Thrust value and tractor propeller for a Hirth F-33

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Ava

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We were playing around with a 313 cc F-33 with a 34mm Bing, slide valve, side draft carb. It is attached to a wire trussed aluminum A frame without wings. Engine RPM readings come from a "Tiny Tach," and the 59" x 10* composite two-blade is spun through a 1:2.5 belt redrive.

To determine thrust, we used the NASA approved method-- no not THAT NASA, I mean the "Northern Assinboine Sport Anglers." We leveled the airframe on its main wheels and tied a rope to the elevated tailwheel. The rope went around a firmly set post trapping a bathroom scale that when compressed gave us a reading of just over 190 "somethings--" ostensibly pounds, probably not accurate, or even linear, but definitely proportional-- as the throttle was opened up.

The Hirth performance on a F-33 chart runs up to 6700 engine rpm, but has a plateau from 5500 to 6500 engine rpm where hp remains pretty constant and torque actually decreases slightly and fuel consumption increases 27%. Thrust isn't measured (and of course is propeller dependent).

We seemed to max out performance wise at just a shade over 5750 (2300 prop rpm). By the Hirth F-33 performance chart, 5800 engine rpm is 2320 prop rpm, 28 hp, 25 ft pounds, and 3.1 gph.

Small carb? Restrictive exhaust? Poor propeller choice? Normal?

Does anyone have better numbers or propeller recommendations?
 
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henryk

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=auer KASPERWING=G-25 (circa 20 HP/7200 RPM ) with two 1.3 m propellers
drived via CRDifferential Reductor (black) ,(circa 3 : +/- 1 ) got us 80 kG static Thrust force...
 

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Ava

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Is it possible to adjust the prop?
Yes, the angle is set by replaceable blocks in the hub. Although we do not have any, they are still available online ($12 pair at AS) 10 degrees (what we have) is the recommended "climb" setting. 11 degrees is the recommended "cruise" setting-- not much of a range. :) Supposedly one more degree of pitch will decrease the rpm 200-300... I assume they mean prop rpm-- the propeller instruction sheet uses the terms prop rpm and engine rpm interchangeably but they aren't.

The engine and propeller came off of an overweight flying MIni-Max that the previous owner obtained a two cylinder powerplant for. So, they worked even if they were not optimized.
=auer KASPERWING=G-25 (circa 20 HP/7200 RPM ) with two 1.3 m propellers
drived via CRDifferential Reductor (black) ,(circa 3 : +/- 1 ) got us 80 kG static Thrust force...
That's 176.37 pounds from two 51.18" two-blade props, minus 5 hours, in 'Merikan. :)
Is 3.0: 1 the reduction ratio? (So, 2.5:1 would ballpark in the n'hood of 210, prob., with slightly less drag due to just having one prop, and slightly more 'cause it's almost 8" longer.)
 
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TFF

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Remember that max thrust should not be on the ground. Max thrust would be at top speed, but it’s pretty hard to measure that with a scale. On the ground it should be a under max power set by the prop.
 

Ava

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Remember that max thrust should not be on the ground. Max thrust would be at top speed, but it’s pretty hard to measure that with a scale. On the ground it should be a under max power set by the prop.
What does it mean that we increase the throttle slowly and the scale reading increases to a point and then falls off just a little. And neither the rpm, cht, or egt have reached maximum values. (After confirming it wasn't a hiccup we backed it down to this "maximum thrust value" of +/- 5750 engine rpm (+/- 2300 prop rpm).
 

TFF

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If you go find a Cessna 150 or 172 pilot handbook there is a static run up rpm you check for on the ground. I don’t know the numbers but let’s say it’s 2200 rpm. That is all the engine can do on the ground with the brakes locked. In the air it will go to,guessing, 2600rpm. A fixed prop is like a one gear bicycle. Hard to start off with, but there is a point where you move pretty good. If you go to slow you work hard because you can’t get rpm and if the bike goes down hill you can’t keep up with it. You can adjust your prop to give max on the ground not moving. That should give best thrust reading even if it’s a 1/4 pound more. If you put it in an airplane, one mile an hour forward and the prop thinks it’s going downhill like the bike. A fixed prop has to be set good enough for takeoff in a reasonable distance, give a reasonable speed, and not tax the engine either too slow or two fast. Bigger planes have constant speed props. You set the engine to an RPM and the prop changes angles to keep it most efficient.

If you want to play with it on the ground, measure the prop and run it. Adjust the prop and run it again. 4-5 times and you should have a pattern of what rpm works best. That rpm is where the prop needs be in the air, not on the ground; on the ground ends up where it ends up. You want the prop to pull and not go into the downhill coast in level flight.
 

TiPi

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Your prop load (absorbed hp by the prop) is an exponential function of rpm at that diameter & pitch. If your max thrust is below the max rpm that your are able to achieve, the prop is starting to stall and unload.
I have added a simple prop load curve to the F-33 power graph for illustration. Change your blade pitch and the curve will move left (more pitch) or right (less pitch), valid while the prop blades are not stalling.
1655975129277.png
 

TLW

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For what its worth, if you are using an ultra prop which it sounds like, one degree of pitch equalled about 300 rpm at the engine on my affordaplane. That was with a kawasaki 440 and an mz201 both around 40-45 hp,
 

MACOWA

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We were playing around with a 313 cc F-33 with a 34mm Bing, slide valve, side draft carb. It is attached to a wire trussed aluminum A frame without wings. Engine RPM readings come from a "Tiny Tach," and the 59" x 10* composite two-blade is spun through a 1:2.5 belt redrive.

To determine thrust, we used the NASA approved method-- no not THAT NASA, I mean the "Northern Assinboine Sport Anglers." We leveled the airframe on its main wheels and tied a rope to the elevated tailwheel. The rope went around a firmly set post trapping a bathroom scale that when compressed gave us a reading of just over 190 "somethings--" ostensibly pounds, probably not accurate, or even linear, but definitely proportional-- as the throttle was opened up.

The Hirth performance on a F-33 chart runs up to 6700 engine rpm, but has a plateau from 5500 to 6500 engine rpm where hp remains pretty constant and torque actually decreases slightly and fuel consumption increases 27%. Thrust isn't measured (and of course is propeller dependent).

We seemed to max out performance wise at just a shade over 5750 (2300 prop rpm). By the Hirth F-33 performance chart, 5800 engine rpm is 2320 prop rpm, 28 hp, 25 ft pounds, and 3.1 gph.

Small carb? Restrictive exhaust? Poor propeller choice? Normal?

Does anyone have better numbers or propeller recommendations?
Get a direct pull "game scale" and you'll be right on the money. Sounds like you are pretty close to normal anyhow. A ground adjustable prop ( I have found that the technical assistance and product quality from Powerfin are top notch) Use ONLY the supplied intake and exhaust components on this motor.
 

Ava

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Get a direct pull "game scale" and you'll be right on the money. Sounds like you are pretty close to normal anyhow. A ground adjustable prop ( I have found that the technical assistance and product quality from Powerfin are top notch) Use ONLY the supplied intake and exhaust components on this motor.
Based mostly on conversations with those I have spoken with who have more experience than myself, I am thinking that this engine is probably set up pretty well. The previous owner sold it after acquiring a higher displacement twin cylinder power plant... I had seen it fly, without paying much attention to it. But it worked... even if not to the seller's expectations... His Mini-Max was "fat," built right up to 278 pounds, perhaps... with a benevolent scale, no canopy, helium in the tires, and a Milo Minderbinder ballistic 'chute :) ).

A project for a snowy weekend in the future is carving a 63" diameter x 51" hypothetical (43.3" computed practical) pitch wooden propeller from a 1930s plan I just acquired. It was intended for a converted 2-cycle 25 hp motorcycle engine turning at 1700 to 2000 prop rpm which is darn close to the F-33 running at 4500 to 5500 engine rpm through its 2.5:1 redrive.
 

MACOWA

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Greetings from the land of dark and rainy nights. I have thought of carving a suitable prop for my little 25 hp Continental twin. But too much other stuff in line first. I'm interested in how you would laminate the blank. Please keep us posted on your progress with all your projects We wish you all success.
 

MACOWA

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If you go find a Cessna 150 or 172 pilot handbook there is a static run up rpm you check for on the ground. I don’t know the numbers but let’s say it’s 2200 rpm. That is all the engine can do on the ground with the brakes locked. In the air it will go to,guessing, 2600rpm. A fixed prop is like a one gear bicycle. Hard to start off with, but there is a point where you move pretty good. If you go to slow you work hard because you can’t get rpm and if the bike goes down hill you can’t keep up with it. You can adjust your prop to give max on the ground not moving. That should give best thrust reading even if it’s a 1/4 pound more. If you put it in an airplane, one mile an hour forward and the prop thinks it’s going downhill like the bike. A fixed prop has to be set good enough for takeoff in a reasonable distance, give a reasonable speed, and not tax the engine either too slow or two fast. Bigger planes have constant speed props. You set the engine to an RPM and the prop changes angles to keep it most efficient.

If you want to play with it on the ground, measure the prop and run it. Adjust the prop and run it again. 4-5 times and you should have a pattern of what rpm works best. That rpm is where the prop needs be in the air, not on the ground; on the ground ends up where it ends up. You want the prop to pull and not go into the downhill coast in level flight.
Well said. Measuring max static thrust is great for engine tuning, Setting things up for actual flight can be quite another thing. I love the bicycle comparison !
 

Lucky Dog

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If you go find a Cessna 150 or 172 pilot handbook there is a static run up rpm you check for on the ground. I don’t know the numbers but let’s say it’s 2200 rpm. That is all the engine can do on the ground with the brakes locked. In the air it will go to,guessing, 2600rpm. A fixed prop is like a one gear bicycle. Hard to start off with, but there is a point where you move pretty good. If you go to slow you work hard because you can’t get rpm and if the bike goes down hill you can’t keep up with it. You can adjust your prop to give max on the ground not moving. That should give best thrust reading even if it’s a 1/4 pound more. If you put it in an airplane, one mile an hour forward and the prop thinks it’s going downhill like the bike. A fixed prop has to be set good enough for takeoff in a reasonable distance, give a reasonable speed, and not tax the engine either too slow or two fast. Bigger planes have constant speed props. You set the engine to an RPM and the prop changes angles to keep it most efficient.

If you want to play with it on the ground, measure the prop and run it. Adjust the prop and run it again. 4-5 times and you should have a pattern of what rpm works best. That rpm is where the prop needs be in the air, not on the ground; on the ground ends up where it ends up. You want the prop to pull and not go into the downhill coast in level flight.
What he said. For a 28 HP (low power) engine, set your prop to "stall" static at 200 RPM lower than its peak power on the chart. You probably won't get much more of an RPM increase once airborne at that power level. It is possible, but use this as a starting point and then experiment with pitch changes to optimize cruise/climb later.
 

Andrewetal

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HXF
Greetings from the land of dark and rainy nights. I have thought of carving a suitable prop for my little 25 hp Continental twin. But too much other stuff in line first. I'm interested in how you would laminate the blank. Please keep us posted on your progress with all your projects We wish you all success.
Henri Mignet drew at least three detailed propeller carving plans-- I know because I have paper copies. I suspect he drew more than three.

One is for a 1.6 meter (~63") long prop cut from a monolithic 70 mm (~2.75") tall x 125 mm (~5") wide blank. It was meant to be used on 20-ish horsepower converted motorcycle engines that maxed out at 4000-ish rpm.

His 1933 plan said that a 64+" x 3+" x 5+" straight, square block of aged hardwood would be easy to find... at a reasonable cost... Were but if that was true today. I note his postwar propeller carving plans call for laminating the blanks.

A second is for a 1.5 meter (~59") long cut from a laminated four part 70 mm (~2.75") tall x 125 mm (~5") wide blank. It was to be used on 40 horsepower Continentals.

A third was designed for 85 horsepower Continentals and is 1.75 meters (~69") long, being cut from a laminated five part 90 mm (~3.5") tall x 150 mm (~6") wide blank.

He used cascamite, a powdered, gap filling, urea-fomaldahyde glue... popular in 1948, 1958, and still used in boat construction today to laminate the wood. It is dissolved in water or mineral spirits, spread, and clamped. Googling around I find that many people use this product to laminate wood that will be in wet conditions (like boat hulls). He also bent and riveted thin aluminum sheet to the leading edge.

Ava
 

challenger_II

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Fisher County, Tx. USA
Cascophen is an excellent glue for sticking wood props together.

 
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