J Hallock

My Feedback: (3)

I have been resisting the urge to experiment with my engine prop combo for a while and now I can't stand it any longer. What are the advantages / disadvantages to 2 vs. 3 blade props? It seems like every time I see a mag article on this topic a professional has a different opinion. I have 2 planes I aam flying right now, the .40 size Model Tech Magic and the CAP 580. I have a .46 OS La with a 3 bladed 10 x 4 on the Magic and a .50 OS FX with a 2 blade 11x7 on the CAP 580. The magic I have flown with a 2 bladed prop but it seems to be more stable in the air with the 3 blade. I am hesitant to try a 3 on my 580 just because it.... well just because I just finished building it and have only flew it 3 times. Do you think there would be a suitable 3 blade out there for that plane/engine combo?

Dr1Driver

The only advantage I can see to a 3-blade prop is extra ground clearance. 3-blade props are less efficient then two-blade ones.

Dr.1

da Rock

There is one other advantage... sometimes.

If you have more power in the engine than the present 2-blade can turn into thrust, and you can't find or carve a wider bladed or more efficient 2-blade, and you can find a 3-blade that actually has more blade surface than the 2-blade and whose blades aren't less efficient individually and collectively, the 3-blade might help the engine get it's power to the ground.... uh get it's power to the air.

But working with what's available to our models, there seems to be little possiblity of all that working out. (Except for the carving deal.)

In WWII, the Corsair was improved when they went from a 3-blade to a 4-blade. I think they did the same with the P-47. But they weren't buying their stuff from the LHS.

Dr1Driver

That's right, darock. The Corsair needed the 4 blade prop to absorb the tremendous power generated by that big 18 cylinder radial.

Dr.1

da Rock

J Hallock,
Actually, stability in the air comes from other things than the number of blades on the prop. You really are heading in a good direction, however, when you're thinking about experimenting with props. They sometimes do make a great deal of difference, and it's often from just changing brands, while keeping the same diameter/pitch.

The other day, one of the beginners at the field was telling everyone how much better his trainer was flying with the new prop. Turns out the new one was same brand, diameter, and pitch. When I looked at the old one, it still had the warning sticker (only slightly surface worn) stuck pretty far out one blade. I put it on my field balancer and that side almost broke when it hit the work bench. It's amazing his trainer was still in one piece. I'm guessing the reason his trainer was flying better was because the engine was running smoother, the fuel wasn't getting SHAKEN like crazy, and the new prop was biting the air along a straigher path (not to mention his receiver not being beaten to death or the gear trains in his servos seeing less abuse). With props, lots of things can change how the model "feels".

Most .40s will turn an 11x5 pretty much like they'll turn a 10x6. Trying them both would be worth the test, real easy to do, and wouldn't crowd the envelope for the engine one bit. And would be a really good thing to do along with testing the 3-blade, if you find a 3-blade that fits into the system. Hey, trying different props is actually a lot of fun.

BTW, every prop change should be followed by a basic readjustment of your needle setting. First thing to do ALWAYS with a new prop is to turn the needle out until the sucker clearly goes rich. Count the clicks out so you won't get lost. Then adjust back down from there. Going out to rich gives you a safe and dependable starting point.

speedster 1919

My Feedback: (3)

I like 3 blades and I love the ground clearence. Funny how us 3 bladers can always find an equal performing prop if they are so inefficent. WW2 was all about speed . Way too bad they didn't know 2 blade was so efficent. They were idiots to put 3 and 4 blades on their planes. LOL

J Hallock

My Feedback: (3)

Right on. I have experimented with props on both of those engines. I guess it just might be that I found a really good match for the .46. I am crazy about the smallest details on my airplanes that improve performance and while my planes are not the biggest on the field, they certainly are very well cared for. Until this point I have pretty much just gone by the engine owners manual as to the engine/prop comb. Now I feel like stepping out of my "by the book" zone and still try to stay within the performance envelope of the engine.

Thanks.

da Rock

J Hallock,
Actually, you're going to find that experimenting with 3-bladers on models is a pretty small little box. There aren't usually many 3-bladers in the hobby shops, or only one or two. The biggest hobby shop in my area happens to have about 5 different sizes, and that's also 5 props total. It won't be easy. There are a couple of small-shop custom prop makers that are around but they're mail order and I can't remember seeing one of them advertising for awhile now. I'd like to suggest that if you plan to be somewhat rigorous in your testing that you look around for a Prather Pitch Gauge. There actually might also be other pitch gauges out there, but I know and use this one and it's a jewel. And after using it for a very short while it'll be fairly obvious that there is a lot more to a prop than the diameter/pitch markings on the sucker and those very often aren't correct for all the props from the same box, and often isn't correct from one blade to the blade on the other side of the hub.

speedster1919,
And merry Christmas to you. BTW, in WWII, the Corsair and P-47 were already faster than just about anything they flew against. But they didn't climb as fast as some. In fact, the designers were working hard to cure that problem. Before the P-47 got it's 4-blade, almost everything with guns in the wings could outclimb that hulk. After it got the 4, it could even outclimb the Spitfire. The design change was really an attempt to get more blade area working and to keep the prop tips under mach speed. They wanted climb and speed but really wanted climb BAD!
And an interesting little tidbit about efficiency of props.... The c/l speed boys have used ONE BLADED props for years and years and years. Can you guess why?

BMatthews

Not idiots, just practical. They already knew that 2 blades were more efficient but the realities of landing gear lengths meant that multiblade prop designs were a neccesary evil. This tradeoff finally culminated with the 6 and 8 blade counter rotating props.

The writeups I've read about the Corsair design all stated that the inverted gull wing was chosen to allow a shorter and tougher landing gear to better withstand the controlled crashes that naval aviation calls landings while still allowing the use of that big 4 blade prop to harness the power of the corncob.

Like most things it's all about compromise.....

So while 3 bladers are not ideal in the power and thrust department they may have other features that work well with one model or the other.

MinnFlyer

My Feedback: (4)

One other thing that BMathews left out is that when dealing with large, Full-Scale props, a 4-blade is WAY more efficient than it would be when scaled down to a size that we can use.

da Rock

GRIN.... As long as we'all are talking about Corsairs......

It wasn't until the F4U-4 model that the Corsair went to a 4 bladed prop. The original version and all versions up to that time had less powerful engines. With the F4U-4 the plan was to use a Twin Wasp R-2800-18W (are y'all writing this down ) and it would have overpowered the area they could build into a three bladed prop under wartime conditions. So "... to absorb the additional horsepower, a four blade Hamilton-Standard propeller was fitted."

And some more fun trivia about the Corsair....

It was the first production fighter to have plastic wingtips. And that, when it was still flying with fabric wing covering! It was the fastest fighter of it's period, and it had fabric covered wings!

And it had balsa in it's construction. (No wonder the Corsiar makes such good models.) End grain balsa was sandwiched between two sheets of aluminum to make the skin of the stabilizers.

BWooster

I thought the Corsair also went to 4 blades to keep the prop tips sub-sonic.

FWIIW

(Sorry, darock, I now see you already mentioned that)

One blade props: to avoid wash from the previous turn?

da Rock

That's partly true about the tip speed.

But when you start pushing a prop around with excess horsepower, the tip speed winds up deciding the prop length, more than deciding the number of blades. But they really work the design out considering a bunch of things that trade off against each other and it's probably more true to say that tip speed MOSTLY decides the prop length. Mostly the decision on the number of blades is decided after they've worked out the general idea of allowed length and figured out what chord (width) blades they can most easily/quickly/economically make. Everything is usually a trade off against everything else.

britbrat

The P-47D Jug could outclimb a Spit Mk IX, but it couldn't even get close to a Spit Mk XII, XIV, or XVIII. Nor could it fly as fast in level flight, nor dive as fast, nor climb as high. You can only do so much with 10 tons & 2,300 HP, regardless of prop.

J Hallock

My Feedback: (3)

So is there any truth to the statement that I hear so often....."3 blade props generate more thrust at lower RPM than a two blade"? I understand that the 2 bladed props are more efficient than 3, but aren’t the 3 blades essentially doing the same amount of work at an equivalent RPM as the 2?

britbrat

Your question is too open-ended because you are misunderstanding an essential point.

To begin with, we are discussing model applications -- so forget Corsairs, Thunderbolts, Spitfires & any other full-scale applications. As soon as variable pitch & constant speed enters the picture, it no longer fits with model discussions.

As generalizations go, diameter & HP are the most important factors in determining thrust (not speed). The only real reason for going to a multi-blade (3 for example) is that you may not be able deliver all of the available HP with a 2-blade. The performance-based reason that you wouldn't use a 2-blade, is because of diametral limitations. Without diametral constraints, the 2-blade will deliver more thrust for the available HP.

Within the diametral constraints imposed by airframe, ground clearance, or centrifugal force, to transmit more power the aspect ratio of a 2-blade prop will normally be reduced (wider blades) before a jump to multi-blades becomes necessary. At some point, as the blades are forced to get wider in order to transmit the available HP, the efficiency of the 2-blade prop falls sufficiently that the weight & drag penalties of the 3-blade are neutralized, because the 3-blade can utilize an efficient high-aspect ratio shape for the same blade area as the fat 2-blade. Beyond that point, for a given diameter, the 3-blade will have an advantage.

The same constraint also applies to a 3-blade. If you can't increase the diameter, you must make the blades wider. Eventually you will again reach a point where a 4-blade gains an advantage --- and so on.

britbrat

They still do use them.

For a given engine load, the pitch can be maximized with a single-blade prop.

britbrat

WWII fighters were compromises in all directions. They were an engineered balance of cost, available construction material, range, payload, speed, turning ability, climb & ceiling. Few, if any of those parameters, were optimized.

Until the advent of the Tu 95 Bear, the world's fastest prop-driven aircraft was the Macchi-Castoldi MC.72 floatplane. With a fixed-pitch 2-bladed prop, it hit 440.7 MPH at sea level, in 1933. I believe that it still holds the world speed record for reciprocating-engine-powered aircraft (floats & all), & still holds the sea-level record for any prop-driven aircraft.

HighPlains

My Feedback: (1)

I thing that only the ailerons were fabric covered. This was done to help prevent flutter.

da Rock

HighPlains,
The outer section of wing, from the fold joint out to where the new plastic tips went, was fabric covered from day one. The LE was aluminum back to the spar. Ailerons were also aluminum structures covered in fabric. and....

Profile Publications, The Chance Vought F4U-4 to F4U-7 Corsair (no 150)

So all through WWII, the Corsairs had fabric wings.

The usual reason for having a fabric covered surface was weight and ease of construction. Also, during wartime, fabric covering utilizes non-strategic materials and cheap, available, already trained labor that comes from outside the high-tech labor force.

da Rock

You know, all this theory is good stuff. But what this thread is about is our model airplanes. And from what we have available in props, we're pretty much stuck with. And testing the one or two available 3-bladers available against the 2-bladers available is going to fall VERY FAR short of ever proving efficiency theory. And theory isn't going to prove a thing, ever.

There is an excellent "signature" quote somebody around here has...... In theory, practice and theory are equal. In practice, they are not.


britbrat,
You're absolutely right. I should have said, "when the P-47 got it's 4, it could outclimb the Spitfires that were then operational". I got that idea from the memoirs of a P-47 pilot who wrote at some length of the mock battles they constantly had over England with the "local boys". He has a piece in his book about the way the Spits would always climb away when he got 'em cornered and how, after he got the 4-blade, HE would climb away. You know, even the longwinded posters like me often don't type enough words sometimes. This time, I have.

rmh

Models- -crank the engine as hard as you can -then try various 2 blade props -- 3 blades will never hit same thrust -static/unloaded
makes no difference
these little buggers have to churn like crazy to get any results
I don't theorize - I fly em and now with my "on board rpm save" feature ignitions - I can read actual unloaded rpm in flight -
The old prop fighters from 60 years ago did the same thing - they flew em and used what was available and in rare cases they got a chance to see what really could improve things .
That usually happened afer WW11-- some of the Trophy racers swapped props and found more speed - but mainly they did the classic model fix - get rid of the weight - and/or add a bigger engine.
All of the theory points to that as being the best fix- and actual practice bears it out.

J Hallock

My Feedback: (3)

Your right about the topic on hand in this thread. I appreciate all the physics and theroy behind the posts. Once again, the beauty of this hobby comes out. It's a match between pilot and his plane. What he likes what he doesn't. My 580 feels really good with the 2 blade just as I feel much more comfortabe with the 3 blade on my Magic. It may be a mental condition but when I am out at my field and have several flawless flights under by belt and a smile on my face..........that's all that matters.

cyclops2

The real small ARF --PBY's are a good place for 3 blades because efficient 2 blades do not clear everything.
Some scale 4 engine bombers force under sized props.
Cheat a little and spread the engines till the right props fit.
-
Super Marine Spitfires went to 5 blades for carrier landing clearence and then finally counter-rotating props because of carrier landings and their special needs.

Hibrass

My Feedback: (1)

Darock,

Your refering to Robert S. Johnson from the Martin Caidin book "Thunderbolt". Johnson was refer to the new Paddle blade prop not a conversion from a 3 blade to 4... All 47's from the C model up that went oversea started out equipped with a Curtiss Electric 4 blade narrow cord prop. By mid 43 Hamilton and Curtis started providing the "Paddle Blade" to improve performance. Also keep in mind that Johnson wasn't running around with a Jug producing stock horsepower. It is documented that his crew chief spent a great deal of time with Pratt techs and his and many other Jugs in the 56th were probable flying around making over 2800HP in Mid 43 and early 44...

Fun stuff!

Hibrass