Propeller Sizes
 

I'm a little confused because different people say different things about prop diameter and pitch. I'd like to know the truth once and for all.

What is going to make my plane fly faster, diameter or pitch? What is going to give it more climbing ability/vertical, diameter or pitch?

Is there someone out there who knows what he/she is talking about?

RE: Propeller Sizes
 

For an example a 11x6 prop would be 11 inches long with a 6 inch pitch, The pitch is how far it would go in one revolution if 100% efficient so a higher pitch would go faster at a given rpm, The reason people talk about diameter is because when you increase pitch it puts more strain on the engine so you remove the strain from the higher pitch by using a smaller diameter. So if I had a 46 that used a 11x6 prop I could put a 10x7 to get more speed and a 12x5 for more climbing power. I hope this helps you understand how it all works a little better.

Dauntae

RE: Propeller Sizes
 

There are two things that must be considered. The propellor produces thrust. When the plane is standing still or moving slowly through the air, but the prop is turning at max speed, you get max thrust from the propellor. How much thrust is determined by the prop diameter, pitch, and the speed your engine is capable of turning that combination. Increasing either pitch or diameter will result in increased thrust at a given prop speed. Unfortunately, increasing pitch for a constant diameter increases prop loading, resulting in decreased max prop speed, and generally results in a decrease in thrust. On the other hand, increasing diameter increases the prop disk area as a square function, so increasing diameter, while increasing prop loading at the same pitch, will increase thrust more.

Pitch on the other hand will determine the maximum speed the prop can move through the air at a given rpm. For a plane with low drag, increasing pitch will increase speed.

What does this mean?

As in everything else, its a series of tradeoffs to get optimum performance.

If your plane is fast, but has poor vertical performance and long takeoff runs, decrease pitch and increase diameter (go one number on each). This will increase your static thrust, but you will sacrifice speed. If your plane has great vertical performance, but you want to go faster, decrease prop diameter and increase pitch (again one number on each). Your plane will fly faster in straight and level flight, but will not climb vertically as well. You will also take longer to reduce speed on landing approaches.

Want to hover? Get the largest diameter, lowest pitch prop that your engine will reasonably handle (12x4 is usually a good choice in the .46 range engines), or even the 12.25x3.75 if you can find them. Want to scream across the field at unreasonable rates? Get the 10x7 or 10x8 (again for the .46 range).

Get a tachometer. Measure your current prop rpm, then download ThrustHP. You can use the measured RPM with the prop pitch and diameter to estimate actual power output. Then play with different combinations to see what they do to max speed and thrust. Then start changing props and find the one that best matches your style of flying.

Hope this helps.
Brad

RE: Propeller Sizes
 

It can be very simply said:

GIVEN THE SAME RPM:
Larger diameter/less pitch = more pull, less top speed. Like low gear on a car.
Smaller diameter/more pitch = less pull, more top speed. Like high gear on a car.

Dr.1

RE: Propeller Sizes
 

Pitch makes you fly faster, diameter makes it pull harder.
Either will effect the load on the engine.
Ususaly if one goes up the other must come down or you will change the loading on the engine.
Too much load generates heat and doesn't let the engine get up to speed.

RE: Propeller Sizes
 

Superairborne, All the information you need is contained in your engine's owner manual. They will suggest a prop to you that will give you the performance you want. Propellers are always going to have trade-offs, as people have already explained to you. The same engine, in a different airplane might require a different propeller, etc. etc...

I'm a true believer in following what the manufacturer recommends. They spend a pile of time and money to bring an engine to market. They test their engines extensively and attempt to get the best performance from them they can. They have the most knowledgeable and skilled people there is in the field to do everything from design, engineer and test their engines. What they have to recommend (from my point of view) is the best advise your going to find. So, pick-up your engine's owners manual and read and follow what they recommend!

People have all kinds of opinions on these subjects. Some opinions are excellent, some are good and others can be detrimental to the health and longevity of your engine.

RE: Propeller Sizes
 

There are more than a few of us here who know what we are talking about -- don't be insulting if you want help. I suspect that your problem is that you cannot interpret what you are being told.

To begin with, your question is far too open -- "the truth once & for all" is quite meaningless, as there is no "truth once & for all" without bounding conditions. Propeller theory is complex & it is "theory", not necessarily fact for any given condition. Real-world propeller performance is affected by many variables and external influences. There are many possible answers for any given propeller scenario.

It is best to take it in small bites.

bkdavey, DR-1Driver & Geistware have made a good start. Mode One's advice is good, as far as it goes, but manufacturers' manuals don't begin to cover the huge range of operational situations. Tell us what you want to know & we will give you explanations & various examples.

RE: Propeller Sizes
 

There is a small program available for you computer that will show you what different size/pitch propellers will do in different cicumstances. As was said above, it figures in the perfect situation but it's a very good tool for determining what different props will do on your plane. You can find it at [link=http://www.bmaps.net/software/thrusthpv20d.zip]Thrust HP[/link]

Hope this helps

Ken

RE: Propeller Sizes
 

britbrat makes a good point.

What I said in my earlier post MUST be applied within given restrictions. Props slip and some blade shapes are more efficient than others. The prop must be correctly sized to the engine AND plane. For example, A 16-8 will fit the Zenoah G-23 on my 14#, 1/4 scale Fokker Dr.1, but will it fly the plane? Probably not, or not very well. It just doesn't move enough air to overcome the drag of the airframe. A 12-4 will fit the .40 on a pylon racer, but will it fly? Probably, but it will be slow and mushy. Phrases to be aware of are "engine loading", "engine over-revving", and "cavitation". All three are very important. The prop recommendations made by the manufacturer are not written in stone.

The "truth", as you put it, is to find the prop that flys the plane the way you want it to.

Dr.1

RE: Propeller Sizes
 

Does this comparison hold true for 3 or 4 blade props?

RE: Propeller Sizes
 

Multi-blade props follow the same basic principles -- increase pitch & decrease diameter for speed, or increase diameter & decrease pitch for acceleration/climb. However, multi-blade propellers have their own particular applications. They are generally used for scale appearance, or where aero-braking is wanted, or low noise is a desired outcome. A very significant application for multi-blade props is the case where physical space limitations constrain propeller diameter & a two-blade prop can't adequately transmit the available engine power within the diametral limitation.

There is usually a performance loss with multi-blade props relative to two-blade propellers, but sometimes the multi-blade prop is the only available solution to the problem.

I have a comment relevant to bkdavey's post. For mid-size engines (40 - 90 size) and 2-blade propellers, to retain approximately the same engine load, a 1" change in diameter is roughly equivalent to a 2" change in pitch. For example, a 10-6 is equivalent to an 11-4, or an 11-6 is equivalent to a 10-8. Propeller disc area increases as the square of the radius. Additionally, the greater wetted area and frontal area of the longer blade increases parasitic drag, whereas a pitch change only affects the blade angle of attack (lift & resulting drag). That is not to say that Brad is incorrect in his recommendation -- the variation in power load with a 1 - 1 change in pitch & diameter is not large, & a 1 - 1 change is a common starting tactic when experimenting with propellers.

RE: Propeller Sizes
 

Dear Britbrat

I know that there are thousands of people at RCU who know what they are talking about. I never insinuated otherwise, else I would not have asked my question here!

I never meant to be insulting. I have been receiving conflicting information, NOT necessarily from RCU members, but from people at the airfield. I wanted to confirm what was correct and what not, hence the statement "the truth once and for all!"

I have no problem interpreting what I am being told, I am an engineer myself. My question was not "far too open at all". I asked exactly what I wanted to know: "What is going to make my plane fly faster, diameter or pitch? What is going to give it more climbing ability/vertical, diameter or pitch? "

I know that there are usually many deciding factors and almost infinite variables concerning these things, but I was only looking for a general rule of thumb, which is what I got other RCU members.

Thank you to those who answered my question. Now I can start to experiment with props to suit my flying style.

RE: Propeller Sizes
 

I never took anything SuperAirborne said as insulting! Nobody else seemed to take what he said as such, either. At least no one else made reference to it.

Britbrat, you stated: "but manufacturers' manuals don't begin to cover the huge range of operational situations". Absolutely true! However, as an example; my O.S. .46AX's owner's manual suggests a propeller range of 10.5X6 to 12X7 for this engine. How far outside these parameters would you be willing to operate this engine, if it were YOURS? How far outside these parameters are you willing to suggest someone else operate THEIR engine? How far outside these parameters are you willing to suggest a rank beginner operate HIS engine?

RE: Propeller Sizes
 

"O.S. .46AX's owner's manual suggests a propeller range of 10.5X6 to 12X7"

Real life operating parameters often conflict with "manufacturer's recommendations". I don't know how manufacturers determine prop sizes, but I suspect it's kinda like auto gas mileage figures on the window sticker - optimum conditions, possibly even a specially set-up vehicle.

A 12 x 7 seems a little large for this engine, IMO, and I've seen .46 engines being run with 9 x 8s in speed planes. It all depends on the application.

Dr.1

RE: Propeller Sizes
 

One thing that should be mentioned or at least highlighted a bit......

The pitch ratings on the props should be taken as "advice", not fact. How well a prop produces it's "pitch" in flight is very often a flip of the coin. If you've ever spent much time with a prop pitch gauge and different brands of props, you know that individual props very often don't even have the advertised pitch. You might assume that the mfg in those cases is advertising the resulting pitch, what you should expect the prop to do in the air. But then you find another identically marked prop to have yet another different pitch on the blades. You also discover props with different pitch from blade to blade. And props with differing pitch up and down the blade.

So don't take the designations on the suckers to mean anything more than a suggestion.

And if you have ever balanced many props you also know that there are reasons those babies give you such different results in use. And don't expect the modern, machine produced props to solve those problems. I just ran into a batch of beautiful, carbon fiber, machine cast lovelies that all balance INSTANTLY at a perfect horizontal in the balancer. But wait!!! a balanced prop ought to hold every position it's placed in without moving???? so these that THUNK right into a horizontal orientation have a heavy side, a VERY heavy side right? so are out of balance right? and can't safely be balanced right? RIGHT!!!

The reason there are so many opinions at the flying field about props is because they're not really very accurate instruments. You can often find the same brand, same diameter, same pitch prop that gives different performance in the air than others of its brand, diameter, pitch. If you take 'em home and put 'em on the pitch guage, and check the airfoils, and check the thickness, width, planform etc, you might discover the why of it, but you might not. And that could be because for whatever reason, one is flexing under load differently than the rest....

So "the facts" about this deal aren't going to be facts chiseled in stone. These things may be called "airscrews" for a good reason.

RE: Propeller Sizes
 

Thanks, Mode One!

And thanks for your input Dr1Driver and darock!

I think I get the propeller picture now! Thank you all for contributing!

RE: Propeller Sizes
 

Good post, darock.

I've seen the pylon boys (who REALLY know props) balancing, carving, sanding, and making their props as perfect as possible. Even given that the manufacturer's pitch numbers are accurate, which they often aren't, pitch is THEORETICAL. It's the distance, in inches, the prop will move through SOLID MATTER in one revolution. Obviously, air is NOT solid, so there's slippage involved. The blade shape, airfoil configuration, and other parameters have a lot to do with just how much slippage there is. For example: The now OOP Rev-Up props were notoriously under-pitched, but were extremely efficient. They were GREAT props.

As has already been said, it's simply a matter of taking some basic facts and theories and applying them to your specific application. Experiment until you find the prop that flys your plane the best.

Dr.1

RE: Propeller Sizes
 

Well I guess that I got my ears pinned back, however ---

-- suggested to me that he was generally passing us off as BS artists --- & were there any "real" cognocenti out there? I certainly wasn't delighted with that choice of words -- but I guess that is just me. My appologies if I offended you.

Mode One --I take each case as it comes & I have no problem at all in stepping well outside the manufacturers' suggested range of propellers -- as long as the load & RPM range is acceptable.

RE: Propeller Sizes
 

The trouble with pitch gauges is they only tell the angle of the back surface of the blade which isn't the true pitch. True pitch is along the chord line which is near impossible to measure and even then you'd really need to know the (negative) angle of attack that gives zero lift before you can say what the true pitch is. Who knows, maybe some manufacturers mark their props with true pitch and others with what you'd find with a pitch gauge?

RE: Propeller Sizes
 

Dear Britbrat

My words "Is there someone out there who knows what he/she is talking about?" , were only an indication of my desparation to receive correct information. I was just tired of the conflicting info I had been getting from other people.

RCU has always been extremely helpful to me and my intention was not to pass you or anyone else off as BS artists! My apologies to you and others who took my words as insulting. No offence was intended!

Thank you for all the help, chaps!

RE: Propeller Sizes
 

SuperAirborne, you haven't mentioned what type of aircraft you are wanting to make fly faster , and indeed how much faster. Before you consider making drastic changes to the prop, find out just how much faster your model will safely handle. All airframes have a terminal spped, and if you over prop it , then the spectators will see an amazing site as your aircraft shreds itself to peices in midair, while you shudder at the waste of money becoming confette at an enormous speed that the airframe couldn't handle. So i suggest you do more research as to what maximun speed your model can handle.
Besides all that , welcome to the wonderful hobby of model flying
bill

RE: Propeller Sizes
 

It is unfortunate that we seemed to start off on the wrong foot. I'm happy to give you what help that I can.

RE: Propeller Sizes
 

Hi!
SuperAirborne...It isn't that easy to tell you what makes your/our planes go fast , you can help us by telling what airplane /engine you have in mind.
For instance in my F3D pylonracer powered by a 6.5cc twostroke engine,weighting 2.2kg a carbon prop ( hand made) that has a pitch of 7.5" and a diameter of 7.1/4" the engine turns this prop at around 30000rpm on the ground. Straight line speed is around 350-375km/h. Straight up speed the same or thereabout , climbing straight up for as long as ...well it disappears so fast that I have not dared to try it for any longer moments.

In my SIG CAP 231 EX (ARF aerobatic airplane ) weighting 5kg I use a MVVS 25cc two stroke that swings a 18x6" Menz prop. Top speed is around 150-175km/h and climbing performance is good but not as good as my F3D racer. The difference is the CAP can stand still in the air with its nose pointing sraight up and will accelerate straight up as far as I can see it when I give full power. Its slow accelerating capabilities are thus much better than the smaller racers.

My small electric aerobatic airplane (Icarus) at 150-160g, powered by a the smallest AXI outrunner motor is swinging a GWS 8x4.3 prop at around 6500-7000rpm and top speed is around 30km/h.
Climbing straight up is out of sight as the other airplanes but climbing speed is much slower. This airplane hovers at 1/2 throttle.

So saying that a certain pitch or certain diameter produces speed is not true..it's as always a combination of things.
So tell us more what airplane/engine you have and we could perhaps help you.

Regards!
Jan K
Sweden

RE: Propeller Sizes
 

Jan, SuperAirborne didn't have specific questions on getting the maximum speed from his airplane.

This is what he asked: "What is going to make my plane fly faster, diameter or pitch? What is going to give it more climbing ability/vertical, diameter or pitch"?

If the Madgenius had only read the whole thread, the topic wouldn't have "Morfed" into "Wow, Man! Like how do I get-like, MAXIMUM WARPAGE from my plane, Man"! (This was meant tung in cheek)!

RE: Propeller Sizes
 

Jaka... of course there are a lot of variables that influence speed and vertical ability. All I wanted to know was what would increase speed and what would improve vertical ability given that all other parameters remain constant.

Thanks