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).
 

proppastie

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What does it mean
perhaps blade stall..... tip stall, blade twisting......I really do not know but you should have lots of fun trying to figure it out......I would think dynamometer less prop would tell you if it was the engine or not.
 

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,
 
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