NX-4L
Coming soon to a lake near me.
A brief question (details follow for the interested).
An acquaintance is building a two-seat parasol that has 183 sq ft of wing area, weighs 500 lb empty and has a 730 to 930 lb takeoff weight. It’s based on an early 1930s design and he will use a 50 hp two-cylinder, two-stroke with a belt reduction drive so his 95 ft lb of torque can spin an 84” wood prop with 34” pitch at 2200 rpm (prop speed) at cruise (802 fps tip speed = mach 0.72). It tops out at 106.83 ft lb of torque at 2460 prop rpm (917 fps tip speed = mach 0.805).
A large part of his plan is predicated on giving the airplane an authentic ‘30s look, a big wood prop driven by a two-cylinder opposed piston engine with the cylinders outside a cowling that hides the reduction belt and pulleys.
This acquaintance knows many times more about the topic than I do. So basically I am just trying to educate myself. I would think that lowering the reciprocating weight of the system would be a laudable goal.
*** DETAILS ***
His plan is to use a Hirth F-23 that produces:
50 hp and 42.73 ft lb at 6150 rpm while burning 4.3 US gal/hr (50 = 42.73 x 6150/5252) max
40 hp and 38.10 ft/lb at 5500 rpm while burning 3.5 US gal/hr (40 = 38.10 x 5500/5252) cruise
We were discussing propeller and reduction drive choices
With 1.8:1 re-drive 6150 (e)rpm is 3416 (p)rpm and 5500 (e)rpm is 3056 (p)rpm.
--- Torque is 76.91 ft lb at 3416 rpm and 68.58 ft lb at 3056 rpm.
--- At 70 mph cruise speed a 62” prop with 24” pitch has 824 fps tip speed (mach 0.74).
With 2.0:1 re-drive 6150 (e)rpm is 3070 (p)rpm and 5500 (e)rpm is 2750 (p)rpm.
--- Torque is 85.84 ft lb at 3070 rpm and 76.2 ft lb at 2750 rpm.
--- At 70 mph cruise speed a 70” prop with 27” pitch has 841 fps tip speed (mach 0.75).
With 2.5:1 re-drive 6150 (e)rpm is 2460 (p)rpm and 5500 (e)rpm is 2200 (p)rpm.
--- Torque is 106.83 ft lb at 2460 rpm and 95.25 ft lb at 2200 rpm.
--- At 70 mph cruise speed a 84” prop with 34” pitch has 802 fps tip speed (mach 0.72).
Of course these numbers are “real” only in a friction-free universe.
A smaller, lighter, flatter, faster-spinning prop, would aid in climbing.
But a larger, heavier, more sharply pitched, slower turning prop would have a larger arc and be a better cruise prop.
IF that higher pitched prop would allow the engine power to be reduced in cruise it would be more fuel efficient... IF... But wouldn't it also deliver more wear to the system?
For comparison, the dual-ignition Continental A-40-5 (O-115) at 154 lb is 83 lb heavier than the 71 lb O-64 Hirth.
40 hp and 81.63 ft lb at 2,575 rpm burning 4.65 US gal hr (40 = 81.63 x 2575/5252)
--- At 70 mph cruise speed a 74” prop with 29” pitch has 832 fps tip speed (mach 0.75).
--- (My Cub book says an A-40-1 can be throttled back to burn a meager 2.8 US gal hr, but it doesn't give an rpm for that power setting.)
No power is lost to the re-drive in the direct drive engine, but the unit is nicely balanced and has a dual circuit electronic CDI ignition so it’s a little bit more fuel and oil efficient (1:40 mix on the Hirth).
--- Cont.: Fuel consum. (40 hp) 0.72 lb hp hr: 4.65 US gal hr. Oil consum.: 0.025 lb hp hr: 0.53 US qt hr.
--- Hirth: Fuel consum. (40 hp) 0.54 lb hp hr: 3.5 US gal hr. Oil consum.: 0.016 lb hp hr: 0.35 US qt hr.
**Numbers input are from Recreational Power (Hirth), Continental’s 1938 A-40 OM, and Culver’s prop calculator.**
Andrew
An acquaintance is building a two-seat parasol that has 183 sq ft of wing area, weighs 500 lb empty and has a 730 to 930 lb takeoff weight. It’s based on an early 1930s design and he will use a 50 hp two-cylinder, two-stroke with a belt reduction drive so his 95 ft lb of torque can spin an 84” wood prop with 34” pitch at 2200 rpm (prop speed) at cruise (802 fps tip speed = mach 0.72). It tops out at 106.83 ft lb of torque at 2460 prop rpm (917 fps tip speed = mach 0.805).
A large part of his plan is predicated on giving the airplane an authentic ‘30s look, a big wood prop driven by a two-cylinder opposed piston engine with the cylinders outside a cowling that hides the reduction belt and pulleys.
This acquaintance knows many times more about the topic than I do. So basically I am just trying to educate myself. I would think that lowering the reciprocating weight of the system would be a laudable goal.
*** DETAILS ***
His plan is to use a Hirth F-23 that produces:
50 hp and 42.73 ft lb at 6150 rpm while burning 4.3 US gal/hr (50 = 42.73 x 6150/5252) max
40 hp and 38.10 ft/lb at 5500 rpm while burning 3.5 US gal/hr (40 = 38.10 x 5500/5252) cruise
We were discussing propeller and reduction drive choices
With 1.8:1 re-drive 6150 (e)rpm is 3416 (p)rpm and 5500 (e)rpm is 3056 (p)rpm.
--- Torque is 76.91 ft lb at 3416 rpm and 68.58 ft lb at 3056 rpm.
--- At 70 mph cruise speed a 62” prop with 24” pitch has 824 fps tip speed (mach 0.74).
With 2.0:1 re-drive 6150 (e)rpm is 3070 (p)rpm and 5500 (e)rpm is 2750 (p)rpm.
--- Torque is 85.84 ft lb at 3070 rpm and 76.2 ft lb at 2750 rpm.
--- At 70 mph cruise speed a 70” prop with 27” pitch has 841 fps tip speed (mach 0.75).
With 2.5:1 re-drive 6150 (e)rpm is 2460 (p)rpm and 5500 (e)rpm is 2200 (p)rpm.
--- Torque is 106.83 ft lb at 2460 rpm and 95.25 ft lb at 2200 rpm.
--- At 70 mph cruise speed a 84” prop with 34” pitch has 802 fps tip speed (mach 0.72).
Of course these numbers are “real” only in a friction-free universe.
A smaller, lighter, flatter, faster-spinning prop, would aid in climbing.
But a larger, heavier, more sharply pitched, slower turning prop would have a larger arc and be a better cruise prop.
IF that higher pitched prop would allow the engine power to be reduced in cruise it would be more fuel efficient... IF... But wouldn't it also deliver more wear to the system?
For comparison, the dual-ignition Continental A-40-5 (O-115) at 154 lb is 83 lb heavier than the 71 lb O-64 Hirth.
40 hp and 81.63 ft lb at 2,575 rpm burning 4.65 US gal hr (40 = 81.63 x 2575/5252)
--- At 70 mph cruise speed a 74” prop with 29” pitch has 832 fps tip speed (mach 0.75).
--- (My Cub book says an A-40-1 can be throttled back to burn a meager 2.8 US gal hr, but it doesn't give an rpm for that power setting.)
No power is lost to the re-drive in the direct drive engine, but the unit is nicely balanced and has a dual circuit electronic CDI ignition so it’s a little bit more fuel and oil efficient (1:40 mix on the Hirth).
--- Cont.: Fuel consum. (40 hp) 0.72 lb hp hr: 4.65 US gal hr. Oil consum.: 0.025 lb hp hr: 0.53 US qt hr.
--- Hirth: Fuel consum. (40 hp) 0.54 lb hp hr: 3.5 US gal hr. Oil consum.: 0.016 lb hp hr: 0.35 US qt hr.
**Numbers input are from Recreational Power (Hirth), Continental’s 1938 A-40 OM, and Culver’s prop calculator.**
Andrew
