Does anyone here understand prop aerodynamics?
 

The relative merits of two and three blade c/s props have been extensively discussed (I have tried to summarise here.) so I don?t want to get back into that except for one area, and that is the stopping effect on landing. Here in the UK, one of the justifications for the extra blade is the disking effect is much greater, and therefore the short field landing performance is enhanced. My own view is that since the ?pull? on takeoff for 2 and 3 blade systems is very similar, if the appropriate prop for the engine is used, the ?push? on landing also ought to be very similar with the engine trying to idle at the same speed, and the prop set full fine (like for takeoff).

Does anyone understand prop aerodynamics?

It's up to you.
 

I have always understood that the drag is dependent on the size of the propellor disk, not the number of blades churning around in it.

The landing distance required for a particular RV is going to be dependent on 'Pilot skill', then to a lesser extent, the headwind and whether it is long grass, short grass or tarmac.
I don't think the number of blades is really going to effect the equation; except the a CSU prop will have more drag the a fixed pitch.
Pete.

Somewhat
 

Steve,
All the really fast Formula one airplanes run two bladed props. I have a three bladed composite prop (fixed pitch) on my RV 6A and I can assure you that there is very little braking effect, probably also since it's highly pitched for max cruise.

I was finishing up another transition training student yesterday and he was amazed at the amount of float and float and float during a power-off, no flap landing. I saw little if any braking action.

On the flip side, a two-bladed prop "reaches" further out from the cowl and has more area for "bite" than the three blade. In our ag work, we found out long ago that in slow speed (85 MPH) turns the Agwagons with the three bladed props couldn't pull a wet noodle out of a chicken's rear!

They were incredibly smooth and cruise speed appeared the same or mostly so. The RVs have been shown to be faster with the two-blades though.
Regards

Props
 

At the same RPMs (big caveat) more blades get you better static thrust (better initial takeoff acceleration) but less net thrust as speed increases. So theoretically the 3 bladed prop could produce more deceleration drag as well when you chop power at high airspeed, but In don't see how it would help much on a slow speed approach for a short field landing.
Now if you had a prop with Beta capbability, a 3 blader would be better than 2 blades after touchdown...

The "drag" from a fixed pitch prop is more dependant on "disc area" than the number of blades. Since with a fixed pitch 3-blade the diameter is smaller, the drag is also less. If the diameter of a 3-blade is the same as a 2-blade, I suspect that the drag would be very similar.
When we compare props, there's a term often used, "all other things being equal." They seldom are!

Props and blades
 

There are many factors besides number of blades, but the general rule is the fewer blades you can get away, to absorb the horse power/speed range, the better. Think of it as a Bi-plane v. "Mono-plane". A Pitt?s special is pretty cool with its short span dual wings, but a mono plane has advantages. More blades on a prop, like more wings, is not always better. There is the sexy factor I know many builder succumb to with multi blade props, it just looks cool. Why would it not be great, all the cool planes like a P-51 have 3 or 4 blades. The difference is the P-51 is making up to 2000 HP. You don't need more than two blades to handle 150, 160, 180 hp or even 260 hp, two blades are plenty.

You NEED multi blades to absorb higher HP and high aircraft speeds to maintain proper or efficient parameters. More blades allows you to vary prop diameter and blade area. You can increase diameter but that increases tip speed. You can increase blade chord or width, but that has drag drawbacks. It is all an interplay of plane, engine and prop. One little factor affects another.

Blades are like wings but the big wrinkle is they are ROTATING and each station along the blade from root to tip is doing something different. There is real aerodynamics behind all of it of course, but it's a little art too. Also the material used is critical. Wood props like the MT (fiberglass covered) are thicker than metal props, which makes them less efficient, especially on a relatively fast plane like a RV. On an aerobatic or utility plane the extra drag is not as big of a deal. Same with the Sensenich fixed metal pitch prop, it is more efficient than other wood props because of the airfoil. Look at a supersonic jet, the wings are thin. Prop tips are transonic, i.e., near supersonic. The good part of wood is they are light and have natural vibration dampening. Metal props can't be home grown with out serious flight test and analysis for vibration and fatigue. However properly designed and installed metal props will last a long time with little or no maintenance.


Other parameters, chord, twist, thickness, cross section shape (airfoil), tip shape, plan profile all affect a prop and must match the number of blades and engine, i.e., RPM, HP and aircraft speed, to work well. It is many factors and compromises to get it all to work together. The reason the Hatzell BA prop works so well is it was conceived, designed and manufactured for RV's. Efficiency of a prop has to be tuned to work for that exact engine and airframe. Many props are generic and work well, but are off that 5% or 6% because they are not optimized for the RV.

If Sensenich or Hartzell wanted to make a three blade for the RV they would, but than weight and balance (CG) would be an issue. The good news is two blades are ideal for the RV from pure efficiency and performance stand point. The "Soft" criteria like looks, noise and ground clearance are all a matter of preference. The #1 aerodynamic rule is there is no free lunch. You will pay a price, not only monetary but performance, to get the three blades. You want to go fast use two blades unless you are near the 300 HP range. I know some of the rocket guys use the three blade, even though they loose 5-8 mph, which they can spare; they note smoother operation, which is worth it to them. I don't think the smoothness factor is as large on RV's with smaller engines. I do know the high tech composite props, all carbon, are so stiff they give up the smoothness factor of wood core / composite props. The wood MT is smooth but gives up a chunk of speed due to the blade thickness, which needs to be thick to account for the wood core. So you could go with a Hartzell three blade metal prop, giving up less performance, but it would weigh a bunch, too much for a RV.

For less than say 300 HP two blades are plenty. Three blades look cool, can potentially give more ground clearance (not necessarily) and potentially less noise (lower tip speed). However some three blade props have the same diameter as their two blade cousin. If you where going to make a three blade prop just for a RV it would use quite different blades than a two blade. The issue I have with three blade props is many times what they offer a RV is the same as what the offer for a Husky (high wing super Cub look a like). Again to get the most out of a prop you need to include the aircraft drag and performance. One prop does not fit all. That is where the Sensenich and Hartzell have it over the competition they not only have the aerodynamic knowledge they intentionally designed a prop just for the RV.

I would not personally choose a prop on how much drag it can produce for landing, but suppose that is a factor to consider for a bush plane.

Hi Steve,

Well I don?t know much about this but I do have back to back experience with changing from an MT 15 two blade to an MT 12 three blade and then back. I did this because MT loaned me the three blade when they had my two blade for an extended time period. This was on a 200+ HP -4, I went in to this with no expectations, Just a prop so I could go flying, the differences where obvious, the three blade accelerated noticeably faster on take off and had so much more drag on approach that it was very difficult to make a power off approach and have time to flair and land with out adding power, this following a very short and steep approach. The two blade definitely has plenty of acceleration and power off drag compared to a fixed prop but the three blade had plenty more of both, the two blade is about 5 mph faster in cruse at the same power setting, I like the two blade better but the three blade was fun also. I think this was a good test being two props of the same brand used on one air frame back to back with no other changes, mix planes and or prop brands or blade styles and you might have different results, don?t know.

What was the diameter of each prop?

Classical theory says that if each prop is designed to be optimized for the aircraft and engine, that the two blade prop will need more diameter than the three blade prop. The larger diameter of the two blade prop will mean more thrust at low speed. The two blade prop will have higher Mach number at the blade tips at cruising conditions, and this will lead to lower prop efficiency. I.e. classical theory says the two blade prop will perform better at low speed, and the three blade prop will perform better at high speed.

Your results are exactly the opposite of what the classical theory suggests. This is a great example of how theory can fall apart in a real-world application. In the real world, it is very rare for a prop to have its design optimized for a particular aircraft/engine combination. That costs too much money, so normally you need to pick from already existing props, and try to find the one that is the best match. I suspect that in this case the three blade prop had a blade twist that was optimized for a speed much lower than typical RV cruise speeds. This hurt it in cruise, but helped it at low speed.

Disking efficiency.
 

Kevin - I started this thread, and found Russ's real world experience interesting, as were your comments. The two MT props he mentioned were both 72".

My reason for starting the thread was to find out if the disking effect of a 2 and 3 blade should differ significantly on landing (more than the thrust differs on takeoff)?

For 160hp, my particular interest, MT recccomend their 11 at 72" (2 blade) and 12 at 71" (3 blade). I find it hard to see there would be much difference in theory, since presumably both are optimised and efficiency would not vary dramitically if the prop is pulling or pushing?

I would welcome your comments.
Thanks.