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Stinson Floatplane Technique

Getting your underpowered Stinson off the water!

Out of necessity, I've spent a great deal of time perfecting my take-off technique with our modestly powered 165 hp 108-2. As a novice, I tried desperately (and unsuccessfully) to force the plane onto the step.  What experience has taught me, is to plan ahead, perfect your technique, and be patient! Under most conditions the aircraft will get on the step, and once on the step one seldom has a problem getting into the air. Here's a few points that may help you:

Keep your float bottoms clean! Performance deteriorates rapidly when green slime, or anything else starts to collect on the undersurface of your floats.
Getting onto the step is easier with a forward C of G. ie. Put the big Pax up front. (best when hot, heavy, calm)
Once on the step, the aircraft will lift-off at a slower speed with a rearward C of G. ie. Lower stall speed. (best under conditions when you're not concerned about getting on the step. ie. nice breeze, cooler conditions)
At high gross weights, take advantage of even the slightest headwind. Stack all the chips in your favor.
Prior to starting your run, center your elevator trim (normal take-off position), lower the flaps to release the elevator stop, pull full up elevator, and retract the flaps fully while maintaining full back pressure on the wheel. This is done for two reasons; with flaps retracted you have much more prop-wash over the elevator and can get the nose up higher at the start of the run, and the flaps have little positive influence below step speeds.
Slowly apply full power while maintaining full up elevator.
Once the nose reaches its highest point, slowly and completely relax the back pressure on the elevator.
Allow the plane to accelerate on it's own. Any aggressive attempts to put the plane on the step, only adds drag and retards the take-off process.
As the back of the floats start to break free of the water, signaling the transition from displacement phase to step phase of the take-off... you'll feel the nose start to bob slightly... carefully lower one notch of flap. This will usually shift the center of pressure forward enough to put you on the step. If very heavily loaded, or on glassy water, slight forward pressure may help to break the suction. This phase is also characterized by the water spray moving approximately a foot behind the front spreader bar bracket, as seen from the pilots seat.
Accelerate on the step, holding the attitude at that sweet spot, until slightly below stall speed.
Pull on full flap, as you raise the nose to yank yourself off the water, then quickly lower the nose to accelerate in ground effect, and retract the flaps

Note: I prefer lifting one float prior to reaching lift-off speed, but this should be avoided in rough water as it places undue stress on the float gear. Also, I immediately place the flap handle in the take-off setting once off the water, then slowly and fully retract the flaps, allowing the airspeed to build to 80 mph IAS prior to initiating a climb. One should resist the temptation to attempt a climb below 80 mph when heavily loaded. I've found the added drag of the floats will prevent the aircraft from adequately accelerating if a climb is prematurely initiated. This can have disastrous results if attempting to clear an obstruction. On the other hand, once the aircraft reaches 80, decent climb performance is assured, and can easily be maintained.

This is the formula used by EDO to determine the take-off time (TOT) for proposed aircraft float combinations. I've presented it here, to show the theoretical difference between the 108-2, 108-3, and larger hp versions of the same aircraft.

HP = take-off horsepower
W = seaplane gross weight in lbs.
S = wing area in sq. ft.
B = total buoyancy of submerged floats in lbs.
K = propeller type factor: fixed pitch = 1.0; constant speed = 0.93
PI = performance index;

Calculate PI
PI = W over HP times the sq rt of W over S

Calculate TOT
TOT=K(W/B)to the power of .3333, x (PI/25) to the power of 5

For the 108-2 165 hp fixed pitch, on 2425s
HP=165; W=2235; S=155; B=4850

PI=2235/165 x sr of 2235/155 = 51.4359

TOT=1(2235/4850)^.3333 x (51.4359/25)^5 = 28.48 seconds

For the 108-3 165 hp fixed pitch, on 2425s
HP=165; W=2500; S=155; B=4850

PI=2500/165 x sr of 2500/155 = 60.8499

TOT=1(2500/4850)^.3333 x (60.8499/25)^5 = 68.496 seconds

For the 108-2 180 hp fixed pitch, on 2425s
HP=180; W=2235; S=155; B=4850

PI=2235/180 x sr of 2235/155 = 47.1496

TOT=1(2235/4850)^.3333 x (47.1496/25)^5 = 18.43 seconds

For the 108-3 210 hp c.s., on 2425s
HP=210; W=2500; S=155; B=4850

PI=2500/210 x sr of 2500/155 = 47.8107

TOT=0.93(2500/4850)^.3333 x (47.8107/25)^5 = 19.08 seconds

For the 108-2 210 hp c.s., on 2425s
HP=210; W=2235; S=155; B=4850

PI=2235/210 x sr of 2235/155 = 40.413936

TOT=0.93(2235/4850)^.3333 x (40.413936/25)^5 = 8.32 seconds.. wow!

For the 108-3 235 hp c.s., on 2870s
HP=235; W=2500; S=155; B=5740

PI=2500/235 x sr of 2500/155 = 42.724432

TOT=0.93(2500/5740)^.3333 x (42.724432/25)^5 = 10.28 seconds



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Copyright 2003 David P. Miller
Last modified: March 23, 2004