special ed
2nd Lieutenant
- 5,667
- May 13, 2018
You will love the YO-3A.
Propeller Design
- Thread starter TheBadger
- Start date
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GregP
Major
Interrupter gears had to have some "error timing" or safe area before and after the propeller edges went past the gun muzzles to ensure you didn't shoot off your prop. Interrupting the prop more times meant fewer shots through the propeller in any given time period. It seems like interrupting for 3-blades of wider chord made for more shots through the prop arc than using more blades.
But, there comes a time when the horsepower generated makes 4-blades necessary, and I suspect the projected 4-bladed Fw 190C V18 reached that level of power, but never got sent into combat. The Ta 152C and Ta 152 saw combat or got delivered to a unit (and not many at that), and both models had 3-bladed props.
Fw 190C V-18 below:
Never saw combat, but is interesting, if only for the view of the turbo-supercharger exhaust. Not even completely sure it was armed, but haven't looked into it to find out since it didn't see combat anyway.
YO-3A Quiet Star below:
Definitely a neat aircraft. It is said you could not hear it if it was only 50 feet overhead!
But, there comes a time when the horsepower generated makes 4-blades necessary, and I suspect the projected 4-bladed Fw 190C V18 reached that level of power, but never got sent into combat. The Ta 152C and Ta 152 saw combat or got delivered to a unit (and not many at that), and both models had 3-bladed props.
Fw 190C V-18 below:
Never saw combat, but is interesting, if only for the view of the turbo-supercharger exhaust. Not even completely sure it was armed, but haven't looked into it to find out since it didn't see combat anyway.
YO-3A Quiet Star below:
Definitely a neat aircraft. It is said you could not hear it if it was only 50 feet overhead!
Shortround6
Major General
I believe, but could be wrong, that you also need more blade area at high altitude than low altitude.
Props that work at low altitude sometimes don't work at high altitude.
Lockheed PV-2.
They were supposed to be able to keep up with a Zero at sea level.
But R-2800s and small 3 blade props are not a good mix.
GregP
Major
I believe you are correct Shortround6. Sort of a Jimmy Durante nose above, but I'd fly it! Check out the covered gun muzzles! Looks like 9 nose guns! Now THAT would not be nice to fly through!
Seems like it needs more propeller to me for the power it could generate, but the performance says it did just fine. A bit over 320 mph as I recall. Not too sure what it could do at sea level.
Seems like it needs more propeller to me for the power it could generate, but the performance says it did just fine. A bit over 320 mph as I recall. Not too sure what it could do at sea level.
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MiTasol
1st Lieutenant
I do not know what those patches cover but they are not nose guns. There were three in a pod under the nose and two up top (like on some Hudsons) on some models of the PV-2 - see
and Warbird Profile: Lockheed PV-2 Harpoon
The first site includes a link to a -1 for the version with no nose guns like shown in Shortrounds photo above.
The first site includes a link to a -1 for the version with no nose guns like shown in Shortrounds photo above.
ThomasP
Senior Master Sergeant
There was a production variant of the PV-2D with an 8-gun nose. There was a large order for them near the end of the war but only 35 were delivered before the end of the war and the contract being cancelled.
The above dwg is from the Warbird Information Exchange website, the thread is about the restoration of a PV-2D.
"Warbird Information Exchange • View topic - PV-2D Harpoon 84062 (Tanker 101) comes back to life"
There are a lot of very good pictures and some very good information.
The above dwg is from the Warbird Information Exchange website, the thread is about the restoration of a PV-2D.
"Warbird Information Exchange • View topic - PV-2D Harpoon 84062 (Tanker 101) comes back to life"
There are a lot of very good pictures and some very good information.
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ThomasP,
Comparing this drawing to Shortround6's photo above it would appear his is the 8 gun version. Just looking at the top left of your drawing, where the barrels numbered 30-33 are, notice the exterior pattern and compare it to SR6s
photo.
Cheers,
Biff
MiTasol
1st Lieutenant
Agreed. Thanks Thomas.
That looks like a nice manual - if you want share I am sure many would like it
That looks like a nice manual - if you want share I am sure many would like it
ThomasP
Senior Master Sergeant
The dwg I posted is from the thread I linked in the post. Unfortunately, I do not have the manual. 
I worked on props and sometimes had to trim damaged blade tips .The trim had limitations for prop diameter. If it was within diameter limits.the blade angles would be twisted at the blade stations ,which also had limitations, thickness and width..Sometimes I would have to change Ham Standard blades on crop dusters, 2D30,12D40,to paddle blades and they would see a marked increase in speed across the field with greater nose up for wing over. "If they missed the electrical cables"…
tommayer
Airman
Sources, please.
tommayer
Airman
Thanks. Am just a dumb old pilot. Never heard efficiency at higher rpm was improved by more blades. More blades for sufficient blade area at high power and rpm, yes, but I always though for pure efficiency the fewer blades the better. More blades and/or wider blades were dictated by tip speeds and ground clearance. But I just fly them.The dwg I posted is from the thread I linked in the post. Unfortunately, I do not have the manual.
tommayer
Airman
Knew a flying priest who bellied a C-210 onto a snow covered strip. Jacked bird up. Sawed off the bent portions of the blades. Flew .5 hr. to shop. I was there. He said the 520 ran up to 3500 rpm at normal cruise speed. Head mechanic told him, "Father, you're gonna kill yourself." Flying friar, "Oh, you know who my copilot is." Mechanic: "Yeah, and you're gonna kill him too."
It's like 3D chess with many ways to win the game. Blade loadings, like wing loadings, become important for efficient performance, and depend upon horsepower, RPM, airspeed, blade area, blade number, and activity factor (ability of a blade to take horsepower). Blade loadings and the desired performance envelope will dictate planform, diameter, and choice of blade material to provide adequate lift (thrust), low drag, flutter resistance, vibration, and manageable tip mach numbers. Curtiss, Junkers, and VDM used asymmetric blade planforms to add area to blades of existing 3 or 4 blade propellers to accommodate increased horsepower or improve efficiency in specific flight regimes.
GrumpyOldCrewChief
Airman 1st Class
There are lots of factors, some of which have been mentioned, but some important ones which have not. I love watching the videos that this guy produces. Here is a very relevant one:
View: https://youtu.be/EKR-Obdq8_E
View: https://youtu.be/EKR-Obdq8_E
Allow me to throw in a few considerations. Assume we're designing a high-speed propeller aircraft from scratch. A single constant-speed propeller is a given. Our variables are propeller diameter, rpm (we have a gearbox that we can design to optimize propeller performance for our engine), number of blades, blade chord, blade planform, and angle of attack. We want to go as fast as possible, because...zoom and boom.
A designer would typically start with diameter, since whatever is choses has to match with the rest of the design concept. A good rule of thumb is:
D = 15.24 * Power^(0.2)/(n^(0.6))
D is in meters, power in horsepower, n is rotations per second. I usually don't mix units like that, but it is what it is.
Going fast requires MOAR POWER!, so that will probably be constrained by whatever the guys in the engine department come up with. Let's just say they have something of P horsepower, and we'll use that. Plug whatever number P is into that equation, and you can develop a curve of D vs. n.
Ah, but we all remember tip speed has to be kept below Mach 1 (in real world terms, maybe 950 ft/sec). Tip speed at any given velocity is determined by D * n...we can't do anything about the high speed we're flying at, but we want to keep D * n as low as possible to keep the tip speed down.
Let's throw some numbers in. D * n^(0.6) = 15,240
D = 1, then n = 9,368,000
D*n = 9,368,000. A little high. (I suspect the units are wrong, maybe it should be rpm instead of rps?)
D = 2, then n = 2,951,000
D*n = 5,901,000. Better.
D = 3, then n = 1,501,000
D*n = 4,504,000
I think you can see where this is going. We want D to be really big and n to be really slow. We'll probably hit some practical limit before we get to a 50-foot propeller, but at first glance, it seems to make sense to design the thing around having as big a prop as possible. It is here that we must introduce "Actuator Disk Theory" or as it is sometimes known, "Prop Momentum Theory".
Imagine that our aircraft has a magic disk, a hoop that accelerates air backward like a jet engine. We can make it as big or small as we like, and pump P amount of power to it, and it will take that energy and give 100% of that energy to all the air that passes through it. What you'll find is that if you make the magic disk really small, it really shoots the air back hard in a super powerful jet, and if you make it huge, it barely puffs an enormous amount of air backward at the speed of a gentle breeze.
And here's the big catch. The energy you give to the air is determined by the kinetic energy formula, KE = 1/2 * M * V^2, but the useful thrust that pushes the airplane forward is given by the momentum formula, P = MV.
So, should you make the hoop small, and shoot the air out fast (really big V) or make it big and shoot the air out slow (very small V)?
If you have INFINITE power, it doesn't matter. But if you are LIMITED on power, you can get more momentum (MV) with less kinetic energy (1/2MV^2) by pushing it out slow. Big disk, here we come!
You've probably figured out that our magic disk is an ideal propeller, one that has no real-world losses from friction or turbulence or anything like that. Now, here's the killer, a graph of the efficiency of ideal propellers as a function of diameter:
Ah. So that looks like we want to make the prop big, but beyond a certain point, it doesn't do any good to get much bigger. And remember, these are imaginary, perfect propeller losses, not real-world losses, which are worse. I guess we might not be stealing that big prop from a Tupolev Tu-95 after all.
So, with a typical WWII-era aircraft engine, we do some design studies, and we get a diameter about equal to what flew on typical WWII-era aircraft. Engine location, fuselage length, landing gear are all designed so that this diameter fits with whatever clearance factor whoever makes that decision likes. We then design a gearbox that keeps the tip speed somewhere between, oh, let's say 750 fps and 950 fps.
Our next problem is ensuring that whatever prop we design can soak up that much horsepower and convert it to useful thrust. Blade angle is taken care of for us, it will be a function of the NACA section chosen for the prop, and they all top out somewhere in the neighborhood of 45 degrees. Go past that, and you'll still make thrust, but not as efficiently, and remember, we're limited to the horsepower the engine guys give us. So, we figure out what angle is the top of the thrust curve for our prop section, and base our decision around that angle at our highest operational speed.
That leaves chord length and number of blades. A long, skinny blade is more efficient than a short, fat one, and fewer blades is more efficient than more blades. So, we start with two absurdly fat blades (big chord, although for the same section the prop will get thicker as well). We then compare with three thinner ones, then four even thinner ones, each time picking the smallest chord that just gets that much power to the airstream at the chosen RPM. We run the design studies, and whichever one wins is our design. I think you can see it's a function of power and diameter. The smaller a diameter for a given power, the more blades we'll need to be supremely efficient.
Why is smaller diameter faster (more efficient?) Because an existing plane (especially an air racer) can't just gear to whatever speed they'd like. The engine has a power curve, and it makes more power at a certain RPM, and that RPM may not be the same RPM that the aircraft was designed for. Sometimes, smaller is faster, sometimes, bigger is faster, rarely is stock faster after you make a lot of engine mods. If you are designing a turboprop and have budget to re-gear, then smaller will probably not be better...unless you add so much power that you need to add a blade, anyway.
Your propeller tips will be rounded for the same reason Spitfire wings were elliptical...it is even more important that you translate every iota of engine power to useful thrust. Why do you see aircraft with square tips? Because they've been re-engined, and it is either more efficient, or nearly as efficient and much cheaper to refit with squared-off blades than it would be to add blades for the higher power engine. Or maybe it was a new design, and it was cheaper to borrow a prop already in production for another aircraft. But you would never design one from scratch that way, it would be a compromise of some sort, probably to maintain ground clearance in a refit.
Just a few thoughts on how all this comes together.
A designer would typically start with diameter, since whatever is choses has to match with the rest of the design concept. A good rule of thumb is:
D = 15.24 * Power^(0.2)/(n^(0.6))
D is in meters, power in horsepower, n is rotations per second. I usually don't mix units like that, but it is what it is.
Going fast requires MOAR POWER!, so that will probably be constrained by whatever the guys in the engine department come up with. Let's just say they have something of P horsepower, and we'll use that. Plug whatever number P is into that equation, and you can develop a curve of D vs. n.
Ah, but we all remember tip speed has to be kept below Mach 1 (in real world terms, maybe 950 ft/sec). Tip speed at any given velocity is determined by D * n...we can't do anything about the high speed we're flying at, but we want to keep D * n as low as possible to keep the tip speed down.
Let's throw some numbers in. D * n^(0.6) = 15,240
D = 1, then n = 9,368,000
D*n = 9,368,000. A little high. (I suspect the units are wrong, maybe it should be rpm instead of rps?)
D = 2, then n = 2,951,000
D*n = 5,901,000. Better.
D = 3, then n = 1,501,000
D*n = 4,504,000
I think you can see where this is going. We want D to be really big and n to be really slow. We'll probably hit some practical limit before we get to a 50-foot propeller, but at first glance, it seems to make sense to design the thing around having as big a prop as possible. It is here that we must introduce "Actuator Disk Theory" or as it is sometimes known, "Prop Momentum Theory".
Imagine that our aircraft has a magic disk, a hoop that accelerates air backward like a jet engine. We can make it as big or small as we like, and pump P amount of power to it, and it will take that energy and give 100% of that energy to all the air that passes through it. What you'll find is that if you make the magic disk really small, it really shoots the air back hard in a super powerful jet, and if you make it huge, it barely puffs an enormous amount of air backward at the speed of a gentle breeze.
And here's the big catch. The energy you give to the air is determined by the kinetic energy formula, KE = 1/2 * M * V^2, but the useful thrust that pushes the airplane forward is given by the momentum formula, P = MV.
So, should you make the hoop small, and shoot the air out fast (really big V) or make it big and shoot the air out slow (very small V)?
If you have INFINITE power, it doesn't matter. But if you are LIMITED on power, you can get more momentum (MV) with less kinetic energy (1/2MV^2) by pushing it out slow. Big disk, here we come!
You've probably figured out that our magic disk is an ideal propeller, one that has no real-world losses from friction or turbulence or anything like that. Now, here's the killer, a graph of the efficiency of ideal propellers as a function of diameter:
Ah. So that looks like we want to make the prop big, but beyond a certain point, it doesn't do any good to get much bigger. And remember, these are imaginary, perfect propeller losses, not real-world losses, which are worse. I guess we might not be stealing that big prop from a Tupolev Tu-95 after all.
So, with a typical WWII-era aircraft engine, we do some design studies, and we get a diameter about equal to what flew on typical WWII-era aircraft. Engine location, fuselage length, landing gear are all designed so that this diameter fits with whatever clearance factor whoever makes that decision likes. We then design a gearbox that keeps the tip speed somewhere between, oh, let's say 750 fps and 950 fps.
Our next problem is ensuring that whatever prop we design can soak up that much horsepower and convert it to useful thrust. Blade angle is taken care of for us, it will be a function of the NACA section chosen for the prop, and they all top out somewhere in the neighborhood of 45 degrees. Go past that, and you'll still make thrust, but not as efficiently, and remember, we're limited to the horsepower the engine guys give us. So, we figure out what angle is the top of the thrust curve for our prop section, and base our decision around that angle at our highest operational speed.
That leaves chord length and number of blades. A long, skinny blade is more efficient than a short, fat one, and fewer blades is more efficient than more blades. So, we start with two absurdly fat blades (big chord, although for the same section the prop will get thicker as well). We then compare with three thinner ones, then four even thinner ones, each time picking the smallest chord that just gets that much power to the airstream at the chosen RPM. We run the design studies, and whichever one wins is our design. I think you can see it's a function of power and diameter. The smaller a diameter for a given power, the more blades we'll need to be supremely efficient.
Why is smaller diameter faster (more efficient?) Because an existing plane (especially an air racer) can't just gear to whatever speed they'd like. The engine has a power curve, and it makes more power at a certain RPM, and that RPM may not be the same RPM that the aircraft was designed for. Sometimes, smaller is faster, sometimes, bigger is faster, rarely is stock faster after you make a lot of engine mods. If you are designing a turboprop and have budget to re-gear, then smaller will probably not be better...unless you add so much power that you need to add a blade, anyway.
Your propeller tips will be rounded for the same reason Spitfire wings were elliptical...it is even more important that you translate every iota of engine power to useful thrust. Why do you see aircraft with square tips? Because they've been re-engined, and it is either more efficient, or nearly as efficient and much cheaper to refit with squared-off blades than it would be to add blades for the higher power engine. Or maybe it was a new design, and it was cheaper to borrow a prop already in production for another aircraft. But you would never design one from scratch that way, it would be a compromise of some sort, probably to maintain ground clearance in a refit.
Just a few thoughts on how all this comes together.
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Nice post but shouldn't it be "fewer blades is more efficient"?
Simon Thomas
Senior Airman
special ed
2nd Lieutenant
- 5,667
- May 13, 2018
Since I am a builder of control line flying models, I tried much of what is in the aerodynamic texts. A fad for a while in speed competition was a single blade prop. I decided to try the theory on a larger plane . A ten inch prop, with one blade off, was carefully balanced. After a successful test stand run, it was mounted on a model and flown. After several flights, I saw signs of engine mount damage. While single blade props CAN be used, the offset thrust causes wear on the airframe as well as the engine shaft/bearings. A shorter prop having thrust closer to the center line may work for specialty events as speed competition where long running times are not needed.
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
I read a book when I was but a lad. It was about a boy who built rubber band powered "ultralight" flying models. The kid built a plane with a single bladed propeller. It seemed odd to me at the time. He also used a very thin covering that seemed to me like using soap bubbles for skin.
