Propellers for Idiots

Capt.Crash'n'Burn

Capt.Crash'n'Burn

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Capt.Crash'n'Burn
So, I'm noticing that on a lot of planes, they have different style props on the same aircraft type with the same engine. Some have 2 blades, some 3, some for and even some with 5 or 6.

Is there a general rule to determine the right number of blades for your application? or is this subject a huge can of worms with no real right answer?
 
lots of factors go into choosing the prop. tip speed is a main one. generally want to keep the tips below supersonic speeds to avoid massive drag as they break the speed of sound. Also ground clearance matters, especially on tricycle gear airplanes. im not an expert but it seems that 2 blad props are typical in sub-300 hp engines and 3 blades or more become more common in higher horsepower applications. It also just seems to me that for cruise efficiency a long prop (larger arc) is optimal.
 
A single blade prop is the most efficient...you'd need one of those redneck planes with 31" off road tires for it to work though!
:)
 
yes plus with fewer blades each blade encounters cleaner air
 
Capt.Crash'n'Burn said:
So I take it, the fewer the blades the less drag??
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You know, I don't remember exactly. It's been far too long ago for me to remember the details.

Keeping the tips well shy of the speed of sound as Tony mentioned is a biggie on our engines though. IIRC a prop starts losing efficiency when the tip hits about 90% the speed of sound. It also creates a lot of unnecessary noise.
 
Generally the lift to drag ratio goes up with the aspect ratio. A single long blade prop has a higher aspect ratio than a four bladed prop, therfore it will a higher lift to drag ratio.

Then, there's a whole bunch of other engineering trade-offs that happen. What is the engine's peak power RPM? Peak efficiency RPM? Ground clearence. And most important: looks.
 
Mooney has swapped back and forth on the Ovation a few times from 2 to 3 blade props. The 2 blade gave it a higher cruise speed but at the trade off of less static thrust (longer take off distances). The three blade gave the Ovation better take off and climb performance but knocked a few knots off the top end cruise. There are many different designs of props also irrespective of the number of blades (wide cord, narrow, scimitar/straight, composite over wood, styrofoam, or solid composite, solid aluminum; and then there's fixed vs. variable pitch...

I guess aeronautical engineers need work too.
 
timwinters said:
You know, I don't remember exactly. It's been far too long ago for me to remember the details.

Keeping the tips well shy of the speed of sound as Tony mentioned is a biggie on our engines though. IIRC a prop starts losing efficiency when the tip hits about 90% the speed of sound. It also creates a lot of unnecessary noise.
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I've read that it's at around 600 MPH tip speed that the drag rises seriously and it starts to make a lot of noise. That's at about 79% of the speed of sound. I'd bet that the speed of the air in the tip vortex is near the speed of sound when the tip is at 600.

Dan
 
tonycondon said:
yes plus with fewer blades each blade encounters cleaner air
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BUT... at a certain point, there is a limit to the amount of HP a blade can effectively convert to thrust. So rather than increasing the blade length or width, they add blades.

All things remaining constant (blade area), a 3 or 4 blade prop can be used to get better prop clearance from a longer 2 bladed prop

Its all a tradeoff to some degree.
 
Less is more efficient, but only to a point. Eventually as you increase the blade length you'll have to gear down the speed of the prop to keep the tip speed at an effective speed. Single blade props will work to about 70 hp then they lose the ability to translate more hp into thrust without getting out of proportion to the airplane as well as adding the weight of a gear reduction. More (shorter) blades typically = less noise which is why they came out with a 3 bladed prop for the R-985 we call a "City Prop". Which prop to use is mostly dependent on your application for the aircraft. Rule of thumb is if you spend most of your time going up and down, more blades are better. If you are spending most of your time pulled back in long haul cruise, less is better.
 
Old props
dcr0059a.JPG




New props
Hawkeye%202000%20E-2C%20front%20nose%20on.jpg
 
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The tip speed has been a problem for the Air Force too. Ever hear of the "Thunderscreech"?



http://en.wikipedia.org/wiki/Republic_XF-84H

The XF-84H was quite possibly the loudest aircraft ever built, earning the nickname "Thunderscreech" as well as the "Mighty Ear Banger".[13] On the ground "run ups", the prototypes could reportedly be heard 25 miles (40 km) away.[14] Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run.[14] Coupled with the already considerable noise from the subsonic aspect of the propeller and the dual jet turbines, the aircraft was notorious for inducing severe nausea and headaches among ground crews.[10] In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.[15]
The pervasive noise also severely disrupted operations in the Edwards AFB control tower by risking vibration damage to sensitive components and forcing air traffic personnel to communicate with the XF-84H's crew on the flight line by light signals. After numerous complaints, the Air Force Flight Test Center directed Republic to tow the aircraft out on Rogers Dry Lake, far from the flight line, before running up its engine.[11]
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steingar said:
And the idiots are not the ones asking good questions.
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Ever heard about a drag racer by the name of "Big Daddy" Don Garlits? It was an idiot who asked him in 1970, why his new rear engined dragster didn't handle good... :rolleyes2:
 
Dan Thomas said:
I've read that it's at around 600 MPH tip speed that the drag rises seriously and it starts to make a lot of noise. That's at about 79% of the speed of sound. I'd bet that the speed of the air in the tip vortex is near the speed of sound when the tip is at 600.

Dan
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Here's a nifty fast site to calculate tip speeds.

http://www.pponk.com/HTML PAGES/propcalc.html

On my experimental my prop is in the .94 mach range on take off. I cruise at .86 mach.
 
You can size about any propeller to work on any setup. For example, the early Malibus had 2-bladed props. I don't know whether the switch to 3-bladed happened with the Mirage or somewhere in between, but the modern 3-bladed PA-46 propeller is a big departure from the early 2-bladed ones. Our resident Malibu pilots would likely have more insight into that.

3-bladed props supposedly give better takeoff and climb performance vs. an equivalent 2-bladed prop with slightly lower cruise speed. I haven't flown enough back-to-back aircraft to make a good comparison, but I do know that when one of my friends bought his SR20 some years ago (brand new), he chose the 2-bladed prop because it was several knots faster in cruise. The 3-bladed twins I've flown do seem to have better get-up-and-go and will really hang on the props when you pull the yoke back, but I've never flown the same aircraft with 2-bladed props, so it's not a valid comparison.

I'll admit that if given the choice, I'd probably buy 3-bladed props just because they look cooler. Although 3-bladed props don't look right on an Aztec, so I think I need to get 4-bladed props for the look cool factor. Hey, at least I can admit that's my reason. :)
 
gismo said:
At what temp?
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Out here in the mountians the temp changes by the minute so that determines my throttle position.
For instance, today its in the mid 30's f now. By this afternoon it will be in the 90's. What will be partial throttle on takeoff now will be full throttle today at 4 pm. Another ol wives tale in you can't spin a prop over 2700 rpms.. That's just plain BS.

Go to that program and enter in various diameters, prop rpms and temps.

Example = 2700 on a 76" dia prop @ 70 degrees is mach .794. That is not even close to tips going supersonic but alot of people will tell you that's the absolute max it can spin.... Yeah, right !!!
you certified guys are stuck with that incorrect thought process and cannot deviate. Even though the facts clearly show the prop can turn a higher rpm. Prop manufacurers and motor suppliers are NOT going to change their specs and you can bet the FAA ain't gonna either.

just my humble opinion.

Ben.
 
On the Hawkeye upgrade pictured above, the 6 8 (doh) blade scimitar upgrade was selected for takeoff performance improvement as the aircraft weight continued to increase with added equipment. The B model in the first photo could deck launch without catapult, lose an engine and still climb away at max gross, IIRC (albeit very slowly). I haven't heard/read what performance the new props restored, nor am I sure about how much max gross weight has increased, but it was significant, I understand.
In this plane's mission, cruise and/or top speed are not important factors; takeoff performance and time on station are.
 
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gkainz said:
On the Hawkeye upgrade pictured above, the 6 blade scimitar upgrade was selected for takeoff performance improvement as the aircraft weight continued to increase with added equipment. The B model in the first photo could deck launch without catapult, lose an engine and still climb away at max gross, IIRC (albeit very slowly). I haven't heard/read what performance the new props restored, nor am I sure about how much max gross weight has increased, but it was significant, I understand.
In this plane's mission, cruise and/or top speed are not important factors; takeoff performance and time on station are.
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I see 8 blades... not six.:dunno:
 
N801BH said:
Example = 2700 on a 76" dia prop @ 70 degrees is mach .794. That is not even close to tips going supersonic but alot of people will tell you that's the absolute max it can spin.... Yeah, right !!!
you certified guys are stuck with that incorrect thought process and cannot deviate. Even though the facts clearly show the prop can turn a higher rpm. Prop manufacurers and motor suppliers are NOT going to change their specs and you can bet the FAA ain't gonna either.
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Ben, I realize that you really don't like certified aircraft, but I also don't see where anyone stated that 2700 RPM meant supersonic tip speeds, or said that's an absolute limit. That is obviously not the case since tip speed is going to be RPM * blade length (i.e. prop radius). Lots of really tiny props for model aircraft spin at 6000 or 8000 RPM.

As to why a lot of certified aircraft spin their props at 2700 RPM or slower, well, the slower props I've seen are quieter, and the engines tend to be optimized for running at those RPMs (there's probably a bit of chicken and egg thing here). If I had to guess, a lot of the driver had to do with noise, which is more than whether or not Mach 1 is exceeded at the tips, but is definitely related to prop speed. Just flying my Aztec at 2300 RPM (my typical cruise RPM) vs. the Archer (for which 2600-2650 RPM is my typical cruise RPM), I notice a difference. Fly a 421 with geared engines and the big, slow props and it's heavenly to me. I love how quiet it is.

I wouldn't expect any engine manufacturers to change their specs to allow higher RPM operation than present. That would result in a change to the type certificate, which would result in lots of time and paperwork. I also don't see the benefit - once again, the engines have been designed around the RPMs they operate at, so you would end up likely seeing engine changes to accommodate the higher RPMs. Nevermind the required change in prop pitch stops and governors. The engine in your plane isn't optimized for rated power at 2500-2700 RPM. Likewise, the engines in my Aztec aren't optimized for rated power at 4500 RPM. Could you change it? Of course you could. However I think the results will be less than ideal. Put in an engine that was designed to run at 4500 RPM and you'd have a different story.

Call me a luddite (you wouldn't be the first), but the 421 is, to me, one of the nicest flying aircraft I've had the privilege to operate, in large part because of the big, slow-spinning props. If I got one that just sat around and I virtually never flew, I'd probably keep it just because I think it's cool. But, I also admit to liking 3-bladed props more because they look cool.
 
Ted....

I agree with 100% of what you said..

Peace brother.

Ben.:cool2:
 
gkainz said:
puts down coffee and holds up 2 more fingers ... uh, yeah - 8... fixed above. :tongue:
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Yeah, the first time I saw that pic, I thought it was photoshoped, but there's 100's of pics just like it on google images. It looks kind of cartoonish.
 
N801BH said:
Out here in the mountians the temp changes by the minute so that determines my throttle position.
For instance, today its in the mid 30's f now. By this afternoon it will be in the 90's. What will be partial throttle on takeoff now will be full throttle today at 4 pm. Another ol wives tale in you can't spin a prop over 2700 rpms.. That's just plain BS.

Go to that program and enter in various diameters, prop rpms and temps.

Example = 2700 on a 76" dia prop @ 70 degrees is mach .794. That is not even close to tips going supersonic but alot of people will tell you that's the absolute max it can spin.... Yeah, right !!!
you certified guys are stuck with that incorrect thought process and cannot deviate. Even though the facts clearly show the prop can turn a higher rpm. Prop manufacurers and motor suppliers are NOT going to change their specs and you can bet the FAA ain't gonna either.

just my humble opinion.

Ben.
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Nobody said you can't spin a prop past .8 Mach. The 185 I flew ran an 86" prop at 2850 redline RPM with a tip speed of 729 MPH or .95 Mach.

But efficiency drops off pretty steeply at that RPM, noise increases enormously, and the centrifugal forces on the prop increase by the square of the increase in RPM.

The lower efficiency means we burn more fuel to go where we want to go.

The louder noise makes us unpopular with folks on the ground.

The higher stresses on the prop wreck it sooner or it needs to be built stronger and therefore more expensively.

The "Thunderscreech" ran its prop tips at supersonic speeds and no test pilot ever flew it more than once. They all refused. Too noisy, ,even in the cockpit. One mechanic could hear it running up at the base when he was at home ten miles away. That's the sort of thing higher Mach tip speeds do, and if you want to do it, you'll make aviation even less acceptable to the general public than it is now.

Dan
 
Dan Thomas said:
Nobody said you can't spin a prop past .8 Mach. The 185 I flew ran an 86" prop at 2850 redline RPM with a tip speed of 729 MPH or .95 Mach.

But efficiency drops off pretty steeply at that RPM, noise increases enormously, and the centrifugal forces on the prop increase by the square of the increase in RPM.

The lower efficiency means we burn more fuel to go where we want to go.

The louder noise makes us unpopular with folks on the ground.

The higher stresses on the prop wreck it sooner or it needs to be built stronger and therefore more expensively.

The "Thunderscreech" ran its prop tips at supersonic speeds and no test pilot ever flew it more than once. They all refused. Too noisy, ,even in the cockpit. One mechanic could hear it running up at the base when he was at home ten miles away. That's the sort of thing higher Mach tip speeds do, and if you want to do it, you'll make aviation even less acceptable to the general public than it is now.

Dan
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I am based out of Jackson Hole. The ONLY airport in the world that's in a national park so I am very sensative of noise. That's why I have big mufflers on my plane. They weigh alot but its my way of fitting in to paradise. I am betting my plane is quieter then most. If you have good speakers on your computer this video demonstrates how "acceptable" my noise level is to the general public.:ihih: This pass was at 4400 engine speed =.946 mach. Prop is VERY quiet.:wink2::wink2:


http://www.youtube.com/watch?v=AOF6eT6FRmY
 
Dan Thomas said:
The "Thunderscreech" ran its prop tips at supersonic speeds and no test pilot ever flew it more than once. They all refused. Too noisy, ,even in the cockpit. One mechanic could hear it running up at the base when he was at home ten miles away. That's the sort of thing higher Mach tip speeds do, and if you want to do it, you'll make aviation even less acceptable to the general public than it is now.
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I would argue that this is the bigger reason than ground noise. Just look at the things like the MU-2. I heard one start up in Texas the other day (while I was in Colorado). A lot of the love of 421s as I understand it had to do with the low cabin noise thanks in large part to the geared engines. I know that's my love for them.

Although I make fun of the MU-2 (and anything Garret powered) for the high ground noise, once they get flying they're pretty quiet. Once again, props. A good example is the King Air 100 that my local FBO charters out. When that thing starts up you can't hear yourself think. When it flies past on takeoff, it's quieter than a Cherokee.

N801BH said:
I am based out of Jackson Hole. The ONLY airport in the world that's in a national park so I am very sensative of noise. That's why I have big mufflers on my plane. They weigh alot but its my way of fitting in to paradise. I am betting my plane is quieter then most. If you have good speakers on your computer this video demonstrates how "acceptable" my noise level is to the general public.:ihih: This pass was at 4400 engine speed =.946 mach. Prop is VERY quiet.:wink2::wink2:

http://www.youtube.com/watch?v=AOF6eT6FRmY
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As you know, prop noise also has a lot to do with the prop design itself, not just diameter. I've run a lot of different props in otherwise identical conditions. Take an engine and put 3 different props on it, you learn quickly which ones are louder vs. quieter. I have done this before and noticed that, with very close prop diameters, there is a big noise difference due primarily to design. Engine muffling is secondary, but does have an impact. Turbocharged engines are definitely quieter thanks to the muffling provided by the turbo system.

I don't have good speakers, so I can't really tell much from the video. But I can tell that I hear the prop noise over the engine, which is exactly what I would expect. Sometime when I make it out to Wyoming I look forward to getting to see your plane in person, I know I'll like it.
 
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N801BH said:
I am based out of Jackson Hole. The ONLY airport in the world that's in a national park so I am very sensative of noise. That's why I have big mufflers on my plane. They weigh alot but its my way of fitting in to paradise. I am betting my plane is quieter then most. If you have good speakers on your computer this video demonstrates how "acceptable" my noise level is to the general public.:ihih: This pass was at 4400 engine speed =.946 mach. Prop is VERY quiet.:wink2::wink2:


http://www.youtube.com/watch?v=AOF6eT6FRmY
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Looks like you had her firewalled there. You must have been doing 70kts maybe 80kts there!!!! :ihih::goofy::crazy::wink2:
 
So can anyone explain to me how the super skinny 4 blade q-tip props work?

big_N4165S-ext10.jpg
 
vontresc said:
So can anyone explain to me how the super skinny 4 blade q-tip props work?

big_N4165S-ext10.jpg
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Near as I can tell most of the 4-bladed part is about looking cool. However they're supposed to be quieter. The part that looks like a prop strike acts just as a winglet (remember, a prop blade is just an air foil). This makes for less turbulent air at the tips and quieter operation.

I've never flown a plane with Q-tip props, nor have I ever seen one in operation on ground or flight. The concept makes sense, however I've only ever seen the props in use on STC'd conversions, specifically the various ones from Colemill such as the Foxstar Baron you picture. I also think RAM is putting them on the Series IV T310 conversions.
 
cessna182b said:
... not to mention the Russian TU-95 "Bear", which had four sets of contra-rotating supersonic propellers. It was supposedly the loudest plane ever built: http://en.wikipedia.org/wiki/Bear_Bomber

Dave
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I've always thought that was a wicked looking plane. I think they are still flying, too. However, I haven't read anything about them having supersonic props. Where did you hear that?
 
Another factor to consider with respect to noise is that the pulses from a 3-blade prop create a higher frequency noise that's easier to block with insulation and/or mitigate with active noise reduction.
 
IIRC, and Kevin Mead can correct me on this, the Malibu had the 2 blade for, as mentioned above, cruise performance. They switched to the 3 for the Mirage. I believe that switch was driven by Piper Marketing folks, like Ttails, but had the definite benefit of shortening, somewhat, the take-off roll. The Malibu wing loves high altitude, but ain't so great at low altitude, and the 2 blade just made it worse.

I will admit that I prefer the 3 blade prop because it just looks better. A Malibu with a 2 blade prop looks kinda funny to me.
 
flyersfan31 said:
I will admit that I prefer the 3 blade prop because it just looks better. A Malibu with a 2 blade prop looks kinda funny to me.
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I tend to think 2-bladed props look more like toys than 3-bladed. Although Aztecs look weird with 3-bladed props, they should get 4-bladers. ;)
 
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