Sport_Pilot

Britbrat,
From the Bolly site.
http://www.bolly.com.au/book/content/22.htm

The reason multi blade propellers often appear inefficient is the need to use considerable lower diameter propellers (in comparison to 2 blades), for the same horsepower available. Diameter for diameter a well designed 4 blade prop will in some circumstances perform better than the equivalent 2 blade propeller.



From McCaully full scale propellers.
http://www.mccauley.textron.com/prop/propframese.html

Propeller diameters are a function of engine and airframe limitations. Larger propeller diameters are preferred for low airspeed operation, while smaller diameters are best for high airspeeds.

britbrat

---and your point is?

Sport_Pilot

That they say a smaller diameter is better for high speed. That multi blade props are no less effecient than two blade props. Read some full scale sites and you will find a lot of modeling myth's debunked.

britbrat

I don't need to -- I've been flying full scale for 50 years, both props & jets --- and I managed a fluid dynamics engineering lab for nearly 40 years. I understand this stuff very well.


Originally you were discussing thrust as the driving component in increasing speed. You said that a multi-blade prop of the same pitch (as a 2-blade) would give more thrust --- which may be correct -- unless the engine RPMs fall far enough that the reduction in power negates the thrust from the increased prop blade area. However, unless you increase the propeller RPM, it will not make the plane go faster -- & you can't increase the RPM without adding power. Essentially you are saying that more power makes things go faster -- correct, but more thrust does not necessarily make things go faster.

You also said that sometimes a smaller diameter multi-blade prop with LESS pitch will increase thust at the same effective speed. The thrust part is possibe, but unlikely because of the diametral reduction. However the "same effective speed" part will not occur without substantially more power, thus increasing prop RPMs to the point where the discharge velocity equals that of the steeper pitch 2-blade. Just changing the prop in that particular example will cost speed. A smaller diameter multi-blade prop of the SAME pitch may produce equal thrust as the 2-blade (if it is a very good one), but again it won't make the plane go faster unless more power is added & the RPM's increased --- because it has the same pitch (discharge velocity).

You seem to equate thrust with speed -- there is a link, to be sure, but it is not necessarily thrust that determines speed, it is both thrust & delta-V. In cases where the airframe is limited by propeller discharge velocity (engine RPM limits), increasing delta-V will increase speed -- even with less thrust. It these cases is actually relatively easy to increase speed by increasing delta-V (smaller diameter steeper pitch prop). For example, a nice clean model with a fixed amount of power, changing from a larger diameter flatter-pitch prop, to a smaller diameter steeper-pitch prop (one that keeps the engine load the same), will make the plane go faster -- but the THRUST may actually be less than it was with the larger diameter flatter pitch prop.

You also seem to equate multi-blade props with greater speed, as compared with simple 2-blade props. The number of blades has nothing to do with speed per se, but it has very much to do with the ability to transmit power. In the sense that more blades can transmit more power you are correct, but simply adding more blades without increasing power will not increase speed. More power generally (but not always) means more speed --- that is the link -- not the number of prop blades.

I'll say it one last time -- more thrust doesn't mean more speed.

This exchange seems to be turning unfriendly -- I don't want that. I will not discuss this further.

Sport_Pilot

You are putting words in my mouth, I did not say many of the things you claim.

The argument was that full scale goes to multi-blade props for ground clearance. While this is sometimes true, it is usually in an effort to go faster, especially at altitude. That is why many small GA planes go to three blade props and why many racers go to them as well.

The delta V of the props is related to thrust. If you put a very course prop on a plane the delta V will be very close to 0 at the max speed, that is because the speed is limited by the prop and any delta V will be equivalent to the drag of the airplane. Thrust is equal to delta V time area. Put a smaller diameter prop on the plane and Delta V speed will be higher at the max speed of the former prop, and thrust will be higher at that speed despite the smaller area. That is true because thrust is always equal to the drag + acceleration.

britbrat

This is my last post on this topic

Go back & read post # 14 & see what you said.


Propeller thrust (F)= delta P*A, where delta P is the pressure differential across the propeller blade(s), & A is the area of the prop disc.

Figure it out yourself.

Sport_Pilot

Delta P * area is correct, but it is the area and pressure is acting on the prop blade. The propellor disk are * delta velocity will give you the same result. Though I may have the areas reversed.

britbrat

Damn -- why can't I leave this alone. From my distant memories, the final simplified propeller thrust equation is F = 0.5 * r * A * [Ve ^2 - V0 ^2] , where r is air density, V0 is the aircraft velocity & Ve is the propeller discharge velocity. Within that equation are the pressure relationships from Bernoulli's equation.

I was getting annoyed with you, but I gave myself a kick in the ***** ---- this is supposed to be fun & informative. I applologize for an overly aggressive tone.

Just one more thing to deal with -- in 2004, the FAI control-line speed champion was a Spaniard, Luis Parramon, flying a model with a SINGLE-blade propeller and the counterweight buried in the spinner. His winning run was 302.5 kph. You can see a very nice picture of the fastest North American entry (286.0 kph) with its single-blade propeller, on the cover of the July 2004 issue of MAAC Magazine.

britbrat

Damn -- why can't I leave this alone? From my distant memories, the final simplified propeller thrust equation is F = 0.5 * r * A * [Ve ^2 - V0 ^2] , where r is air density, V0 is the aircraft velocity & Ve is the propeller discharge velocity. Within that equation are the pressure relationships from Bernoulli's equation.

I was getting annoyed with you, but I gave myself a kick in the ***** ---- this is supposed to be fun & informative. I appologize for an overly aggressive tone.

Just one more thing to deal with. Getting back to your posts #14 & 19, wrt single-blade propellers -- in 2004, the FAI control-line speed champion was a Spaniard, Luis Parramon, flying a model with a SINGLE-blade propeller and the counterweight buried in the spinner. His winning run was 302.5 kph. You can see a very nice picture of the fastest North American entry (286.0 kph) with its single-blade propeller, on the cover of the July 2004 issue of MAAC Magazine.

Sport_Pilot

Britbrat,
You can also convert the propellor area and speed to come up with mass of air pushed by the prop. The mass of air and the delta velocity will also equal the thrust. Sorry if I am not communicating well.

Doing a bit of research, the advantage of multiblade propellors is mostly at altitude with constant speed props. So more than two blades is probably a disadvantage for speed. With controlline overcoming the drag of the control line is also a factor so a larger diameter single blade prop would help with the thrust to overcome that. They have abandoned single blade props in the racers, the ones that race each other, but that may be a factor or replaceing props and pitting problems. Though I do recall seing some of those half wing planes in MA with two blade props.

britbrat

I'm aware of that (I'm beginning to remember this stuff). The simplified thrust equation is unable to deal with variables such as tip losses & shear losses, which substantially complicate the calculation of propeller thrust. In reality, there are empirical correlations to thrust calcs for each particular propeller configuration. These are tabled & provide "block" corrections for design purposes that are close enough for workable engineering & manufacturing solutions.


Full scale propeller configurations are only loosly applicable to models, with the simple fixed-pitch types being the only ones in the model ballpark. Even they can be misleading, unless they are the old wooden prop types. Modern full-scale fixed pitch propellers do not have anything like the root-transition losses that model props have. This is why generalizations based on full scale propeller experience are often very misleading in discussing model prop performance.

Are we wearing this out yet?

dasintex

My Feedback: (10)

I have enjoyed this thread; lots of in depth info; different opinions all valid; I agree and have read enough to convince me that perhaps 4 blade props are less effecient; I use a 3 blade prop and can say it performs without any problem, I have not run a 2 blade prop on the same engine and plane to make a comparison; I too am interested in running a 4 blade prop on a new scale project using the same engine, I realize that the 4 blade prop may be less effecient, but it will fly the plane right?, maybe not as fast or may have poor verticle, I don't know, I love to fly scale so high performance is no interest to me, where scale is; it may work out that the diameter of the 4 blade prop may be too small and not close to scale for this new plane; that I may run a 3 blade or 2 blade; but the main question regardless of the effectivness of a 4 blade prop is that it will still fly an airplane, the pattern guys use them without any complaints. My 2 cents worth.

britbrat

You'll have to suck it and see. There is a fair chance that for a scale diameter 4-blade, you may have to reduce the pitch too much for decent flight performance -- but maybe not -- depends on the engine. If you have stuffed lots of motor into it, things may work out reasonably well.

MustangFan

The Cool Factor is High
The aesthetics factor is high

4 blades may not be as efficient as 2 ... but ...

If I'm flying a Warbird around in circles and doing limited aerobatics ... what really does it matter ... I'm not in a Dog Fight !

Most of us overpower our models ... Sooooooo ... put on that cool 4 blade and have fun.

saramos

Those were the reasons for my selecting an RCV .90SP engine for my TopFlite Spitfire. Being able to turn a 15.5 x 12 4 blade prop with an engine that can be hidden inside the cowl of a Spit was too much of a draw for me to resist. I ran my RCV .90 on the test stand with an 18 x 12 2 blade and it drew quite a crowd. I can't wait for the day I complete my build and start it up with that big 4 blade out at the field and watching all the heads turn.

The Cool Factor will be High indeed![8D]

Scott

britbrat

Talk about ***** envy!! [8D] My TF Spit screams around with a Tower 75. Goes fast, but hasn't a shred of cool. [:'(]

Red B.

I don't want to get into the heat of the battle but simply point a couple of things out:

The official (FAI) seaplane speed record for piston engined aircraft was set in 1933 by an Italian Macci Castoldi MC.72 (picture below). The speed was 709 km/h or 440 mph!
The Supermarine S-6b was certainly fast at 655 km/h or 407 mph but the record, which has not yet been beaten, still belongs to the Italians

The aircraft used a 3000 bhp Fiat engine and two contra-rotating two blade propellers.
If there had been anything to gain by using more than two blades on this aircraft I am convinced that such a propeller would have been used. Four blade props were not uncommon in the thirties.

One blade propellers are certainly the most efficent but not "much more efficient" as somebode wrote in a previous posting. The performance gain compared to a 2-blade propeller is less than 10%. Going from a two- to three-blade propeller would incure a efficiency penalty of approximately 5%. In the picture below the one-bladed propeller of the power assisted full size glider Silent 2 is shown.

/Red B.

britbrat

You are right Red. However, if I remember correctly, the Macci set its record after the Supermarine had previously established its own record & permanently captured the Schnider Trophy. Both were record breakers.

The contra-rotating prop was an effective but complex fix to the serious problem of torque reaction with these planes. The Supermarine S4 was heavily damaged in a "water loop" during take-off.

The salient point is that 2-blade props were used.

Red B.

Britbrat wrote: Yes that's correct. In 1931 J.N. Boothman captured the Schneider Throphy for the UK permanently by winning in a Supermarine S.6B. The winning speed was 547.633 km/h.

/Red B.