This was a great series of videos Gregg. I studied blade element theory in university so I can try and explain what is behind the curtains without going into the math, if this is of interest to you. BET is a simplified theory that basically says that if you have a prop blade and you look at a thin slice of it - if you know the airfoil properties of this slice (the blade element), and the parameters that define its motion relative to the airflow (its pitch angle, rotational speed, flight speed and downwash), you can then calculate the aerodynamic forces that act on it. Then, if you do that for the next slice and then the next and so on, and sum it all together (and multiply by the number of blades) you now managed to calculated the thrust, absorbed power, torque and efficiency of the prop. In effect, this theory assumes that each slice is unaffected by, and is unaffecting, any of its adjacent elements (in other words it assumes a constant downwash across the blade). It's a simple theory, but that also means it can be solved by pen and paper. More advanced methods (like 3D potential flow based solutions) require a computer. Despite being simplified, BET gives decent results and is still sometimes used for initial design of props/rotors. The reason that it works is that the solenoid nature of the spiral vortex wake of a prop means that the downwash is not far of from being constant over the prop disk, so it's not a bad approximation. Regarding 3 v 4 blades, here's another example that matches the point you made in the video. There's a report by Hamilton standard (available online) that contains measured propeller performance maps of many different parameters - "generalized method of propeller performance estimation". From a quick look at one example, comparing two props of equal solidity and using the same airfoil, a 4 bladed prop is very slightly more efficient than a 3 bladed one - by about 1 % at peak efficiency. On a related note, you may remember that a while ago I made a drag and maneuverability comparison between a Mk IX Spitfire and an FW-190A following one of your previous videos. As part of this, I used the aforementioned Hamilton Standard report to estimate the prop efficiency of both aircraft. This is not an apples to apples comparison, as these are two different aircraft with different engines, but nonetheless the estimated efficiency of the 4 and 3 bladed props came to an identical figure (85%).

Greg, as an instructional designer of 40 years experience, I admire your native abilities as a very effective "explainer." It is always a tightrope walk between technical detail that informs and technical trivia that overwhelms. You do a great job of walking that wire!

Also remember that, if you add another blade, you increase weight. You might gain small amounts of efficiency and power capacity, but you add overall weight to the airplane, as well as rotating mass to the propeller, which increases its gyroscopic effect and puts more stress on the engine and its bearings during tight maneuvering.

The power vs solidity is a good commentary. Almost all modern open-propellers, for newer airplane designs, tend to use six-to-eight bladed Scimitar blades. As in the video, the reasons for this vary, depending on powerplant diameter and power input. The efficiency losses for any propeller (discounting prop-skin drag and hub/motor losses) are mostly dominated by the tip-speed losses, which grow exponentially as supersonic velocities are approached. As such, any propeller cannot be made to go supersonic, or it will create enough drag to actually rip-off the propeller from the engine. This was demonstrated in the post-WW2 developments, where jet engines were fitted to propeller airplanes, and when enough thrust was generated by the jets, to get the planes into supersonic range, the propellers (and engines) actually broke off the plane, due to the (mostly) asymmetric drag spike. At least one pilot was killed during these tests, as a result. Scimitar blades reduce this problem by increasing the "pushed area" of the blade, at a subsonic speed. This allows more air mass to be propelled by the blade, at a lower tip velocity. That is thrust. Shrouded fans have a much higher efficiency (by as much or more than 30+percent), since they use dozens of blades, and the tip shock-waves are contained and actually forced backwards, so as to add some minor amount of thrust. Unfortunately, the shroud adds weight, size, diameter, and external skin drag to the airplane, so there is a tradeoff between efficiency of the blades, versus the power pushed into the air, versus the speed of the shroud+airplane itself. Modern turbofans have pretty much maximized these parameters, and easily fly at about Mach-0.8, without having any issues with tip-failure. Open-propeller airplanes cannot fly much beyond Mach-0.4 (maybe 0.5), even with scimitar props. Modern turbojets can easily fly beyond Mach-3.0 since the engine size/drag/diameter losses are low enough to allow enough air-mass to be pushed through the engine, to compensate for the skin/drag losses of the rest of the plane. This analogy also applies to underwater propellers, and all modern submarines have shrouded high-efficiency low-noise propellers. Another good example of this is the 1920's farm windmill pumps water pumps. They use dozens of blades, very efficient at very low wind speeds, using open hubs to allow useless air passthrough, but adding a large-area outer turbine. This allows minimal material to be used in manufacture (ie: cheap, cheap, cheap), while retaining about 85+percent of the useful wind energy. Simple & effective.

Greg - back many years ago, I remember conversations at the glider port I grew up on about buying 3-bladed propellers for the tow planes. Nothing to do with power, but the extra blade reduced noise and the hope was of pacifying the neighbors for as long as possible. It doesn’t fit the theme of this channel, but in such a thorough review of propellers, it seemed like an omission worth pointing out.

Big props to Greg for his great explanation of prop design! Greg is teaching me stuff I didn't even know that I wanted to know. But after watching these videos my understanding of airplanes went up 50 levels.

The guy who invented the cooling cuffs was a member of the same EAA chapter I'm in. He passed away a couple of years ago, but he was an interesting guy...

VDM stands for "Vereinigte Deutsche Metallwerke", which can be translated "United German Metalfactories".

That Cartoon was brilliant. I’ll never get that out of my head…“draufhalten“ Also that little rhyme is nice…something along the lines of: The pilot above all else cherishes The position where his aim-lead vanishes Yeah, it is a bit forced...cherishes-vanishes, I’m obviously not a poet.

TWO propeller videos within 24 hrs?! That basically makes my whole weekend and I'm not sure whether that says more about how good your content is, or how much of a nerd I am.

I know there was at the very least a proposal for a Bf 109 K-14 with a four bladed prop, since by that point the prop was bottlenecking engine power delivery - and they couldn't make the prop diameter any larger because of ground clearance issues; but it never went past the drawing board before the war ended, so this should be a pretty interesting video.

I used to work on the C130 propeller control. Most of the time I found that the hunting prop was caused by the constant velocity potentiometer and I soldered a new one on. It was nearly the most boring job in the world.

I suddenly realized that I could have watched two of Greg's videos instead of the last episode of Game of Thrones all those years ago.

The amount of research you put into these videos is amazing.

Guessing by late war the fact that you could make four 3 bladed propellers for every three 4 bladed propellers was also a serious consideration. The Germans weren't flush with fancy metals by that stage.

at the video 33:30 time mark you talked about the Corsair three and four blade prop. My father worked for LTV for over 42 years. I remember him telling me that the Corsairs had a vibration problem with the four blade props. Many went back to the three blade prop because of that.

Wow! I binged and watched all 3 parts this evening. Magnificent stuff. I may not be an expert aeronautical engineer in the prop area now, but you've left me with a much more solid understanding of both the history and technology. Thank you for a most enjoyable and fulfilling evening.

It is as Doug Hanchard said it. We should also keep in mind that a prop is quite a heavy component and concerning weight - there are clear limitations on an aircraft. With a four blade prop the hub is certainly heavier and there is a fourth blade whose inner portion does not contribute very much to thrust but adds a lot on weight. German prop fighters were based on much earlier designs and therefore the technicians were constantly struggling with weight and CG. The prop is the foremost component on an airplane and has a significant impact on CG. At that time there was no technology based on composites. This might have been the reason to use wood as blade material (and also lesser vibrations). Vibrations crack metals - therefore preferably uneven blade numbers (3, 5, 7, 9). This is the choice of the experts !

Another one of your consistently great aviation videos. Thanks for taking the time to create and share!

Two prop videos in as many days. Greg you are to kind. Fantastically informative as always.

This channel is thoroughly fascinating! Extremely well researched, incisive. 🙏🏼💛

That's made for an interesting weekend; 2 propeller videos. Thank-you again for your work.

Thanks Greg for providing some technical interest to liven up my Sunday morning coffee. Great job as always!

Awesome! A subject I have been curious about for a long time. Did a little research on my own but listening to you is so enjoyable.

Thanks! I've always been fascinated with the wooden 3 blade prop on the FW 190.

Oh man back to back. Can't wait to hear greg's well researched thoughts on this. I always thought it had to do with the german guns shooting through the prop arc but let's see.

really enjoyed this series of videos on propellers, greg, really thorough.

Love listening to these and learning. Great work, Greg.

Very well done, thank you for sharing your time and knowledge with us.

Greetings Greg, you are the only person who covers and adequately explains this era of aircraft. Kudos.

My grandfather was ordered to pick up the prototype 4 bladed propellor for the spitfire and deliver it to it's destination where some big wigs were to watch. It didn't work that day and there was an accident on the start up. It kicked two chaps sitting on the back of the plane up in the air and the propellor dug straight into ground, he was later ordered to bury the parts.

Always a very enjoyable and enlightening study, Greg. I learn a lot. Thanks.

I used to go with the' interrupter gear argument' so it was nice to be corrected and educated. The Germans were well advanced with jet power so probably realised, correctly as it turned out, that there was more to be gained developing turbojet aircraft, rather than wringing another few knots with four- or five-blade props. Excellent info as always.

Greg's really the GOAT theres no other way to put it. The content is just too good.

Love how War Thunder is becoming an easy source for 3d models of these planes.

Totally fascinated by this topic and this is a masterly series explaining the different propeller designs.

Hi, Greg - Thank you for another great video, and an extra thank you for this opportunity to finally use what I vaguely remember from the semester-long course in propeller design that I took as an undergraduate student of naval architecture. Everything you said about the relative merits and demerits of three-bladed vs four-bladed propellers is spot on. To that, I would add this: The lift-versus-span profile of a typical propeller blade essentially is a parabola, because as you and your intrepid followers know, lift is proportional to the square of the blade velocity, which increases linearly with span. Of course, tip vortices and other phenomena screw up the flow at the terminus of the blade, so the parabola stops short of the tip, but still the bulk of the lift is, in theory, generated by the outer sections of the blade. My contention is that when a long-bladed propeller operates in the cold, thin, slippery air encountered at high altitudes - and especially when that cold, thin, slippery air has been disturbed by the preceding blades - true steady-state laminar-ish flow - the kind presumed by subsonic lift calculations - never fully develops in the distal blade sections: the local blade speeds simply are too high for the air’s dynamic viscosity, which decreases at low temperatures. This, I believe, results in severely high local blade loads yielding, among other problems, diminished acceleration and climb rates. One way to counter this is to “flatten” the lift-versus-span profile by adding area to the proximal sections of the blade: lift also depends on blade area, so although the blade speed at a proximal section is lower, and the lift per unit area is less, the overall area is larger, and the section picks up more load. That appears to be the approach the Germans took: the propeller on the FW-190 shown around the 35-minute mark in your video has a pronounced mid-span beer belly, yet we know what a terror it was, even at high altitudes. The Americans used a similar approach to improving the climb of the P-47 Thunderbolt, which you, better than anyone, know was initially equipped with a four-toothpick propeller, as some of its pilots derided it. Republic gave the P-47 a somewhat fatter four-bladed prop, though not as fat as the three-bladed German variety, made a few other improvements, and voila! The P-47’s climb went from sub-mediocre to respectable. As in life, you pay for fat with efficiency, which was a prime motivator for American engineers trying to fulfill the requirements of a long-range escort fighter, but less so for Germans defending their backyards. Again, thanks again for the much-needed and much-appreciated geek-fix.

34:20 for what it’s worth, the planned 109K-14 was intended to use not only a 4 bladed prop, but a 2 stage supercharged version of the 605, both for better high alt performance

Always fun to learn something new in the exciting world of aircraft stuff. The photo of the CW P-40 assembly line was quite telling, given the fact the P-47s were on the line also with nobody working on them. Of course we all know how that turned out! Thanks again Greg.

The BV 155 only existed as a prototype. Was intended as a sole high altitude fighter. Had a four bladed prop. Love your videos. I actually go years back and watch them again.

Great videos Gregg, kudos on the work! Glenn Curtiss was a wild fellow. As you point out, it was odd the Curtiss and Wright merged after so many confrontations in court. Perhaps the powers that be decided it was the only way to stop the madness. Based on the importance of propellers in performance it is terrific to learn more about them from you. Thanks Gregg.

I do appreciate listening to an expert on such topics.

Great video Greg. I often wondered about the two vs three vs four blades as well as the variable pitch, dual pitch, fixed pitch questions. Good info, very good analysis, and not difficult to follow. Thank you.

Great presentation as usual...I always manage to learn or confirm watching the results of your research. Thanks again

"Gentlemen, we do not stop till we win the war." "What about prop blade? "You've already had it." "We've had three, yes. What about fourth prop blade?"

This prop series got better and better with every instalment, and with a double whammy to boot. What a treat!

Yet another brilliant video Greg. You manage to make such detailed topics really understandable to people like me with no engineering knowledge, thank you.

Great video Greg! Nice job and loved all the photos illustrating your points. Especially the Paul Allen FW-190D-13 and its huge, fat blades.

Dear Greg: Thank you so much for making these videos for us engineering nerds. Have a Merry Christmas.

I just checked the Swiss 109E3 they had a VDM unit in 1939! Some units had an indigenous Escher Wyss constant speed unit. The same that was used on the licence built Morane Fighters that had a centreline canon too. Escher Wyss startet with variable pitch Ship Propellers by the way. Reference: "Die Flugzeuge der schweizerischen Fliegertruppe of Swiss Air Force.

Very sober analysis. For some reason I was having good time following this, thank you.

I'm loving these videos. Thanks, Greg!

u have no clue how far i had searched for this answer

Another great video Greg. Another plane worth investigating with different propellers is the Reno racer Rear Bear Bearcat. It ran both 3 blade and 4 blade in its career with Lyle Shelton. You can look at the Reno air race forums for more great into but I believe the 3 blade was blades modified (cut down) from a P3 Orion. According to John Penny it was a handful to fly around the Reno course. I think they set the climb record and top speed level records with the 4 bladed I recall correctly.

Another excellent video series. Well done sir!

When I watch your videos sometimes I feel just like applauding. When you present it is just like hearing a friendly jovial professor speaking to graduate students at an excellent university. The assumption is made that the trivial is known to the audience and that the audience can follow an explanation even if the subject is somewhat difficult. The students then leave the hall at the teaching university and they know a lot more than when they walked in. Thank you professor, I now have interesting questions answered well enough that unless I someday go into building propellors I know all I need to know and questions that had piqued my interest have answers that are now part of my education. I am looking forward to your next discourse sir.

I have been waiting for this for months

Do remember reading yrs a go about a Jug pilot in WWII talking about flying around England and Spitfires practice dog fighting them from time to time and how they (the P-47) were always out-climbed in mock combat, but when the 4 paddle blades were introduced they were able to out-climb them easily.

Amazing amounts of research went into this video. Thanks.

Fascinating! Greg never disappoints.

That was quite interesting. I actually got to the end of your video this time. My dad was a P51D fighter pilot in 1945 fighting the Japanese. I remember him saying something about the variable pitch propeller but I didn’t understand much about it when I was a young kid.

Great video Greg!

That is a question that I've always had in the back of my mind. Thank you so much for answering it.

Well done with excellent visuals! Thank you.

For referenceI have experience with 3 vs 2 blades. I race an RV4 in cross country SARL races and have tried three props while racing. All were Catto propellers, and you may know that Catto driven airplanes hold something like 6 world records. My last prop from Craig (Catto) was a 2 blade that was a bolt on 3 knots faster than the three blade which is a lot when TAS is 197 knots in an RV. I have not studied prop design in detail, but I have studied wing theory. In the smaller racing airplanes I believe the general consensus is that 2 blades are better than 3. And, I always assumed that the reason for this is that a third blade will add some drag due to airflow over the blade itself. Also, it may just be the hp range of the engines we run blade drag is a bigger factor than a 2000 hp engine. I also have noticed that the Hartzel composite blades are much wider (more chord), and are very fast. FYI Catto props are fixed pitch, so only really good for cruise and racing, although initial climb rate is around 2000 fpm and it will maintain better than 1000 fpm all the way to 10,000 msl, good enough for me. Really enjoy your videos Greg. Keep them coming!

From the world of wind turbines: We have similar issues with blade number, disc solidity, tip speed, etc. You might notice that almost every wind turbine has 3 blades. The reason is 3 blades run the smoothest, especially when changing direction. You never see 4 blades on a wind turbine. Three is all you really need, power-wise. Two-bladed wind turbines extract almost the same amount of power as 3-bladers, but two-bladers have a BIG problem: During rotation, when the blades are aimed vertically, the entire turbine can change directional aim (yaw) more easily, since, like a figure-skater pulling their arms in during a spin, the yaw moment of inertia is reduced. Just 90 degrees later during the "propeller" (rotor) rotation, a 2-bladed wind turbine rotor has its blades oriented horizontally, with its highest moment of inertia with regard to direction change (yaw, or aim). So it resists changing direction with the blades horizontal. 90-degrees later still, it can once again change directional aim easily. Etc. So the two-bladed rotor shakes like the dickens when changing direction. The result is a severe "hammering" of a 2-bladed rotor against its bearings and support frame when changing direction. Therefore, two-bladed wind turbines literally shake themselves apart due to this terrible phenomenon, known as "yaw judders", "chopping around the corner", etc. The only cure is a "teetering hub" for a two-blade rotor, which entails its own set of problems, such as tower-strikes. Also, 2-bladed rotors (lower rotor solidity) typically operate at higher tip-speed ratios, which makes them louder. Two-bladed turbines are too noisy. And they look weird when running. And the high speed causes more leading edge wear due to dust in the air, a huge problem in itself. So, 3-bladed turbines are so smooth, and quieter, with less abrasion from dust, they have replaced two-blade turbines. Why mess with a two-blade turbine that will destroy itself, piss off the neighbors, and wear out its blades, when adding one more blade solves these problems? Four blades do not run quite as smooth as three, and add weight and cost, while not adding more power. I'm imagining these factors also influence the choice of number of blades in airplane propellers, especially for a fighter changing direction fast. The real reason for three blades versus two might be hammering when changing direction quickly. Three blades run smoother than two, while four blades are not worth the extra material and complication.

Thank you Greg for presenting in such well detailed and digestible manner, the mystery of prop pitch control and blade count. This is also thr first time I've heard of the term "solidity", and I'm thinking about the jump in solidity with the introduction of 8 blade semitar props 🙂

Great job on explaining a complicated subject. Even I understand most of it. I never knew a 1 bladed prop was even a thing, but I always "knew" that 4 blades were better and faster! Well you learn every day.

Boy am I early.. Usually I'm watching videos with a few years under their belt. Great work, as always!

I remember as a kid seeing single bladed props on competition rubber powered models. Often wondered why. Efficiency

Excellent as always.

Excellent presentation Greg and great reasoning that supports your theories. I would love to see a presentation on 5, 6 and contra rotating props if possible.

Great video Greg. I was reading up on the Mars helicopter propeller design. Mars has 1/100 density of the earth's atmosphere and has to operate at a very low Reynolds number. The craft uses two bladed contra rotating propellers, about four foot long. The blades look very "weird" to operate at such low fluid density. I love the symbolism of the helicopter taking an artifact from the Wright flyer and using it on the Mars copter. The Wrights created the first workable propeller and their legacy has us flying powered propeller craft on Mars. I love aviation and space tech. I love this channel.

Single blade prop was news to me, thanks for the video.

Absolutely love your videos. Your voice is one of a kind too.

Outstanding video and presentation.

Hello Greg. Great stuff! I recognize some of the NACA formulas from my work in cylinder head flow bench fluid dynamics. A fluid is a fluid. So many non-technical/ political/ cultural factors were involved here as to the evolution of front-line warplanes (I am working on an outline for a book on this). You mentioned the competing industrial bases and capacities. Add to that the German move to underground manufacturing. Not a time to make changes? I had a number of years in contact with the engineers at Porsche & Audi back in the late '70s. I ran into an almost monolithic attitude of "NIH"(not invented here...). Chrysler engineers ran into the same thing in the early to mid 2000s while Daimler-Benz owned them. And I quote: "Americans know nothing..." Many aviation engineers went to the automobile industries after the war. I conversed with many (not captured by the Soviets). Funny stories there...

As an aside, the one-bladed Everal propeller was also a self-adjusting, constant-speed propeller. It balanced aerodynamic loads with centrifugal force to automatically change pitch. Everal helped improve performance on the early Taylor and Piper Cubs that only had 40 horsepower. The later Aeromatic, two-bladed propellers sold in significant numbers during the late 1940s. Aeromatic used a combination of balance weights and springs and aerodynamic loads to produce a self-adjusting, constant-speed propeller that needed no extra components aft of the prop hub.

Thanks for all the info in this video !!!

Excellent video once again!

I'll actually be able to catch this on time, happy days.

I would love to see a series this in depth on Wings, I think that would be really interesting, especially with the Flying wing experimentals.

awesome as always - thanks Greg 2% difference between 3 and 4 blade props got me

Feels so good with 2 videos in a row

You don’t have to censor nudity because it’s educational content 😂😂

This also makes sense when you look at the B-36's ENORMOUS 3 bladed propellers, whos blades had a huge chord that maintained thickness from halfway down the blade, all the way to the later flat tip. Thanks for making these Greg; we all love them so much. Oh, some of my mates and I have recently started playing Il-2 recently; mostly on the Finnish Virtual Pilots server. Where do you normally fly?

The real gem here IMO is the 109 pic at 35:00, which demonstrates like no other the armament arrangement and the cannon’s cohabitation with the pilot in that tiny cockpit !

great stuff greg

Completely fascinating. I have always wondered about the difference in width and shape between Germen and Allied propellers. I wonder no longer. Thanks.

Premieres at 10PM Japan time. I have to go to bed at 10PM Japan time :-D See you tomorrow

The explanations here are first rate, thanks Greg. If anyone is interested, Periscope films has a WW2 era film on propellers that shows some of what the piolts and I'm guessing the A&P's would have seen on the principles involved back then.

Thank you for posting. Great video.

i love the history of all of this. i also love the detail on how all of this worked. as i work on boilers with a forced air burner ( ok it is almost a jet engine ) and all of this work out like the flow for air and water. it is all fluid dynamics and it ties together .

A luftwaffe general named Jahnke, faced with production, fuel, amaterial short age ordered that there would be no 4 propeller planes, which meant no heavy bombers... Love your channel

Curtis-Wright have a facility close to my home in Brea, California that as a kid I naively assumed produced Aircraft but unfortunately isn't involved in any aviation products anymore but instead a Power Plant equipment supplier. However, a number of other Aviation and Aerospace companies are located here in the Orange County area still. Marine Corps Air Station Tustin no longer has active aviation activity, but still retains the 2 massive hangers built to house World War 2 era coastal patrol blimps, and while MCAS El Toro was decommissioned in the 1999, it was the home of the Marine Corps Aviation on West coast. Built in 1943 it was home of the 3rd Marine Air Wing, and attracted quite a few defense aerospace companies to Orange County in the 65 years it was operational.

Fantastic subject! Thank you sincerely!

Maybe Hans 15yo was breaking Panther's final drive because his mother didn't let him read that manual?

Ever consider doing a video just on Glenn Curtiss? You cover the early aviation stuff better than anyone else I've seen, and Curtiss was an interesting and important figure, with plenty of innovations to cover (and not just in aviation). Curtiss just doesn't seem to get any recognition in aviation overall. Most pilots and fans of aviation I meet have never heard of him, but that also gives me the chance to talk to them about him too.

great vid greg

I just love the fact that this 40 minute video about something some other KZfaqr would spend 10 minutes on, Thanks Greg for giving us the facts.