Selecting the “right” prop

As promised in the selecting the "right" airfoil thread here is a corresponding write up on propeller selection. This is a subject I covered briefly in my physics tutorial (originally posted in 2007). Here I’ll dig a little bit deeper and provide more background.

Selecting the “right” propeller for a plane is probably the biggest factor in improving its performance. On the other hand, picking a “wrong” prop is best way to ensure you have a dog in the air.

Most R/C propeller manufactures have a chart or table matching the correct sized props to engines. APC, however, totally passes the selection off to the engine manufactures and does not make any recommendations for propeller use. Their disclaimer specifically refers to written instructions/recommendations that are provided with most motors and directs the consumer to contact the motor manufacture for suggestions.

Master Airscrew provides tables to “give the beginner a starting point for best performance”. The rest of MA’s disclaimer, “Modelers who have some experience develop a feel for the best size propeller for different model/engine combinations”, however, is pure BS. This statement is of course where I will be putting some actual engineering up against “developing a feeling”.

These tables are from Master Airscrew’s web site and I am only providing them here so RealFlight designers can go to one spot to find all the information.

Propellers for 2-Stroke Engines:
.049 to .051: 5.5x4, 5.5x4.5, 6x3, 6x3.5, 6x4
.09 to .10: 7x3, 7x4, 7x5, 7x6
.15: 7x6, 8x3, 8x4, 8x5, 8x6, 8x7
.20 to .25: 8x6, 8x7, 9x4, 9x5
.29 to .35: 9x6, 9x7, 9x8, 9.5x6, 10x4
.40: 9.5x6, 10x4, 10x5, 10x6, 10x7, 10x5, 10x6 10x8, 10x9
.45 to .50: 10x7, 10x8, 10x9, 11x4, 11x5
.60: 11x4, 11x5, 11x6, 11x7, 11x7.5, 11x6, 11x7, 11x7.5 11x8, 11x9, 11x10
.71 to .80: 12x4, 12x5, 12x6, 12x8, 13x5
1.08: 14x6, 14x8, 14x10, 15x6, 13x6, 13x8, 13x10, 14x8 15x8, 16x6
1.20: 14x8, 14x10, 15x8, 15x10, 16x6, 16x8, 16x10
1.5: 16x8, 16x10, 18x6, 18x8, 18x10
1.8: 18x8, 18x10, 20x6, 20x8
2.1: 20x6, 20x8, 20x10
2.7 to 3.5: 22x8, 22x10, 22x12, 24x8, 24x10, 24x12

Propellers for 4-Stroke Engines:
.20 to .25: 9x4, 9x5, 9x7, 9x8
.40: 11x5, 11x6, 11x7, 11x8, 12x4, 12x5, 12x6, 12x8
.60: 11x8, 11x9, 11x10, 12x5, 12x6, 12x8
.90: 12x8, 13x8, 14x6, 14x8, 14x10
1.20: 14x8, 14x10, 15x8, 15x10, 16x6, 16x8

You’ll notice there is not a lot of difference with the 4-Stroke table, the mid-sized engines just have a slightly increased maximum size due to the greater torque of these engines compared to a 2-Stroke of the same displacement.

Top Flite provides a chart (attached below), which lays out the same information visually. Spot checks show the two manufactures to be pretty close on any particular engine, so use which ever format feels the most comfortable to pick a prop range.

This prop range is important because all engines want to operate at a particular RPM where they can reach max power. Using too large a diameter and/or too high a pitch may cause the engine to not rev up to the best power band. With too small a diameter and/or pitch, the engine will over-rev and not deliver the best thrust.

So picking any prop within its posted range will work with your engine, but there is a very big omission here and that is what type of plane is the prop being installed on. The prop manufactures have all alluded to this omission in their disclaimers, but none specifically address the issue other than by generalities. The following is quoted directly from Master Airscrew’s website – “Often, heavy and slow airplanes use a large diameter and moderate pitch, while a fast plane will have a smaller diameter and a higher pitch. Hovering and lifting applications use an over-sized, low-pitch propeller.” Those of you who find that statement completely useful, in that it gives you everything that you need to know, may stop reading at this point.

Anyone else, who’d like a little more information in order to make a more informed prop decision please continue.

The differences within any prop range for an engine is best shown through an example. Let’s take two scale planes both with .51 engines. One plane is a scale bi-plane with full rigging and guy wires and the other plane is a low drag pattern aircraft with retracts. Even if by some wild stretch of luck these two planes weighed exactly the same, you would still not expect them to fly anywhere near alike. So why would you expect the same propeller to work for both airplanes? The answer is, of course, it won’t.

The way out of this dilemma is to use a ratio of diameter to pitch to help rate propellers to determine the type of aircraft to which it will be best suited. Therefore, for best aircraft performance use a prop that is first within the power range for your engine, but then match the prop ratio to the type of aircraft that the engine and its prop will go on. Use the following chart for a guideline:
Draggy/3D - 3:1
Trainer - 2:1
Sport - 1.5:1
Racing - 1:1

Going back to my example above, a .51 engine can use any prop ranging from 10x7 to 11x10. This range includes the following commercially available props (rather than repeating these in another table, I’ve also included the prop ratio in parenthesis):
10x7 (1.4:1)
10x8 (1.25:1)
11x4 (2.75:1)
11x5 (2.2:1)
11x6 (1.8:1)
11x7 (1.57:1)
11x8 (1.38:1)
11x10 (1.1:1)

The biplane can easily fall into both draggy and 3D descriptions, so by my table the prop closest to a 3:1 prop ratio would be best. For this engine displacement the 11x4 with the 2.75:1 ratio would give the best performance. For the pattern plane a 1:1 ratio is desirable. Here I would also take into account the type of engine. For a four-stroke .51 that can swing a larger prop I use the 11x10 at a 1.1:1 ratio, but for a two-stroke of the same displacement I go with the 10x8 at a 1.25:1 ratio for the higher RPM. Interestingly enough, these selections fit within Master Airscrew's generalities but I had a defined method to make the actual selection.

A few other tidbits for prop use:
The above dissertation focuses on two-bladed props, but if you want to use a three-bladed one the conversion is easy. To convert from 2 blades to 3 blades you want to decrease the total blade area and increase the blade angle of attack (or pitch) to overcome the increased drag of the third blade. The general rule is to DECREASE propeller diameter by 1-2”, and INCREASE by 1-2” the propeller pitch. It is, however, all right to keep the same pitch when going from 2 blades to 3. A three-blade is slightly less efficient, but can be useful if you need a bigger bite and do not have the ground clearance to use a larger diameter two-bladed one.

In real life, all R/C props should be balanced to minimize the vibrations which can quickly destroy a model airframe.
To Balance: Take a 1” piece of masking tape and place it on the tip of the light blade. Retest for balance and add or subtract tape as needed. The amount of masking tape on the blade will tell you how much material you will need to add or remove for final balance.
In most cases, if the weight of the tape is so slight it won’t show up on a gram scale (say 1” or less of tape) the prop is within spec and can be flown without adding or removing material.

To Add Material: For g/f nylon props, place modeling clay or silver solder in the holes in the back of the prop hub, on the side of the light blade, until it balances. For wood props, try adding paint or nail polish to the back of the light blade. Industrial enamel or nail polish can also be used on g/f nylon props.

To Remove Material: For g/f nylon and wood props, use sand paper to remove material from the heavy blade and bring into balance. For g/f nylon props, use a pocket knife to trim the edges of the heavy blade.

My personal preference here is to avoid the modeling clay & solder tip, while the manufacture says you can do it I've had the "stuff" fall out of my spare props sitting in the flight box. I'll usually add paint or nail polish to balance props (note that both change weight slightly as they dry). The exception being the grey APC props, where I'll shave/sand the leading edge of the heavy blade.

For those poor deranged souls, who don't get off on the smell of nitro at the crack of dawn - here is a spreadsheet that can aid in the prop selection for electrics.

Hey now.... Watch it!! For me its like dogs... I enjoy them if they belong to someone else but I do not want to clean up the mess or deal with the maintenance costs.

A good ball park calculator that I use (and many others too) can be found here:
http://adamone.rchomepage.com/calc_motor.htm

Its not the exact science that motocalc can produce but it gets you in the right area. There are lots of tricks that you can do with electrics that you simply cannot do with gas or nitro. I know folks who are just manic about scale RPM and could you ever do something like this with an IC engine? dropping a cell to give bigger diameter and higher pitch to make a more scale RPM can be a good thing if you are a good enough pilot. Its all trade-offs.

Joshua,
That's a nice tool. I couldn't find anything like it in 2007, which is why I put together my own.

If you go to the root site:
http://adamone.rchomepage.com/
there is also a CG and wing loading calculator. For others following this I'll not get into these as there are MANY different way to calculate these... especially if there are multiple wings. Some do it the simple way (me) because it gets you to the right ball park. You have to trim nearly any model so getting it exact on the first try seems a waste of time. I usually maiden my builds nose-heavy anyway for better in-flight control. Better nose-heavy than tail-heavy.

Off-topic over... lets get back on-topic