Tell me everything there is to know about propellers

I won’t go into the details here but I’m working on a model airplane, built from scratch, for a cargo (ie, weight) lifting competition.

I need to know everything there is to know about prop selection and prop engine matching; specifically pertaining to model aircraft.

Links to websites, titles of published papers or books would be great. Of course, any personal knowledge, experience or advice would be great too.

You might be more specific, like what kind of engine are you going to use? Internal combustion? 2-stroke? Glow? Diesel? 4-stroke? Electric? Direct or gear drive? Rubber? Most engine and motor manufacturers will have guidelines as to which prop is the best to use. With rubber it’s more a matter of tailoring the rubber and prop to the particular plane. I’m sure there are resources around but you have to throw out a few parameters first.

[Nosey bugger alert]
What kind of competition? What are the rules? Can you use a hot air balloon?

:smack: Sorry.

Its an OS 0.61FX engine…obviously internal combustion, and 2 stroke. Starts with a glow plug. Direct drive. Rubber? What do you mean by that?
I know manufacturers give suggested props, but those are geared towards aerobatics whereas I need a design geared towards heavy lifting. I’m looking for a generic answer. I don’t want a simple “use this” reply. I’m an engineer; I want to learn the engineering aspects behind choosing a prop. And keep in mind the goal is heavy lifting, so that’s what the prop choice should be geared towards.

“Rubber” refers to rubber powered airplane, were a rubber band is twisted (winded) to store the energy, and as it unwinds rotates the propeller to produce thrust. Heck, I´m just about to finish one of those, the only thing left to do is the propeller. :smiley:
As for your question, the engine manufacturers had charts with all the information you need. In this particular case what you´ll be looking for is for the lowest pitch and largest diameter; thus providing more low speed thrust.

Just what it sounds like. “Model airplane” is pretty generic. Since you wouldn’t give any parameters in the OP there was no way of telling what kind of engine you needed a prop for. Electric motors and rubber bands are all perfectly legitimate types of motors.

The info helps. I wouldn’t go too far from the props they reccomend for that engine. You may go a bit larger in pitch and diameter but not too far as you’ll just defeat your purpose if you bog the engine to below its power band. If you aren’t familiar with model engines I advise you find someone who is to help you break it in and get it running correctly. Learn to listen to the engine break from four cycling to two cycling as you lean the mixture. Since you seem pretty serious about this you might consider investing in an optical or audio tach to make sure the engine is at the peak of the power band with whatever prop you choose.

Okay, great, thanks…but I want an explanation behind this. Believe it or not, I’m not just a dumb kid :slight_smile: WHY does low pitch + large diameter = low speed + more thrust? I want to see the formulas. Not a simplified elementary school answer…I want graphs, charts, formuale, numbers…hard data. OR Papers or books that could provide such information.

This isn’t just a fun thing we’re doing…we need full design schematics showing a complete “engineering process” was used to build the thing. Everything we design needs to be quantified with formulas/research/data.

For heavy lifting low speeds are better, because as speed increases the induced drag raises quite fast; so you better have a slow plane.
A large low pitch propeller is better suited for that because of how a propeller works, it´s kind of tricky to explain without diagrams, so if you like I could upload some to my website tomorrow. Besides that would give me time to dig out the link for a great site dedicated to propeller design.

You’re an engineer and you’re asking us to do your homework for you?

Unfortunately I don’t think you’re going to find such detailed information for model engines and props. You’ll be lucky if you can find the HP and torque curves for your engine and I can’t remember seeing one of those since Peter G. F. Chinn wrote engine reviews in Model Airplane News. You’ll find lots of data for full size aircraft but you’ll find it doesn’t scale well, particularly props and motors. .

Besides that a prop doesn’t work in isolation. How it works on a static test bench is very different from how it flies. I assume you’re going to attach an airplane of some kind to this 10cc beast. Are you making a low speed kite with a big fat 20% Clark Y wing or for higher speed? Radio control, control line or free flight? I’m assuming nothing here. Are you a good pilot?

Is this page too elementary for your tastes?


This isn’t homework. I’m a member of an extra-curricular club called Aerodesign. We are competing in a University-level international model airplane heavy lifting competition that will be held later this year. The airplane is designed and built entirely from scratch (yes, that means everything. We have to build any and all moulds, and in our case, do all the composite laminating.) My University has no aeronautical engineering department. The best I/we can hope for is mech students who are focusing on aeronautics. In my case, I’m actually in electrical but happen to love airplanes. We do however have a graduating student (our captain) who is a mech. engineer with focus on aeronautical engineering and has been running the team for several years. However, I’m the only member who is also a member of the SDMB…ergo, my rather un-informed sounding posts :slight_smile:

We haven’t designed the rest of the plane yet. We are first trying to determine the max power we can extract from this engine. And, as you pointed out, we have been unable to locate any information on model airplane prop theory. I don’t expect “you” to do my homework…hence my request for papers or books on the subject.

amore ac studio: I haven’t read it thoroughly, but that looks to be precisely what I need.

So all that stuff on "The Flight of the Phoenix"where the model airplane guy re-designed the broken plane to fly them out of the desert was all just BS? Another heroic vision crushed!:frowning:

Trigonal Planar: It might help if you asked a little nicer. Your OP sounds more like a demand than a request, and your subsequent messages seem to have a little attitude to them as well.

You’re really not giving enough information to get good answers. The correct prop pitch really depends on the usage, the airframe its being attached to, etc.

I’ve never actually designed propellers, so I’m basically guessing at the rough parameters of the design. So take this with a grain of salt, but here’s the approach I’d take:

For heavy lift, you’ll want to design an airplane with a big, high lift wing, flying at a slow speed. Once you’ve calculated the best L/D for the airplane, you’ll know what speed you will be flying at. From that, you can size your propeller. The pitch of the prop on a model, if I recall correctly, is the measurement of how many inches the prop would move forward in a full revolution, neglecting slip. Off the top of my head, I think you’d want to look at the power curve for your engine, and find out the RPM for max power. Once you have the RPM you’ll be turning, and the speed you want to fly at, you can calculate the theoretical pitch you need. Then you have to calculate propeller size, and I’m thinking that’s going to be more trial-and-error. Basically, you want the biggest prop you can turn at the RPM you need. Go too big, and the engine will lug and not make rated power.

From that rough guide, you could do a lot of fine tuning. Perhaps you want to go to a slightly bigger prop and slightly less RPM and more pitch to get better prop efficiency. Maybe you want to go the other way. Then there will be slip, so maybe you want a slightly smaller prop with a coarser pitch to maintain best L/D at max power.

Perhaps the easiest way to do this, since model props are so cheap, would be to buy a range of propellers of various sizes and pitch according to the engine manufacturer’s recommendations, and fly them. Collect some data, draw some curves, and optimize your design.

A Scale Model HMMM… How about a 1:1 scale C-5.

Actually for heavy lift I would think the airframe and lifting and stabilization surfaces would need to be finalized before choosing a specific powerplant. I mean you proably already know it’s piston powered, but what power rating at what RPM? how many engines? Is the heavy lift for payload or just a heavy aircraft?

Well because of structural issues rather than aerodynamic. A well equipped airframe shop would have a hard time building a flyable Phoenix out of a C-119. You don’t just plug in an outer wing panel like it was a tinkertoy. I’m willing to give it some suspenstion of disbelief bcause it’s a decent adventure movie. The only downside IMO is that the flying mockup of the Phoenix was the death of Pilot Paul Mantz.

Trigonal Planar, I think people bristled because you seemed to say “I don’t care about reality, I want to know what the book says.” Fair enough. You’re taking an engineer’s approach. With sufficient parameters I’m sure you can determine what is the best prop for your purpose. Problem one, you have no parameters. Problem 2 you’ll still have to settle for a commercially available prop at the hobby shop. I’m sure you’re considering making your own prop but consider that one that comes apart at high RPM can be fatal. In the end I doubt if you’ll be much better off than if you had just followed Sam Stone’s and just experiment with some different props. IMO good engineering is determining if the effort is worth the benefit.

I’m assuming this will be an RC plane. Is anyone on your team an experienced pilot?

Did you all miss the point where he said everything must be made from scratch? He can’t go out and buy a propeller to use.

If that’s the case why isn’t he mining the bauxite and iron or to smelt the metal for his engine? These kids today and their store-bought engines. When I was a kid we had to travel to the Amazon to tap rubber trees for latex to make the rubber bands to hold the wings on. The radio is going to be a real bitch. :smiley:

He may have different requirements but in the modeling world “scratch built” means the airframe. No precut wood, no premade foam cores, no premade fiberglass components etc. Powerplants, control componetnts are purchased. Rubber band folks are the only ones who routinely make their own props. I’m certain someone could build a router fixture to correctly mill helical pitch propellor blades I really don’t think he’s going to go to that much effort.

You may want to check this site for aerodynamics study

By the way, you may want to consider adding a reduction gear/belt to the engine, that usually increases the eficiency.