Shortened props.

Tom-D

Tom-D

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Tom-D
What would be the aircraft performance results of removing 1" from the prop tips. ?

Standard fix pitched prop.
 
Met a guy in a Maule when we were on our way to Alaska who had curled his tips back an inch or so. He didn't seem to think it made much difference. :rolleyes:
 
Tips are usually clipped when the meat of the blade gets too thin. Thin blades are poor performers. Clipping would make little difference at that point. I wouldn't expect to see a healthy prop clipped so the performance change would be a guess. But on the other hand, a 180 seaplane prop is 88" and the same prop on a 185 is 86". I doubt there's much difference at similar pitch and power application.

Air racers may prefer shorter props but STOL guys want longer. Average ops probably wouldn't recognize any difference. The early Valdez winners used 92" props as opposed to the maximum legal 82" that most Cubs have. Huge difference. For what that's worth. These days comparisons are hard to make since composite props are all the rage.
 
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Don't now about shortening it but beveling a cherokee prop at 45 deg. front to back increases efficiency allowing a 2 inch pitch increase keeping the same rpm (Art Mattson STC). I went up 10 HP and did the prop tip mod. Repitched 4 inches and was still redline at WOT. Art is no longer with us and I don't know who has his STCs.
 
When you shorten the prop, doesn't it lessen the load, lessen the torque and add rpm.?
 
Tom-D said:
When you shorten the prop, doesn't it lessen the load, lessen the torque and add rpm.?
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Yes, it does. The idea is to get a bit more HP by increasing the RPM. I shortened the wooden prop on my Jodel from 76 to 74 and then to 72, and ended up with more RPM and less performance. Takeoff, climb and cruise all suffered. Should have left it alone. The designer called for a 76 x 44 on an A-65 in that airplane, and he was right.

The strange thing with that prop, originally, was that it had no measurable slip. I had read somewhere that a prop without any slip is usually too long. Wish I had not read that. It was the perfect length before I shortened it.

Calcuating slip: prop pitch times RPM gives a distance in one minute. Doing the calculations to convert from inches to feet to miles and hours gives the zero-slip MPH, and that is what I was getting. Just like some airfoils that generate lift at slightly negative angles of attack.

I should mention: The TCDS for the airplane will give the propeller or propellers certified for it, along with max and min lengths, in most cases.
 
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Just FYI the cargo ship I was on in the Navy swung a 25 foot by 25 foot 4 blade prop. At flank speed we also calculated almost no slip. That's for a 455 foot 8000 ton ship plus 4000 ton cargo.
 
Here's a dumb question that has been nagging me for years: why aren't engine/prop combinations rated in lbs of thrust?

Using hp as a figure of merit seems bassackwards to me, like rating lightbulbs in watts or vacuum cleaners in amps. What am I missing?

Tom-D said:
When you shorten the prop, doesn't it lessen the load, lessen the torque and add rpm.?
Click to expand...
 
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Tom-D said:
When you shorten the prop, doesn't it lessen the load, lessen the torque and add rpm.?
Click to expand...

There are a couple of different definitions for torque, which one do you mean?

If I have a plan where I go to red line with the long prop, I wonder how the shortened prop will help me? I agree the load is reduced, and I reach redline sooner.
 
Cap'n Jack said:
There are a couple of different definitions for torque, which one do you mean?

If I have a plan where I go to red line with the long prop, I wonder how the shortened prop will help me? I agree the load is reduced, and I reach redline sooner.
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But which load...... Sure thrust load will be reduced a bit, but gyroscopic loads will increase as the higher RPM's will exhibit greater oscillations because of faster tip speeds.. IMHO
 
N801BH said:
But which load...... Sure thrust load will be reduced a bit, but gyroscopic loads will increase as the higher RPM's will exhibit greater oscillations because of faster tip speeds.. IMHO
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Let me ask another question- what determines red- line RPM? The prop or the engine?

Assuming, in my earlier question, that the engine is what limits red-line RPM, isn't the load reduced on the engine? How does a shortened prop help?

Please note I'm not an engine guy!
 
Is this a fixed or variable pitch.

Margy's flight instructor used to fly cargo on a DC-6 or something along those lines and said they always had problems with one prop wanting to overspeed. Finally a mechanic measured the blades on that engine and found that someone (undocumented) had removed a few inches all the way around.
 
A good prop shop knows what the relationships are between prop length and pitch to make a desired RPM. With a fixed pitch prop if taken to a shop for work that may include trimming the tips, blade profiling, etc they'll ask how your static RPM is and they'll offer to re-pitch to adjust. I've had my props pitched to raise RPM as well as lower it as I searched for my ideal pitch. What's being discussed is prop shop work, not casual hangar maintenance. There's more to be addressed than just trimming tips.
 
Tom-D said:
What would be the aircraft performance results of removing 1" from the prop tips. ?

Standard fix pitched prop.
Click to expand...
It would either improve or get worse depending on how well the propeller was designed to match the engine.

There should be an optimal length for any given operating condition that could be shorter, longer, or the same as the original prop.
 
Stewartb said:
A good prop shop knows what the relationships are between prop length and pitch to make a desired RPM. With a fixed pitch prop if taken to a shop for work that may include trimming the tips, blade profiling, etc they'll ask how your static RPM is and they'll offer to re-pitch to adjust. I've had my props pitched to raise RPM as well as lower it as I searched for my ideal pitch. What's being discussed is prop shop work, not casual hangar maintenance. There's more to be addressed than just trimming tips.
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Many props have a limit as to how many degrees they can be twisted in their useful life. When that limit is reached they seek other methods of tweeting the prop to meet the RPM needed.
And in some cases the prop that gets the tips damaged, can be shortened and used on a different application.
case as an example. Grumman props are a short version of a 172 prop.
 
When I upgraded my 180 engine and prop my 88" 2-blade was clipped to 80" and installed on a buddy's Hawk XP. The tips were getting thin so it was a great way to keep that prop in service.

My Cub props were what I referred to earlier. I tried everything from 8244 to 8240. I settled on 8242 and the biggest single performance improvement came from switching from a many times profiled "toothpick" to a brand new prop with more meat and more chord on the blades. Size matters.
 
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Cap'n Jack said:
Let me ask another question- what determines red- line RPM? The prop or the engine?

Assuming, in my earlier question, that the engine is what limits red-line RPM, isn't the load reduced on the engine? How does a shortened prop help?

Please note I'm not an engine guy!
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The engine is designed for a specific redline. It could turn faster, and the racers do run their engines well past redline, but they lose some life and risk other damage. Both Lycoming and Continental publish service bulletins regarding overspeeds and, IIRC, a 10% overspeed for five seconds or longer is cause for special inspection. They will also tell you that the prop needs to come off for NDI and other inspection.

Which brings us to the prop. That thing is probably the most highly stressed part on the whole airplane. There are tens of tons of force pulling outward due to centrifugal forces, and a 10% overspeed results in a 21% increase in those forces. There isn't the same margin of safety that the rest of the airframe has, either. There are thrust forces pulling the blades forward, countered by the centrifugal forces. There are drag forces bending the blade against the direction of rotation. There are centrifugal twisting forces trying to flatten the blades' pitches, and aerodynamic twisting forces trying to increase the pitches.

The tip speed can't go much past 600 MPH without a terrific increase in noise and drag. Wasted power, that is. So the RPM gets lower as the blades get longer.

The prop manufacturers publish tables for every prop they make. They have specified blade widths and thicknesses for various stations along the blades, and minimum blade lengths. Once the prop goes past a minimum of any of them, it is junk. Blades can flutter when they get too thin, and failure becomes a risk that could tear the engine off the airplane. Similarly, they publish data for repairing dings and nicks, since damage or a poor repair creates stress risers that can lead to a crack and maybe a blade failure. If the engine comes off the airplane, it won't glide.
 
Cap'n Jack said:
Let me ask another question- what determines red- line RPM? The prop or the engine?
Click to expand...

The Engine/application sets the limits, the prop sets the load to stay within the limits.
 
GeorgeC said:
Here's a dumb question that has been nagging me for years: why aren't engine/prop combinations rated in lbs of thrust?

Using hp as a figure of merit seems bassackwards to me, like rating lightbulbs in watts or vacuum cleaners in amps. What am I missing?
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Engine and prop combinations arent rated in HP. That horsepower number only corresponds to the engine. Two guys with identical aircraft and identical engines but different props will see different performance but they both still have 200 HP, or whatever the engine is rated for.

Lbs of thrust statistic would be interesting to see though.
 
Could you tie the tail down with a spring scale and see what it reads at static?
 
Stewartb said:
Thrust numbers are available from some aftermarket shops.
http://www.pponk.com/HTML PAGES/propellers.html
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My understanding from ponk's site is the most efficient length for any application is when the blade tip speed is just a tad sub-sonic with full power.

Anything over that is wasted (or non-existent) thrust no matter how much power is behind it.

In almost every application, the 88" prop that the STOL flyers like is NOT the most efficient prop, not even for seaplane's, so I've never understood why they dominate the field like they do. 88's make a lot of noise on take off if you like that sort of thing .... :dunno:
 
88s dominate 180 seaplanes but aren't approved on 185s (86" max). Anyone around seaplanes knows a 185 is louder on takeoff than a 180. Prop speed/prop length/atmospheric pressure all play into prop efficiency. Northern pilots know to spin the prop slower on cold days. Ponk has a calculator for that. And for what it's worth? One summer I had a mis-adjusted prop governor that had my 86" prop spinning to 2900. Anyone who says 2600 or 2700 offers the best take off performance is reciting what they read, not what they tried. I can testify that my plane was a rocket off the ground at the higher prop speed. Noisier than it should have been but it sure worked well. And yes, it's correctly adjusted now.
 
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Capt. Geoffrey Thorpe said:
Yes you could.

Quote:
Originally Posted by Silvaire
Could you tie the tail down with a spring scale and see what it reads at static?


How would that correlate to propeller performance during climb or cruise?
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I did that with my slightly overpowered experimental.. I have a digital strain gauge and used it to measure thrust.... Hooked it to a steel pipe and to my tail hook and landing gear... That way it spread out the load...

I got 1100 lbs pull with my first 84" Ivo paddle blade prop... Keep in mind this test was at 7000MSL and the motor was still really detuned with a restrictor plate and lower RPM chips in the MSD ignition boxes...

I estimate it was putting out 360 HP @ sea level, and back out the loss = 3% for every 1000 feet above sea level = 284 HP..... The prop loses bite too because the air is thinner...

On one flight I did test the true STOL capacity of the Zenith 801... I lined up at the end of the runway, locked the brakes, deployed full flaps / flaperons, all the way back on the stick, went full power and wanted to see how fast it would lift off...

Plane rolled maybe 20 feet , climbed 50' agl and proceeded to do a uncommanded roll.... Planes wings were almost vertical to the ground.. By shear luck I reduced power, full aileron, full rudder and nose down, all at the same time.... Thank god it rolled back flat and I was able to not smack back into the ground.... I landed, taxied back and cleaned out my shorts..:eek:

That very day..... I removed the prop blades, sent them back and had them cut down to 76" as I assumed the additional leverage of the large diameter is what rolled the plane... It was just like building a toy plane with a rubber band powerplant and holding the prop and the plane just spins on the centerline axis.......

Note to self.... NEVER try that trick again.....:no::no::no::no:

I have since removed most of the HP limiting devices and now have over 400HP and LOVE my toy...
 
Stewartb said:
88s dominate 180 seaplanes but aren't approved on 185s (86" max). Anyone around seaplanes knows a 185 is louder on takeoff than a 180. Prop speed/prop length/atmospheric pressure all play into prop efficiency. Northern pilots know to spin the prop slower on cold days. Ponk has a calculator for that. And for what it's worth? One summer I had a mis-adjusted prop governor that had my 86" prop spinning to 2900. Anyone who says 2600 or 2700 offers the best take off performance is reciting what they read, not what they tried. I can testify that my plane was a rocket off the ground at the higher prop speed. Noisier than it should have been but it sure worked well. And yes, it's correctly adjusted now.
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Calculate your horse power at 2900 RPM see what you get. Of course it will climb faster with more horse power.

But was the prop at it's max efficiency? how much of that blade was simply making noise not climb.
 
When I bought the MT, they did not want to sell me their three blade for my 0-470. They said it was a little light in the pants for their three blade. The two blade was what they recommended and they said it would handle the power if I ever upgrade the engine to more horsepower.

MT were some good folks to work with because I wanted a three blade for the looks alone and had cash in hand. They left money on the table to fit me with what was better and not just what I wanted.

It isn't as sexy as a three blade, but I'm glad I got the two blade. The extra 9 lbs. right off the nose seems to be a really good fit particularly for skywagons since they're a little forward c.g. to begin with. The lightness of the composite prop, it's spinning up faster and deceleration faster and lighter overall weight is a match that is good for a STOL plane. I think I'm a little faster on top than three blades with all things being equal too.
 
Stewartb said:
88s dominate 180 seaplanes but aren't approved on 185s (86" max). Anyone around seaplanes knows a 185 is louder on takeoff than a 180. Prop speed/prop length/atmospheric pressure all play into prop efficiency. Northern pilots know to spin the prop slower on cold days. Ponk has a calculator for that. And for what it's worth? One summer I had a mis-adjusted prop governor that had my 86" prop spinning to 2900. Anyone who says 2600 or 2700 offers the best take off performance is reciting what they read, not what they tried. I can testify that my plane was a rocket off the ground at the higher prop speed. Noisier than it should have been but it sure worked well. And yes, it's correctly adjusted now.
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A 180's redline is 2600 (the last iteration had it at 2400). A 300 HP 185 has a redline of 2850. The 185's prop will be shorter that the 180's. All the engine/prop numbers are in the TCDS:

http://www.airweb.faa.gov/Regulator...5f2ca218625756e006e0b64/$FILE/3A24 Rev 39.pdf

http://www.airweb.faa.gov/Regulator...8e1e465986257810004f5a7e/$FILE/5A6 Rev 67.pdf

An A185F can have an 86" McCauley prop on it. AT 2850 RPM those tips are at 729 MPH. Lots of noise, and that noise is little sonic booms off the tips because the airflow over the cambered side of the blade is faster than the tip is travelling. It is just a rotating wing, after all.

Moving forward, the speed of the tips increases even if the RPM stays the same. Rotational tip speed squared plus forward speed squared equals the net tip speed squared.
 
Dan Thomas said:
The engine is designed for a specific redline. It could turn faster, and the racers do run their engines well past redline, but they lose some life and risk other damage. Both Lycoming and Continental publish service bulletins regarding overspeeds and, IIRC, a 10% overspeed for five seconds or longer is cause for special inspection. They will also tell you that the prop needs to come off for NDI and other inspection.

Which brings us to the prop. That thing is probably the most highly stressed part on the whole airplane. There are tens of tons of force pulling outward due to centrifugal forces, and a 10% overspeed results in a 21% increase in those forces. There isn't the same margin of safety that the rest of the airframe has, either. There are thrust forces pulling the blades forward, countered by the centrifugal forces. There are drag forces bending the blade against the direction of rotation. There are centrifugal twisting forces trying to flatten the blades' pitches, and aerodynamic twisting forces trying to increase the pitches.

The tip speed can't go much past 600 MPH without a terrific increase in noise and drag. Wasted power, that is. So the RPM gets lower as the blades get longer.

The prop manufacturers publish tables for every prop they make. They have specified blade widths and thicknesses for various stations along the blades, and minimum blade lengths. Once the prop goes past a minimum of any of them, it is junk. Blades can flutter when they get too thin, and failure becomes a risk that could tear the engine off the airplane. Similarly, they publish data for repairing dings and nicks, since damage or a poor repair creates stress risers that can lead to a crack and maybe a blade failure. If the engine comes off the airplane, it won't glide.
Click to expand...

Thank you for an interesting reply.
 
Tom-D said:
What would be the aircraft performance results of removing 1" from the prop tips. ?

Standard fix pitched prop.
Click to expand...

(grabbing hacksaw) Lets find out!! :D
It may or may not degrade. It may or may not be enhanced.
 
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