Prescott Pusher Notes

Some of the more significant modification made to my Prescott Pusher N40LE.

Safety Improvements:

Control & Stability improvements

Other Improvements

Performance Improvements

Safety improvements:

1. Add a stall warning system. The airplane has no aerodynamic or other warning of an impending stall. My system also incorporates an AOA indicating system
2. Change the fuel system to increase reliability of fuel delivery to the engine.
a. Add small electric fuel pumps (feed pumps) as close as possible to the wing tank outlet
b. Modify the fuel selector to incorporate a “both” selection. This will be the only selection used in normal use.
c. Add warning lights to indicate failure of a selected pump to deliver fuel pressure. (Be aware that skidding with relatively low fuel quantity in a tank, may throw fuel away from the pick-up point)
d. Add warning lights to indicate lack of fuel pressure at the engine
e. Establish an SOP to operate both feed pumps for t.o. and landing. En-route operate only one pump at a time, switching as required to keep the fuel balanced. In case of electrical failure, fuel will be sucked through the feed pumps by the engine driven pump, but a landing before fuel levels possibly become unbalanced is required.
f. Add an electronic fuel management and fuel flow indicating system.
3. Add warning lights for other essential functions. (Voltage or generator, vacuum, door latching system.)
4. Add a standby or essential electrical system. I use sealed batteries in the nose compartment to power one comm. radio, a handheld GPS, the propeller RPM function, and the gear down function and lights. The weight of the batteries is offset by a reduction in ballast weight. Batteries are charged by the regular system.
5. Add a small damper to the elevator trim tab. This is to prevent destructive flutter in case of failure of tab control rod, its attachment and trim motor gears and crank.
6. 100 W. (each) adjustable landing lights have been provided with circuitry to turn off these lights when gear is up.
7. Add switch and circuitry to sound the gear warning horn if gear is up and flaps are more than half extended.
8. Gear horn volume has been increased.
9. Change flap travel switches position slightly to prevent coast down travel from engaging the mechanical stops.
10. Provide a shoulder harness anchor in the ceiling between front and rear seats.

Control and Stability Improvements:

1. Directional Stability
a. Add 8” or more to ventral fin.
b. Add app. 3” to lower trailing edge of rudder tapering to zero at the top of the propeller arc.
c. Add a fixed triangular shaped filler to the horizontal stabilizer at the top of the rudder to fill the gap between rudder and stabilizer.
d. Eliminate the narrow triangular spaces each side of the rudder just forward of the lower rudder. This is where closeout panels 11045-007 & 008 would otherwise be installed. It requires narrowing the closeout ribs under the rudder, part # 11045-001 & 003 to app. 6.7” at the joint. It also requires cutting the shell where it attaches to the vertical stab. to slope down and aft app. 13º to a point app. 3½” above lower edge of rudder.
2. Pitch stability
a. Change elevator gearing by moving the horizontal connecting rod up app. 3” on the trapeze.
b. Change the position of stop bars as required.
c. Add an elevator down spring.
d. Change the trim tab limits for more tab down travel. (Required to enable trimming with full flaps and forward C.G.) A larger trim tab with dimensions similar to later Prescott Aeronautical design is provided.

Other improvements:

1. Add a stinger with a roller blade wheel to the ventral fin to protect the prop against over-rotation. Stinger should be long enough to accommodate flexing of shell and still protect propeller. App. 2-3" propeller to ground clearance is desirable.
2. Fix the engine-cooling problem by moving the air intakes forward to the area outboard of the baggage compartment. I use 10x2” NACA scoops with a small projection (scoop) at the aft edge of the intake. Total cross-section is about 27 square inches each side. This is ducted through the firewall using 6” dia. stainless steel ducts and SCAT ducting to the engine air-box with essentially no directional changes to the airflow. Because the firewall is penetrated, consideration has to be given to fire protection.
3. The design location for the oil-coolers has been retained, but with an aft pointed exit to prevent reverse flow through the coolers at high angles of attach. The coolers are plumbed in parallel rather than in series. The coolers also provide cabin heat, and this function has been substantially improved with movable doors in the air-intakes changed to taking air from inside the compartment, as well as blowers in the air-box above the coolers. Fit of the flapper valves and air leaks have been addressed. Future plans include remote adjustment of the movable intake doors. They are presently ground adjustable only. 3.Install a Dawley designed and built exhaust system with tail pipes terminating in the wing root areas.
4. Move the pitot tube to the very nose of the airplane. (I don’t at this time have a perfect place for the static ports.)
5. Change the arrangement in the nose compartment to enable the ballast to be moved to the extreme front, and to provide support for the pitot tube. To do so, I moved the electrical relays to the back wall of the compartment and made the ballast and pitot tube supports structural.

6. Nose gear improvements. (Prescott nose gear)

a. Add two ball plungers to drag link 14232-13 and drag arm -031 to prevent gear from folding when hydraulic pressure is not available. Avoid interference with steering damper when gear is retracted.
b. Provide additional wear surfaces around the kingpin (14231-075) holes in strut (14231-57) to prevent king pin wear. I used ¼” thick material about 7/8” dia. welded to the strut top and bottom. Also requires a longer kingpin,

c. Reinforce drag link 14232-31 tube in top right corner leading to the hydraulic actuator

d. Increase dia. of stud 14233-017 in part # 14233-021 to 5/16”. These parts transfers steering inputs to the strut
e. Add lubrication fittings to the four points where the gear mates with the airframe. (The above changes (a-d) are addressing actual problems or failures that have occurred on my airplane or others.)
7. Add relay and circuitry to the “gear up” function to keep the hydraulic pump operating till the gear is up. (This is a mirror image to the relay and circuitry that P.A. added for the “gear down” function.)
8. Double the number of nut-plates and fasteners used to attach the wing tips to the wings. (In the area aft of the fuel tank only)

Performance improvements:

1. Add outer gear doors to close up the holes at the top of the main gear legs when retracted.
2. Change wing tip design. I like the ones made by Craig Easter. (Precision Design, Weatherford, OK. 580 772-2140). They reduce the stall speed by two MPH, improve the feel (stability) of the airplane, particularly near the aft C.G. limit. Since they move the center of lift aft by app. 0.6”, it enables moving the aft CG limit further aft by 0.6”. On the negative side, they are about 9 lbs heavier (for two) than the P.A. tips and increase the wing bending moment slightly. This mod requires the additional fasteners in item 7 of previous paragraph.
3. Provide “canoe” fairings for the aileron counterweights.
4. Change propeller to an MT hydraulic constant speed unit. This is a three blade unit, model MTV-12-B/LD 162-24. Diameter is 63.77” which gives me more ground clearance. Performance improvements are limited to a 200” shorter t.o. roll, cruise speeds are not improved, but the positive control of RPM at altitude is really nice. The RPM control is by means of a pedestal mounted switch operating an electric servo hooked up to the Woodward governor.
5. Install vortex generators (VG) behind the wing-walk (and similar on the right side) and app. 8” in front of the ailerons. Effect is a lowering of stall speeds and improved lateral control at the stall. Note that the VGs behind the wing walk only, had to be next size up to reach into undisturbed air.
6. In addition to these more significant changes, I have a number of smaller changes incorporated during the building process. Some of these changes involve cutting lightning holes in non-structural parts such as fillers where feasible, others provide better fitting parts or provide for easier servicing.
7. Installed avionics makes the airplane suitable for light IFR. This includes a three-axis autopilot with altitude hold. Later installation of a storm scope has been provided for:
a. The original propeller was a Price fixed pitch unit. It was changed to a light constant speed unit in 2003.
b. I use a Lyc. 0-540 engine with an estimated output of 280HP. It is among the lightest in the 540 series. It is still heavier than I like;: however, it is more reliable than the IO-360 pushed to provide 225 HP, which is about the minimum required to get acceptable t.o. performance.

I use 2600 lbs. for my normal max gross weight; an alternate gross weight of 2820 lbs. can be used if four people must be carried, but this weight requires a max zero fuel weight of 2600 lbs, a max landing weight of 2710 lbs., it further requires a reduction of the max speeds and different maneuvering speeds, as well as a reduction in G-tolerance.

An SOP of approach and landing speeds based upon weight is in effect. (Vref speed system) The above set-up enables me to take off in 2100’ (to 35’altitude) at 2600 lbs. at sea level, standard day, and no wind. Take offs under other conditions can be calculated. Cruise speeds have been obtained and documented for fuel flows ranging from 11 GPH to 15 GPH and range from 178 MPH TAS (3000’, 2750 lbs. and 11 GPH) to 205 MPH TAS (12,500’, 2100 lbs. and 13 GPH). These speeds are obtained during summer flying in Illinois and approximate conditions of Standard + 12º C.

 

Sincerely,
Ole