Prop pitch, rpm, and performance

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Armilite

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As I’ve related elsewhere, I’ve been experimenting with a new prop on my Hatz. I don’t want to dilute that discussion, which is more about data collection, while here I want to talk more generally about rpm, pitch, and performance.

First, the engine, which is a Lycoming O-290-D which has a “rated power” of 125HP at 2600 rpm. The manual also lists a “takeoff power” of 130HP at 2800 rpm, but nowhere in the manual does it say what “takeoff power” means. I’ve heard of “5 minute takeoff power” ratings for other engines, but there’s no mention of anything like that in the Lycoming manual. (Edit, it's in the TCDS Dan posted, strange it's not in the manual).

So, the prop: The original prop on the plane is a 74x50 Sensenich metal prop. Static rpm is 2480, around 2500 in a 60kt climb, I cruise 70kts at 2250, and it maxes out at 2600 rpm WOT at 80kts in level flight. The new prop is a 74x47 Sterba wood prop. Static rpm is 2440, 2425 at 60kt, and it maxes out at 2565 at 85kt in level flight (the full throttle rpm averages 75 lower over most speeds). Even though the new prop has less pitch, rpm is down and speed is up… but takeoff and climb performance are worse.

My goal with the new prop was to get a little close to the 2600 rated rpm for more power during climb, hopefully without losing too much airspeed in cruise. That the reduced pitch didn’t achieve this points to the other differences between the props. The wood prop has a different blade shape (it’s wider than the metal prop near mid blade but narrower closer to the hub) and a thicker airfoil section. I will be sending the prop back to Sterba for repitching, but how much? According to Ed an inch of pitch is good for about 50 rpm.

I’ve seen references to pitching the prop so it’s at redline rpm at full throttle in level flight, but what is “redline”? The manual makes no mention of any limiting rpm. The old prop maxes out at the 2600 “rated rpm”, but of course I won’t be cruising at full throttle. I’m thinking reducing the pitch by 3” to pick up, say, 150 rpm (so it’d make 2575 during climb) would be about right. It would bring the pitch down to 44” which is what two other prop makers recommended (Sterba recommended 50 but I thought 47 would be a better place to start). That would (presumably) lower the cruise speed at the same rpm, or raise the rpm needed to get the same speed, which is OK if it’s not too drastic. It also means the rpm would go above 2600 (some might say “overspeed”) at full throttle in level flight, but the answer to that is “don’t do that” (or maybe it’s OK for 5 minutes?). I’m not looking to get up to the “takeoff power” rpm of 2800, I suspect that a fixed pitch prop that allowed that would perform very poorly in cruise.
=========================================
Using this Prop Calc, and 77F Temp:
http://godolloairport.hu/calc/strc_eng/index.htm

125HP at 2600 rpm. Pitched for 2500rpm:
(2) Blade 74"x 50 = 502.51 lbs Static Thrust, needs 159.158 hp
(3) Blade 74"x 50 = 703.51 lbs Static Thrust, needs 222.822 hp
(4) Blade 74"x 50 = 854.26 lbs Static Thrust, needs 270.570 hp

(2) Blade 74"x 47 = 502.51 lbs Static Thrust, needs 149.609 hp
(3) Blade 74"x 47 = 703.51 lbs Static Thrust, needs 209.453 hp
(4) Blade 74"x 47 = 854.26 lbs Static Thrust, needs 254.336 hp
------------------------------------------------------------------

Ideal 74" Prop seems to be for 125 hp Pitched for 2500rpm:
(2) Blade 74"x 39 = 502.51 lbs Static Thrust, needs 124.144 hp
(3) Blade 74"x 28 = 703.51 lbs Static Thrust, needs 124.780 hp
(4) Blade 74"x 23 = 854.26 lbs Static Thrust, needs 124.462 hp


=========================================

130HP at 2800 rpm. Pitched for 2700rpm:

(2) Blade 74"x 50 = 586.12 lbs Static Thrust, needs 200.494 hp
(3) Blade 74"x 50
(4) Blade 74"x 50
------------------------------------------------------------------
(2) Blade 74"x 47
(3) Blade 74"x 47
(4) Blade 74"x 47

Ideal 74" Prop seems to be for 130 hp Pitched for 2700 rpm:
(2) Blade 74"x 32 = 586.12 lbs Static Thrust, needs 128.316 hp

(3) Blade 74"x 23 = 820.57 lbs Static Thrust, needs 129.118 hp
(4) Blade 74"x 19 = 996.41 lbs Static Thrust, needs 129.519 hp


 

TFF

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High static ground thrust means you will fly through max thrust at low speeds and the prop then becomes a brake. Unless STOL competition only, you want max load at climb or cruise or somewhere in between. Before you start moving means it’s always loosing.
 

Lendo

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Propeller design is an art. I look at everything I can, I managed to get a copy of Jan Carlsson's Prop program, while he was alive. I would love to know his background, as a lot of work went into this.
However it wasn't the be all and end all as his new program was a step up with the latest Prop Theory. He only used this for his consulting work.

I also love Paul Lipps work and spoke to him via e-mail only briefly - he felt I understood his basic philosophy and theory.

Both men were on the right track to achieving the optimum prop design, I have developed my own theories on the subject, but that's all they are just theories.

All I can say is best of luck with all this, the Big Boys have the Big computers and best programs money can buy.

I say this tongue in cheek, if only the bloody things could morph in the best shape and pitch for the conditions :).

George
 

proppastie

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Basic question:?

85% prop....so thrust x density of air x volume of moving air = 85% of engine hp: torque x rpm?...
 

BJC

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Basic question:?

85% prop....so thrust x density of air x volume of moving air = 85% of engine hp: torque x rpm?...
Example of 85% efficient propeller: 100 HP out of engine into the propeller produces 85 HP out of the propeller available for propulsion. Note that aircraft speed plays a big role in prop efficiency.


BJC
 

proppastie

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Example of 85% efficient propeller: 100 HP out of engine into the propeller produces 85 HP out of the propeller available for propulsion. Note that aircraft speed plays a big role in prop efficiency.
BJC
how is it measured
 

BBerson

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85 thrust hp used by the prop. Measured in pound of force over time.
One hp is 550 pounds per foot per second.
 

BJC

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Efficiency is output divided by input, i.e., Thrust X aircraft velocity / HP into the prop.


BJC
 

Vigilant1

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FWIW:
If your speed is in MPH and you want thrust in lbs, then:
Thrust = HP x prop efficiency x 375 / airspeed

So, if we want to know the thrust at our 67 MPH climb speed and we have a 22 HP engine, and the prop that allows it to turn at max HP RPM at 67 MPH is 65% efficient at that RPM and airspeed, then:

22 x .65 x 375 /67 = 80lbs of thrust

Observations:
1) Propellers generally get more efficient at higher airspeeds (until their design speed is reached, if it is fixed pitch)
2) Despite the above rule, thrust generally decreases with airspeed (as would be expected due to that airspeed term in the denominator of the formula above)
 

Dana

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85 thrust hp used by the prop. Measured in pound of force over time.
One hp is 550 pounds per foot per second.
Power is thrust times velocity, one HP is 550 lb-ft/sec, which is lbs times ft per sec, not pounds per ft per second.

That's thrust HP (THP), as opposed to shaft HP (SHP), which is what the engine actually produces (often called brake HP (BHP) in the automotive world).

Prop efficiency is THP divided by SHP. Note that by definition, THP (and thus prop efficiency) equals zero if the aircraft isn't moving.
 

BBerson

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Power is thrust times velocity, one HP is 550 lb-ft/sec, which is lbs times ft per sec, not pounds per ft per second.
Hmmm. I just know that 550 pounds lifted in one second is one hp. (/ is read as per, I thought)

Anyway, it's interesting that the 375 (in the post 73 formula) is derived from the hp formula. (550x60=33,000 and 33,000 divided by 88fps is 375)
So I just remember that at 375 mph, one hp is equal to one pound thrust at 100% efficiency. Can't do better than that.
 
Last edited:

Pops

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I just know enough to know that I would be wasting my time designing and building a prop. Local friend of mine designed and built and sold props for several years for different hp 1/2 VW engines.
 

BJC

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All those odd magic numbers based on the length of a king's foot...
...would disappear if you used SI units.
Anyone can calculate in SI; it takes a mastery of obtuse units to work in Btu’s, feet, inches, miles, knots, water that turns solid at 32 degrees, slugs, pounds force, inches of mercury, etc. We do, however, share time units with SI.


BJC
 
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