a speed/configuration
that gives the best glide ratio and, in that configuration, (usually
neutral trim, no speedbar) you'll fly the farthest distance in calm
wind. Mind you, the speed at which best glide occurs will be
faster at heavier weights but the glide ratio remains the same.
A heavy pilot will fly just as far from 100 feet as a light pilot on
the same rig but the heavy pilot will get there faster. Both his sink
rate and forward speed will be greater.
Note that power required
is based on sink rate and weight so the heavier pilot, not surprisingly,
requires more power to stay aloft than the lighter pilot.
