so whats the deal with these triple prop (3 fins) things? Do they make a diff in performance versus the standard probs with just 2 fins? does it create more thrust? Would you recommend it on a trainer?

In general 3 bladed props are less efficient than 3 bladed props. How much less is a matter of debate. Since many of our planes are overpowered, this slight loss of efficiency is often meaningless. When you switch from a 2 blade to a 3 blade, the rule of thumb is to reduce the diameter by 1 inch. The only uses I can see for them is to provide more ground clearance or cosmetics (some planes may look more "scale" with a 3 blade prop).

Like piper said...less power...but thats generally not an issue.

I cringe every time I see a Corsair with a 2-Blade on it...

Everything piper said is correct but I can think of two other things:

1) A 3-Blade prop is quieter
2) A 3-blade prop gives a better braking effect


There have been some very long and good debates here on RCU over the years as to which one is best, 2-blades versus 3-blades. Just do a search, grab your favorite beverage and start reading.

As Piper mentioned it's also a matter of preference and in the case of competitive flying a matter of need. In competition, having a plane that is quiet does effect your overall score so 3-bladed props and specialized exhaust systems, also known as canisters, are the norm.

Quiet planes helps all of us retain our fields as noise is probably the most common cause of fields being shutdown. Another thing is the bigger the diameter of the 3-blade prop the less efficency is lost which means there is more time for the air to "recover" (meaning less turbulent air) between the blades

HUH? Assuming no difference in RPM and both are 3-blade, the TIME (or frequency) between blades remains unchanged.
In addition, the blade tips of the larger props will be traveling faster than the smaller prop which will cause more noise.

Bruce,

I can prove the difference which I just did on my Yak.

Engine and exhaust setup is:

DA50
KS Canister
MTW Header

2-blade Mezjlik 22X8 92Db at 7200rpm
3-blade Mezjlik 21X11.5 90Db at 7050rpm

The rpm figures are max, not richened for flight

Now here is the same engine prop combo's with a Slimline Pitts muffler
2-blade Mezjlik 22X8 104Db at 6900rpm
3-blade Mezjlik 21X11.5 100Db at 6750rpm

In the case of the Pitts muffler I would not have made any friends at my field due to noise. In competition, sound is scored but not measured so it boils down to a judges "ear". I just flew this Yak in a simulated round this past Monday evening at a judging school and outscored everyone in the noise department. I also found in my flying tests that the 3-blade gave worse downline braking and made me redo how I land this plane do to the braking effect difference. I saw no difference in any other area of flight performance between the 2 blade and 3 blade

I could be way off base on everything else, but it seems to me that noise it better with a 3-blade because of the slower rpm, I believe the blade style has everything to do with it. I also did a test for noise on a NX 22X8 wood prop and the Mejlik 22X8 CF prop and the NX was way quieter. We did not take measurements but it was definitely noticable. This engine could rip the 2-blade Mezjlik easily but could not rip the NX at all. It also cannot rip the 3-blade Mezjlik at all.

]
Have a look at these comparisons by Gene from Troybuilt. They are at the bottom of the page

http://www.troybuiltmodels.com/Propeller.htm

Bubba, you are correct WRT the noise & tip velocity, but your comment about "recovery" isn't correct.

There is a popular myth regarding multi-blade props that their relative loss of efficiency comes from travelling in the wake turbulence of the preceeding blade -- it doesn't happen, other than occasionally with steep-pitched props in a static situation -- but not in flight.

If you have a 3-blade prop with the same dia as a 2-blade, & turning at the same RPM -- it won't be quieter unless it is specifically designed for noise suppression, but it will make a different sound -- a higher frequency sound.

I stand corrected, thanks

Different blade shapes also make entirely different noises. This is one of the reason full scale prop mfg spend millions on prop research
http://www.ramaircraft.com/Aircraft-...s-Hartzell.htm

The navy has also spent millions(billions) developing quieter props for our submarines.

so you guys suggest reg 2 blade props

Getting back to Mchwindchimes original question -- if it turns your crank to have one, by all means do so. On a trainer it doesn't matter -- enjoy yourself. However, it will cost you in various ways.

Greater weight

Greater cost

Poorer performance than a suitable comparitive 2-blade.

For the same prop manufacturer, same construction materials & same blade geometry as the appropriate 2-blade, your engine will make the same power, but it will produce less usefull work with the 3-blade prop..

Yep

Yup

One other "issue" with 3 blade props is the selection of spinners is quite limited.

i just thought the 3 blades give you more thrust or something, but i guess not

They can, but they also may not . like most things in life -- it depends.

There are several reasons for buying a 3B prop.

1) It's cool & you like it.

2) Scale appearance

3) Noise suppression -- some clubs & competitions have stringent noise limits

4) Aero-braking -- this is the common application to trainers

5) Geometric limitations prevent the use of a 2-blade prop that can handle the full output of the engine -- eg. insufficient ground clearance, insufficient airframe clearance.


The use of multi-blade props incurs performance penalties -- such a loss of performance is the price for meeting the other relevant criteria.

Regarding efficiency --- staying with the same manufacturer, the same material of construction, the same prop blade geometry (aspect ratio, shape, etc), a 3 blade prop for the same power load will always be less efficient that the 2 blade -- it isn't debatable -- it is simple fact & physics. There is more mass, more wetted area, more frontal area, one more root-transition zone, and for a prop of the same pitch -- less diameter. All of which simply means more parasitic power losses from drag, plus the loss of disc area in the case of smaller diameter. The engine makes the same HP (same load), but it just produces less usefull work.

However, as soon as you start mixing & matching the prop performance factors noted above, you can have some 3B props that actually "outperform" some 2B props -- but you are comparing apples to oranges in those cases & this is where much of the confusion & endless debate arrises.

The more efficient the 2B prop design, the smaller the performance differences will be between the particular manufacturer's 2B & 3B props. Conversely, the poorer a given manufacturer's 2B's perform, the greater will be the disparity in performance (poorer) of the equivalent 3B.


Guidelines for 2B - 3B conversion are simply that -- guidelines. In the prop size-ranges that are appropriate for 40 - 90 engines, for a particular prop manufacturer, using the same material of construction & with props of the same blade geometry (shape, aspect ratio, etc) --- the approximate equivalency is: going from 2B to 3B, reduce the diameter 1" for the same pitch, -- or, reduce the pitch 2" for the same diameter.

In the case of a typical BB 46 engine, an 11-5 2B, 11-6 2B, or 10-7 2B is about right for general flying. All of those props will let the engine pull acceptable RPM's & generate reasonable thrust. Staying with the same prop manufacturer etc, an appropriate 3B would be a 10-5, 10-6, or 9-7. If you could find them, you could also use an 11-3 3B, or an 11-4 3B, but airspeed may begin to suffer unacceptably.

If you mix manufacturers, materials, blade shapes & aspect ratios, it becomes very difficult to use anything other than suck-it-and-see testing to determine which 3B prop is equivalent to your favorite 2B, or vice-versa. The guidelines become usefull only for ball-park guessing.

Just curious.....how do you balance a three blade prop?

Same as a 2 blade, get it to the point where it doesn't always stop at the same position. I balance 3 blade boat props fairly often.