Question about prop characteristics
 

In my Box-O-Fun (flight/charger tool box), I've got an extra prop and I spent a few minutes studying it and comparing it to the prop already on the plane. This lead to a couple of questions that I've got running around in my head.

The only plane that Doc ever owned was the Senior Falcon with a .61 engine (the plane I have now) so I know BOTH of these props were used on the same plane and the same engine. Here are the specs for the props (both are wooden):
Prop #1 (currently mounted on the plane): "Top Flite", "Super M", 11.8". The tips on this prop are rounded
Prop #2 (I know it's been mounted b/c it's got a few minor scuffs on the prop bolt position): "Rev-Up", "Pro Series 4", 9.6". This one has "clipped" flat ends on the blades (almost squared off)

My question is this: What are the different flight characteristics for each of these 2 VERY different props? One is 2" longer and has rounded tips while the other is shorter and has squared off "clipped" tips. If they're both flying on the same plane and the same engine, how would each one change the plane's behavior in the air? Is there a quick "sticky" on different types of props, materials, angles of attack, how the shape will affect the climb and turn, in general -> where can I find out more about these magical devices that make our wonderful flying machines either "fly" or "fall"?

RE: Question about prop characteristics
 

I've been TRYING to understand most of the following thread in which some members of RCU which seem to know a lot about aerodynamics discuss different prop myths:

http://www.rcuniverse.com/forum/m_51...tm.htm#5124751

The basics you need to know for regular sport flying are the characteristics that you'll gain when changing either pitch or diameter of your prop. The greater the diameter, the greater distance that the prop will tend to pull the plane with each turn. High pitch props tend to give more speed whilst lower pitch tends to give better acceleration.

RE: Question about prop characteristics
 

The 9x6 is TOO SMALL for a 61.

11x8 or 12x6 suits most 61s.

RE: Question about prop characteristics
 

Spacemonkey,

Let me add a couple of things to what has been said.

First the 11 stands for the diameter of your prop and the 8 is the pitch. The same thing about the 9X6, it has a 9" diameter and a 6" pitch.

In theory if you have an 8" pitch for each revolution the prop moves the airplane forward 8". A larger propeller requires more power to turn it so a lower pitch prop maybe required to bring the engine rpm into range. A smaller diameter will turn up faster and will require more pitch or bite to control the rpm of the engine.

What propeller you select has to do with a number of considerations. What is the rpm range of the airplane's engine. How much ground clearance do you have to try to avoid prop ground strikes? What type of performance do you want out of the airplane.

Here are some thoughts to keep in mind. If you want takeoff and climb performance a flatter pitch is going to rev-up more quickly getting you off the ground faster, but be slower flying once airborne. The opposite is true with a higher pitch its going to take more bite out of the air each revolution moving the airplane forward faster but have slower climb and takeoff performance. In model aviation most airplanes have more than adequate power so even with a higher pitch prop the performance is most often good.

Its a good idea to reference your engines specifications and use a tachometer to see what the engine is turning at full throttle. Its desirable to be somewhere close to the top end of the engines rpm range for best performance, but not over the rating. Its not a good idea to constantly overspeed your engine, its always a good idea to use power management by reducing power in dives often to idle depending upon the maneuver.

Thats the gist of it! Sometimes it gets to be a balancing act to come up with the right prop to give you the exact performance your looking for.:)

RE: Question about prop characteristics
 

Props are air foils, there's different series of the same brand you can buy.
It also depends on what type of airframe you will be mounting a prop on.
Obviously you would prefer a larger diameter prop for a model such as a YAK or Corsair becuase of the bulky cowling.

A prop also acts as a torque converter or wieght that an engine require to run or rotate.
It is recommend that you put suficient load on an engine, to prevent it from operating beyound it's deisigned operating rmp therfore prevent damages to the engine.

Different engine will have dfferent peak power band. Peak RMP dosn't necessary mean peak power. Once peak HP threashold have been achieved, the HP
will drop if the rmp continue to climb. In other words..you'll just be making prop noise and not peak power at peak rpm.

RE: Question about prop characteristics
 

To expand on what others have said, the propeller is your airplane's transmission. By selecting a propeller, it's very much like selecting a gear on a stick shift car.

Each propeller is described by 2 numbers. In your case, it was 11x8. That means that the prop is 11" in diameter, and in a perfect world, each revolution will carry the prop forwards 8 inches. Increasing either number increases the load the engine sees (dragging the RPM down), and decreasing either number decreases the load the engine sees (allowing the RPMs' to climb).

A propeller with a large diameter and a small pitch will act like 1st gear in your car. It'll accelerate well, go uphill well, but top speed will be quite low. Conversely, one with a small diameter and a large pitch will act like 5th gear. Acceleration for take off will be very slow (if it goes at all), but once in the air, the speed will be quite impressive. It's climb performance will also suffer.

When we choose a prop for a plane, it's seldom done just by the numbers, tho they're important. An engine manufacturer will give a recommended prop size for starting out with an engine. For a .61, something like a 12x6 or 11x8 might be suggested, but that's by no means the only size we can run. The manufacturer is simply giving us an idea of the sort of load the engine will require. We can move around these numbers a bit to achieve what we want from our plane.

If we want to fly 3d aerobatics, then we want excellent thrust and vertical performance, but high speed is not desirable. We might find the 12x6 to provide too much speed and inadequate vertical. So, to get more thrust, we'd move up to a 13" propeller. In order to keep the engine's speed up, we'll have to lower the pitch. Each inch of diameter is worth 1 to 2 inches of pitch, so a 13x4 or 13x5 might be good things to try.

If it was a speed plane we were setting up, we may find that while our vertical and take-off perfomance is great, our top speed may be disappointing with the 12x6. So, we might try an 11x8, or a 10x9 or something like that.

When swapping props about, its often a good idea to use a tach to verify that the engine's turning RPMs within a reasonable range. For most middle size 2c glow engines (say .40 - .91), between 10,000 and 13,000 is a good rule of thumb. The larger ones will be on the slower end of that range, and the smaller ones tend to want to turn faster.

That 9x6 may well have been run, but I sincerely doubt it was on a .61. If the .61 had started at all (the small prop is not much of a flywheel, and so the engine may have refused to run), it would have grossly overspeeded, and probably come apart in short order. A 9x6 is appropriate for something in the .25-.35 range.

Hope this helps.

J

RE: Question about prop characteristics
 

Thanks for all the help on this, guys! That makes alot of sense: stick to the powerband where the proper amount of torque is applied in order to achieve your desired result. Too small of a prop with not enough overall mass could over-rev the engine and basically turn fuel into noise without achieving any significant results. Too big of a prop and your engine can't swing it around fast enough to give you any thrust. I was curious about the small " - " between the size of the prop and the other number. I assumed it was in tenths of inches instead of the more obvious "pitch" identifier (I gotta start thinking about things in terms of "aerospace engineering" instead of "industrial engineering"!!!)

It sounds like the smart money is on checking out the engine mfg specs and it's effective operating range and listening to what they say. Use that spec as a "baseline" for experimenting (within reason....) to get different results. Ex: What would be the effect of keeping an 11" prop on a .61 engine but increasing the pitch to spin slower but bite harder? or decreasing the pitch to cause it to spin faster but bite less? and how both situations would compare against the baseline spec. Instinctively, I'd bet money that they pay a couple of aerospace engineers quite a bit of money to crunch the numbers that would give the consumer the best "bang for the buck" in prop specs. Economically, if you supply the consumer non-optimal prop specs, your engine "appears" to run poorly and you get a bad rap for building and selling junk engines instead of just screwing up on the prop specs ;)

I can't help it.... I have an engineering degree from Ga Tech...... I like to monkey around with stuff :D

RE: Question about prop characteristics
 

Hi!
And to make things even more easy for you...APC plastic prop is one of the best props on the market. Much better than old Top-Flite rounded 11x8 props and Zingers
There are others equally good props , like the British RAM series of plastic props.

RE: Question about prop characteristics
 

It looks like these guys have already cleared it up for you, but here is a page on my website that explains the characteristics of different size propellers.

Most of which has already be explained on this thread.



[link=http://www.hooked-on-rc-airplanes.com/model-airplane-propellers.html]Model Airplane Propellers[/link]

RE: Question about prop characteristics
 

Thanks a MILLION, wings!!!!!!! That document is GREAT !!!! I've converted it into a Word file for future reference in my "field notebook"

==> Ever since I embarked on this hobby/adventure I've been keeping a notebook of documents, questions, and topics that I've run across that are of particular interest or importance; stuff like course corrections using aileron vs rudder, pre-flight checks, trimming and evaluating CS's, crosswind take-off & landing techniques, basic flight tips, how to perform specific flight maneuvers, etc.... Some of it is from constructed cut/paste documents (like the one I created for this subject), others are downloaded PDF's with manufacturer's specs on engines/servos/Tx/Rx, etc... while others are the printed threads from this board. Everyone on this board has been incredibly generous in sharing what you know and your experiences with the new guys like me. I'm very, very grateful to everyone for all of your help. I've already learned enough to know that there is ALOT that I don't know ;) (the 1st step is to SEE the elephant in the room....)

The notebook is getting pretty thick :D but it's the kind of stuff that I know I'll want to reference in the field>especially the sections on stick inputs, preflight checks, and troubleshooting! I'm hoping I won't have to look up the data in the "repair" sections too often :eek:

Thanks again!

((actually wings, your whole site is fantastic in terms of the info you've provided and how it's layed out!!! I hope you don't mind me printing hardcopies of some of it and keeping it in my field notebook ;) You've really put alot of work and thought into the site and it shows; it's basic enough to not feel "overwhelming" to us new folks, and it doesn't suffer from the "RC Flying for Dummies" feeling. Keep up the good work! I know there are plenty of people like me who can really benefit from a site like yours to get our foundation level "meat and taters" 411 !!! [sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif]))

RE: Question about prop characteristics
 

I don't mind at all. That's what it's there for! [8D]

I truly appreciate the positive feed back. It kinda makes all the work really worth it when I see that people are truly benifitting from it!

If you have any more questions or recommendations let me know.