Yes Master Airscrew. I was under medication at the time of writing.... It's wearing off now.
Thanks for bringing me back.

PS. maybe it hasn't completly worn off... it's the MA classic series....you know the ones that look like they belong in a museum... the scale looking one's.
I've been flying all day... in minus 11 deg conditions. Yep, it numbs things!

Thanks for the info Bla Bla and to others for your positive comments and related expereinces! I'm finishing an engine test stand that will measure engine temp., RPM, thrust (in pounds of force) and fuel consumption. It's going to be interesting to see how various prop, fuel and engine combinations stack-up in getting closer to a more scale-like RPM and sound. We'll never get the same sound and RPMs as the scale jobs, but in my book, the farther away from sounding like a nest of hornets the better.

I kind of like the sound of a high speed revs of a 2 stroke engine. Sounds odd on a scale job though.

When at full throttle with 4 blades and maintaining 5.5k rpm, your idle will be non-existant. The motor will NOT idle! This has been a problem since the beginning of time. We know that with the increase of blades at the hi rpm's put out by the engines, efficiency greatly decreases. For a 4 blade prop to be effecient, the rpm's need to be below 5.5k as you are aware. Needless to say nothing single cylindered except the RCV's will run like this. Even the RCV's are less effecient with 4 blades but the result is closer to scale and what you are after. Good luck in your testing!

That is completly untrue. The optimum speed is determined by the tip speed, a multiblade prop has the same tip speed at the same RPM and diameter as a two bladed prop. The efficiency does go down, but not enough to be noticeable. People assume this because there are not enough multiblade prop sizes to match their best two bladed prop performance. On high speed planes multiblade props can actually improve performance. Some WW II fighters went from three blades to four for this reason. But, this is not a factor on our models.

That is completly untrue. The optimum speed is determined by the tip speed, a multiblade prop has the same tip speed at the same RPM and diameter as a two bladed prop.

Nah dude, the prop speed only stays the same if you are at the same RPMs after switching from a 2 blade to a four blade. The 3 and four blades are less effecient than the 2 blades because when the 1st blade or a 4 blade propeller passes a certain point, then the 2nd blade passes right after it, it's riding right in the 1st blades wake and doesn't scoop up as much air as if it were a 2 blade where the blades a 180 degrees apart.

Sorry scone, try again. Of course if you add extra blades it will change the loading of the engine, but if you add more power to bring the prop up to the same RPM the tip speed is the same. However there are more tips with their resulting losses.

Your explaination is completly wrong. For the simple fact that the next blade never passes through the wake of the second blade, even when the plane is stationary. The air is moving through the prop and backwards, thus the wake is behind the prop when the next blade reach's the same position.

Hugh,

This is very true.

The assumption that more blades create airflow patterns that interfere with each other, created the counter-weighted, single blade, control-line speed props.

Even with migratory birds, this is untrue and the airflow from the the bird flying diagonally in front of another, actually aids the rear bird.

Some WW2 planes, with powerful engines resorted to 5 blades (Supermarine Spitfire XII, Hawker Tempest and Fury).
Many modern turboprop planes, the C-130K among them, have very efficient 5-12 blade props.

And nobody said anything about the "inefficiency" of the multi-blade configuration. APC and other manufacturers have the ability to make efficient props, with as many blades as they want.

Actually the single blade is more efficient because there is only one tip, not because of the wake. But not so much that you see single blade CL speed planes anymore. The last few I have seen had two blade props.

But... on real aircraft rpm is lower than on our glow motors and also you have to account for variable pitch props on the real deals. The blade causes a vortex and on multi blade props the next blade enters the vortex thereby not pulling much air. A lower rpm allows the air to recover and also by varying the pitch creates a shorter term vortex.
We normally go down in diameter for every blade we add to counteract some of the above but still loose efficiency!

Deputydog,
RPM is not that important, airspeed is. The tip airspeed of full scale is similar if not a bit higher than on our models. The second blade does not pass through the first blade's vortex at all. The desturbed air is behind the prop before the next blade gets to the same position. The air does not recover until well behind the aircraft. In fact the disturbed air is the cause of P-Factor , a major reason for adding right rudder during take off. This effect is the same if not stronger on full scale aircraft.

Many of you are repeating what I call modeling myth. Some of them are also in full scale aviation such as the down wind effect causing the plane to stall when turning base or final.

Loss of efficiency of multiblade props is due to the added drag of the tips and the prop to hub intersection. There may be other reasons, but passing through the wake or votex is not one of them.

Did you know that one of the reasons turbins can have so many blades is because the tip losses have been eliminated (or almost) by the duct? They are right next to each other yet they never pass through the wake or vortex of the adjacent blade.

This is true again.

But despite the shrouding provided by the duct, many modern turbofan engines have fewer, wide chord fan blades, replacing more numerous, narrow chord blades.
Examples are the Rolls Royce Trent and 535 series, GE 90, IAE V2500 and others.

They all achieve specific fuel consumption levels unsurpassed by any of the narrow blade engines.

AHhhh..........but props and turbines are 2 different concepts whereas props bite the air and turbines compress the air. This is why props are more efficient at sea level and turbines are more efficient at altitude. And I thought P factor was torque. Oh yea, I remember... P factor is that first big drop on a roller coaster . Removal of blade tips prevents them from going supersonic thereby getting more rpm. This doesnt mean you can snip you blade tips(unless you put it in a shroud!). The new blade designs used on the highly efficient turboprops(pushers!) are the ultimate way to go. Our subs use the same design.

Samuel,

To get the terms correctly, a turbine is spun by a moving medium. It compresses nothing, except for raising the pressure of the medium that has not yet passed through it (Pre-turbine pressure is always higher than post turbine pressure, or there would be no flow of medium.).

The fan stage in a turbofan engine "bites" air, like a model ducted fan, or a propeller would.

In a turbine engine, it is the compressor that compresses. The turbine only spins that compressor (and the fan, in a turbofan and the prop, or the rotor(s) in a turboprop, or in a turboshaft).

I was emphasizing the fact that even turbofan engine manufacturers, have resorted to fewer blades, to achieve greater efficiency. This is probably due to the fact that the wide chord blades operate in a higher Reynold's number environment.

I should have said turbofan. However the turbine blades are not that much differant. Differant blade airfoil and pitch, and differant materail to take the extra heat. Same concept as a fan or prop in reverse.

Also turnines are not more efficient at altitude. The efficiency actually drops at altitude. The comperssor stage is made so that if given full power it is able to over compress with a very lean mixture, that is much more air than an ideal mixture. A piston engine would knock and overheat with such a lean mixture. As the altitude increase more jet fuel can be added to keep the power at or near the same level. So they are burning more fuel. However the plane is moving in thin air so that it is moving faster which overcompensates for the higer fuel flow. Thus turbine powered planes get better fuel milage at altitude, not because the engines are more efficient but because they slip through the air better. High performance supercharged piston engine aircraft have the same gain at altitude. This was first used to great effect to increase the range of piston powered B-29 aircraft to bomb Japan from distant islands. Jets just get better benifit by being able to climb faster and higher.

BTW another reason many full scale aircraft add more prop blades is to get a better bite in thin air. In fact I think that is the reason the C130 went to six blades. Better performance at altitude.

Hugh,

Jet engines really do become less efficient at partial power settings.

At its T/O power setting, a modern turbofan uses about 0.3 pounds of fuel, for every pound of thrust, per hour.
If it is making a maximum of 20,000 lbs static thrust, it will use 6,000 lbs of fuel per hour doing it.

At a cruise setting, at 36,000 ft, making about 25% of its rated T/O thrust, at 0.8 Mach, it will be using about 0.55 pounds of fuel, for every pound of thrust, per hour. At 5,000 lbs thrust, it will use 2,750 lbs of fuel, per hour.
At this altitude, the thin air allows this 25% power cruise setting to give the plane a very high cruise speed.
Often it is over 90% of the plane's maximum speed.

This would seem the engine is a little more than half as efficient at cruise power, compared to T/O power, but actually it is different, because the cruise thrust is rated is at cruise speed, it is not static.
A turbofan is part jet, part ducted fan. Its thrust is reduced as speed is increased, somewhat like a propeller's.
Its output is rated in thrust, when actually, in part, it is has "horsepower" characteristics.

1 want to respond to the overpropping issue.

i put a three-bladed MA 16/10 on a saito GK 80 ran it on coolpower 20%; Type F plug. Ran surprisingly well; full throttle gave about 7 k as I recall; idle was good, around 2k; high speed needle had to be set rich. Definitely had a fatter exhaust note & thrust was noticeable reduced compared to the usual MA 13/8 I would run on it. No obvious ill effects after about 6 10 minute flights.

Hi!
Aren't you looking for scale size props for your Hercules..?? Isn't that you are after ?! Scale rpm is not realisticin our model world and at the same time have good performance.
On my Marutaka DC-3 (Scale 1:13,7) that I'm building I plane to use two OS .26 FS engines.
I know that these engines turn a MA scimitar 10x5 prop at 10000 using 5% nitro and therefore I think is possible to use 3-bladed scale props for this bird. I have been planning of making these props of carbofiber with a diameter of 9,8" and a pitch of about 5". I hope that these props will pull as good as the MA Scimitar 10x5 twobladed ones have done in my other airplanes ...only time will tell.......the first mold of a single blade was done today and it looked good. Now I only have to make two more blades and put these togheter with the centre section and made a molding of the entire prop...phuuu!
Regards!
Jan K
Sweden

Please share your prop making method. Do you make an impression of another's prop blade?

So 130,
Have you made a final decision on what you are going to use?

I also have questions on my bipe. I fly with a Saito 150. and am a bit overpowered. I was thinking that a 3 bladed prop, being less effecient, would tone back the power a bit, If I use the same size prop(16x8) three blade.

If muti-blade props are actually less effecient is there a reason to use them other than they look cool, and or scale for some projects?

RC Jones,
They are also quiter, the pattern flyers have used them to meet their noise requirements. That is about the only advantage for our use.

RC, I have run a MA three blade 16x8 with one half inch cut off the tips, it turned in the 9,200 rpm range. My Saito 150 is an early high compression one. The high compression has never been a problem, but it commands respect. It will actually run on 5% nitro very well. But, I run it on 15% mostly.

I'd like to thank everyone for their input, especially those who provided their real-world experience. Currently I am experiencing some issues with foam cutting the wing cores for the c-130. So, progress on the test stand and engine testing is on a momentary hold for a week or so. It's been a little interesting to sit back and read differences in opinions on the propping question! I think there are a few people who misunderstood the original intent of the propping question, so let me clarify some points.

1. Using a four bladed propeller to look more realistic is a goal only if thrust can be maintained at an acceptable level.
2. Lowering the engine RPM is a goal in as much as I fly (take-off too) my scale ships at around half throttle (T-28, P-51, P-47, etc.) and I see much wasted potential at higher (unused) RPMs that do little for the desired scale speed - high speed may be great for P-51s but does not do much for transport planes. Ya, I know, I may be out of the most desirable power band but I don't have time to stroke and re-time the engines
3. An associated goal of lowering RPMs is to lower prop speed (noise) and to lower the exhaust note and perceived frequency (again noise).
4. Available multi-blade (3-4 or more) props on our model airplanes will lower efficiency, however, speed is generally most affected by this efficiency loss and thrust to a lessor degree.
5. There never was a free lunch. So the inevitable inefficiency, power loss, heat build-up, transition issues and idle problems are all balanced against noise gains (desired sound) and desired RPMs. Yes, there is only so far you can go!
6. No, this thing never will sound like a full scale C-130 - I hope that fact is just too obvious. I'm around them enough and live in the approach path of the only C-130 air base that routinely serves both the North and South poles (last I checked). So I have a good idea what they sound like.

The simple answer to all of this is larger engines (52FS not 30FS) that can turn the less efficient but larger four bladed props at a more desirable RPM (no, I didn't say cruise at 2,200 RPM). Of course one of the trade-offs here is weight and fuel consumption - no free lunch. "More realistic" does not mean "exactly as", but closer than what you have now!

Thanks again to all who contributed and please keep the info coming!

Excellent information! I am in similiar decision making process for a Jet Hangar C-130 (133").....will be using as an Air Tanker (1 Gallon Slurry Drop).....weigh in around 50 LB's. Have 4 Saito 180's (too much power).....seeking info on an appropriate 4 blade prop for an this size engine (16 x10 4 Blade...APC or Graupner?).

Thank you

Bob
a.k.a. C-130 Junkie!

(Gallery Has Pic of C-130 Build in Process)

Bob, check out zingerpropellers.com, they have a chart that will get you in the ballpark and have some awsome fourblades that are not unduly expensive, I ust ordered a three blade 12x7w and it was 21 bucks.