Static Thrust in lbs = 2.83E-12 x RPM^2 x D^4 x Cp x ((In. Hg)/29.92) x (528/(460+deg F))

Where:
RPM = revolutions per minute;

D = propeller diameter in inches;

Cp = propeller coefficient, use 1.00 for Zinger wood props or APC nylon props. See note below for Master Airscrew, K-series, black nylon props and others;

(In. Hg) = barometric pressure in inches of mercury;

deg F = ambient temperature in degrees Fahrenheit

NOTES:

(1) Don't worry about not having a pitch term. I questioned that too. The basic equation is from the Marks Mechanical Engineer's Handbook. The equation in the handbook does not include pitch as a variable, but expects the user to empirically determine a coefficient (my/your Cp) that is adjusted for variation in propeller geometry. Although my data is limited, I have found that pitch variation in the 6" to 10" range has no appreciable affect on thrust prediction of props of 11" to 15" diameter, tested below 10,000 rpm. Test with a new, unblemished Zinger prop and you will be a believer! There are great variations in propeller geometry though, so you should determine your own Cp for the style of prop that you prefer to use (refer to note 5 below)

(2) Don't be concerned about the units not working out to lbs. The constant 2.83E-12 isn't dimensionless. It includes standard air density (lb/ft^3) divided by the gravitational acceleration constant (ft/sec^2). It also contains units conversions, to allow you to input rpm rather than rps, and prop. diameter in inches rather than feet. In addition, it contains an empirically determined value that allows thrust prediction using a Cp value of 1.00 for Zinger or APC props.

(3) If you don't want to mess with temperature and barometric pressure just leave out the terms following Cp.

(4) I have a reasonable amount of data on Zinger and APC props, but just enough data on Master Airscrew (MA) K-series Nylon props to show that they are definitely a different breed. Data taken on the same day, with the same engine and the same fuel showed a Cp of 1.33 with a 13x8 MA when compared with a 13x8 APC (baselined at Cp=1.00). My engine rpm dropped considerably (1000 rpm) running the MA, so, though thrust was higher overall performance was not. The bright side is, if you need more engine load because of ground clearance problems use a MA prop. You will get the thrust, if not the airspeed.

(5) Determine your own coefficient (Cp) for your props. I have found certain sizes/pitches of my Zingers and APC's don’t precisely hit the 1.00 Cp. I am sure that there are variations in all brands and sizes. All you need is a good thrust measuring technique. Using a fish scale and a hard level surface, you should be able to measure thrust while the engine is mounted on the plane.

(6) Results will not be predictable for any prop that has been deformed, shortened, or reshaped in any way. My experience is that the prop efficiency will always be reduced if you mess with it! But then, I don’t design props, and some folks are masters at prop carving.

Horsepower = RPM x Pitch/12 x Thrust/33000

Where:
RPM = revolutions per minute

Pitch = propeller pitch in inches

Thrust = lbs-force from my static thrust calculation or your thrust measurement.

Velocity in miles per hour = 9.47E-4 x RPM x Pitch

Where:
RPM = propeller speed in revolutions per minute.

Pitch = propeller advance rate in inches/revolution.

For towing banners I have a release bracket mounted on the bottom of the fuselage just behind the landing gear.

I first powered the Kadet with a 0.61 glow and later a 0.61 diesel. I switched to the diesel because I was having flameouts with the glow. Neither are strong engines but plenty strong enough. I used a 12x4 prop on the glow and a 15x8 on the diesel which runs about 1/2 the rpm of the glow.

The Kadet will fly slow without concern of stalling and gliders like to fly slow.

A launch assistant held the glider and ran a few steps with it until the tow plane reached glider flying speed (long before liftoff of the tow plane). We reached a higher altitude in a climb-out followed by a 180 degree turn than the glider pilot normally gets with his power pod. The glider pilot swings out wide on the turns as a water skier would to avoid getting slack in the tow line.

Here is the tray with a transmitter installed.

Mat’ls:

• 1 ea. board, 1/4” x 6” x 10” long
  
• 2 ea. eyebolt, 1/4” x 8” long
  
• 4 ea. fender washers, 1/4” x 1”
  
• 4 ea. hex nuts (preferably hex stop-nuts), 1/4”
  
• 2 ea. electrical cable clamps, 1/4”
  
• 2 ea. machine screws, #6, 1-1/4” long
  
• 2 ea. washers, #6
  
• 2 ea. blind nuts, #6
  
• 2 ea. wing nuts, #6
  
• 2 ea. Tygon® tubing, 5/16” i.d. x 6” long
  
• 1 ea. neck strap
  
• A few drops of Loctite #290 Threadlocker

The tray shown is 5-7/16” wide (actual stock width) solid lumber, cut to a length of 9-5/8” which allows room for a timer or frequency pin to be clipped to the tray above the transmitter. Drill 1/4” diameter holes at each bottom corner, 5/8” inboard from the edges. Saw a concave curvature in the bottom edge, beginning and ending 1” from the corners. This will help stabilize the tray against your stomach.

Bend the two eyebolts 20 to 30 degrees, beginning just beyond the threads. Slip the Tygon® plastic tubing over the shank of each eyebolt, then temporarily install each in the 1/4” holes using a nut and fender washer on the front and back sides of the tray.

Place your transmitter on the tray and mark the location of the two electrical cable clamps that you have selected to anchor your transmitter to the tray. Drill 5/32” holes at the marked location for the cable clamp installation. Also, mark the location of your buddy-box cable connection and charging connection so that you can carve an opening for them (see half-moon notches in pictured tray).

The cable clamps shown have been fitted with pressed-in blind nuts. To press the nuts into the clamp you can use scrap lumber as a base, drill a 5/32” hole in the lumber and pull the blind nuts into the clamp with a screw from the backside. To install the clamp (fitted with a blind nut) on the tray, insert a machine screw with a wing nut installed backwards (installed with the wing side facing the screw head) and a washer from the backside of the tray. Thread the screw into the blind nut only until the screw begins to protrude through the blind nut. Use Loctite threadlocker to secure the screw to the blind nut threads. The wing nut is then the item used to pull the clamp tight against the transmitter handle and to release it when necessary.

After installing the transmitter, and painting/finishing the tray, reinstall the eyebolts, using Loctite threadlocker on the nuts (unless stop-nuts are used) to preclude loosening.

One rather short (in height) piece is at the base, the other, taller piece, is several inches closer to the apex of the triangle. The latter must be taller than the axle height of the plane. It must also be 6” or so wider than the landing gear to assure safety.

Note: This device is only meant to be used with tricycle gear airplanes. If you use it with tail draggers you must hold the tail down when throttling up to avoid nose-over prop bashing.

The restraint was designed with many uses in mind. Foremost was allowing engine startup and needle setting without someone having to hold back the plane to keep it from leaping forward and chopping someone or something into bite-size pieces. Secondly, it greatly reduces the amount of debris that the propeller picks up and throws at the carburetor and engine operator. If your engine’s thrust will slide the restraint, the restraint itself must be pinned to the ground by driving a spike through a drilled hole provided in the restraint, and into the ground below. I use a concrete penetrating nail if asphalt is involved.

The restraint doubles as a thrust measuring platform for tricycle gear airplanes by allowing the operator to pull the plane backward (by the tail feathers) while reading a fish scale. The average reading, as the plane is pulled back and then allowed to return to the wheel stop, is taken as the thrust measurement. The hard, smooth surface of the restraint allows good measurements.

The platform of the restraint also catches a great deal of the oily exhaust residue, especially for side-mounted engines. The bulk of the residue is kept off of the grass and is easily cleaned up with the same solution used to clean the plane.

The handle for this setup is two ropes, each spanning the narrow dimension of the tray. The ropes are located at opposite ends of the tray (or near the ends). By adjusting the length of each rope, the tilt of the unit, when picked up by one hand grasping both ropes, can be adjusted to make it level.

Use metal bottles for long term storage of diesel fuel to avoid loss of the ether; I use Sigg bottles. They don't have caps that can be adapted for fuel servicing so I transfer the fuel to a translucent plastic bottle before going to the field.

Back to the fueling operation. You need 3 each, 3/8” diameter wooden dowels, 4 ft long. Bundle the dowels and wrap a #64 rubber band around them (double it several times) about 8” from one end. A shoestring is shown in the photo but a rubber band gives you more flexibility in setting the tripod legs. Add a second rubber band for security in case one breaks. Spread the dowels at the far end and stand them tripod-like over the airplane.

Tie a string with a loop on it to the neck of the fuel bottle. You can drop the loop over the S-hook or dowel as appropriate. Hang the fuel bottle high. Route a long line from the clunk line connection to the plane’s fueling connection.

A nice alternative to the above tripod is a davit made of 3/4” pvc pipe standing in a socket on your field box. For a socket, I used a fishing rod holder with a section of foam rubber pipe insulation inserted. The pipe will fit snugly in the insulation. I incorporated a holder for my planes in the figure below.

Now, you can go about the business of preparing the aircraft for flight; fueling will take care of itself.

When fueling is complete, you will notice that the vent line will be filled to the level of fuel in the bottle. Lower the bottle to the ground and watch as fuel in the vent line begins returning to the aircraft fuel tank. After it reaches the tank disconnect the fueling system, sealing all lines as necessary.

To deservice, leave the servicing bottle on the ground below the aircraft, hook up the fueling line only, squeeze the bottle, block the bottle vent, and release the squeeze. When fuel reaches the fueling bottle reopen the vent. Disconnect and seal all fittings

Deservicing is not necessary for glow fuel. Just disconnect the airplane's fuel feed and pressure lines and connect them to each other.

Buy a glow plug connector from your local hobby shop and two bulb sockets and two bulbs from your local auto parts store. Wire the sockets such that both filaments (hot and cold) from one bulb and the cold filament only from the other bulb are in parallel, supplied by the 12-volt battery.

Don't use all four filaments -- your glow plug will burn out in 4 seconds.

Buy one of those clock-like timers at the hardware store (less than $10). The kind that you plug directly into a wall socket. It should have sets of pins that can be placed for "ON" and "OFF" times. Make sure it can be set for TOTAL on-time of as little as 30 min. Hang it on the wall and plug in your transmitter/receiver charger. Hook up the transmitter and plane and turn it on with a 30 min. setting. You will get a 30 min. charge every 24-hrs.

When you come home for flying hook it up again, spin the clock dial around until you can see that you will get an appropriate number of hours before it reaches the "ON" pin, then manually turn it "ON". It will charge until it reaches the "OFF" pin; then will charge 30 minutes every 24-hrs thereafter.

If, when you check the charge on your transmitter or airplane. it is not fully charged, it is time to replace the battery because, in 24-hrs, it is leaking down more than a 30 minute charge will replenish.