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.

Russ McCutcheon said:

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.

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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.

Yes both where 72" but there blade design where very difrent to my lay eye.

Steve Sampson said:

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?

Click to expand...

Steve - I don't think there is a simple answer to your question. The two props you are considering only differ by one inch of diameter. If both props were optimized for the application, I would expect a bit more difference in the diameters. Just based on the prop diameters and numbers of blades, it is plausible that the three blade prop might have a bit more drag at idle, as there is more blade area swinging around, and the size of the prop disks is almost the same. But, there are other big variables that could significantly affect the result - blade twist and low pitch stop setting.

If performance, cost and weight are driving the decision, I would be looking at a two bladed prop. Other priorities will lead to other choices.

prop theory

Hi, another factor in prop design is the solidity ratio, i.e. the area of the blades within the disc area. Posters here have alluded to changes in drag proportional to disc area, but what we should actually be looking at is the actual blade area. If a 2b and a 3b were designed using classical theory for the same design condition, and had the same diameter, then the 2b would have fatter blades than the three blader, to absorb the same power. I think, due to construction limitations and standardisation, many 3b props use 2b blades but on a 3b hub. This results in more blade area (greater solidity ratio), thus the extra drag at idle. The 3b prop will run at a lower AoA at any given power setting. In climb (low speed, high power) the loading on a prop disc moves inwards towards the hub. This is an inefficient area, but 3 blades working inefficiently at a lower AoA is better than two, so more low speed thrust will result.

I did my BE thesis on light aircraft prop design, some time ago. I found that theory and practical reality don't correlate very well! As stated earlier, very few props are designed for a specific condition and a specific aircraft. Thus rues of thumb and empirical design procedures, with experience, usually work better than a theoretically correct design. Hope this helps!

Russ McCutcheon said:

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.......

Click to expand...

Very interesting information on 2 blade vrs 3 blade MT prop, Russ.

I can confirm your observation re the 3 blade - great for take off, not so great for cruise, and definitely a speed brake.

I have the MT-7, 72" three blade electric. It is a huge drag device when at idle power and fine pitch. In fact, from altitude one can descend at 3000 fpm without gaining airspeed. On final, very seldom is the throttle at idle unless it is an unusually steep approach and then a little power is needed going into the flare, just to make sure it will flare before running out of speed. It definitely will slow the airplane after flare at idle power, it never floats. That is with 70-75 knot approach speed.

Take off is robust, and sooner than later.

Last year, launching out of wet grass strip with full aft stick and while fiddling with the throttle so as not to overspeed the engine due to the slow response of the electric pitch change motor, I found myself flying, literally hanging on the prop, as the machine came off the ground much sooner than expected. A gentle relaxation of back pressure resolved the dilemma without plopping back into the soft turf, glad to say, and the airplane accelerated quickly into a normal climb attitude.

I, too, do not understand all I know about prop dynamics. The MT 3 blade is great at low speeds but falls off as speed increases. Some days it feels like you're up against a cement wall. You can be cruising at 8 gph, increase power to 10 gph and TAS reluctantly increases a few knots. On the return from OSH this year, reducing rpm to 2100 from 2300 resulted in a couple knots INCREASE in speed and .4 gph LESS burn. (?) I did not believe it, so reset power and did it again. It is true.

This business in interesting. We never have all the answers.

David Domeier
Troy, MO
RV-7A
Subby H6

It is interesting in the MT prop world that several people with different aircraft and engines have confirmed through flight testing what the MT engineers told me when I was picking a prop. Top speed and high speed cruise with most of their designs are usually achieved at rpms well below 2700 despite the engine probably making less hp there.

Since cruise was my most important zone on my RV10 Subaru, we chose a different blade profile (3 blade) and a 2550 rpm governor/ controller (electric). By changing the reduction drive ratio from 1.93 to 2.04, torque at the prop shaft was boosted slightly and MT said the lower rpm would result in about a 3% gain in prop efficiency.

The price tag was ah gulp but the thing is a work of art. I hope it works as well as it looks but I had no other alternative available for this engine at the time.

Thanks for the input

Just a note to say thanks for all the input to everyone who tried to further my knowledge. Cheers!

Climb performance?

This is a great thread, very educational to me anyway. I have a somewhat related question. Does anyone have a rough idea of what kind of climb performance differential you'd see with a fixed pitch vs constant speed prop. I'm building the -7, and am interested in the tradeoffs between the two. Obviously a CS prop will weigh more, which would have an effect on climb performance, but it should climb better than the fixed pitch all other things being equal. Anyone have any comparisons. I'll be flying out of CO, so density altitude becomes an issue in the summer.

Thanks much,

Beer30?

Beer30? said:

Does anyone have a rough idea of what kind of climb performance differential you'd see with a fixed pitch vs constant speed prop.

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You mention Colorado, so I'll assume 7,000 ft altitude. You don't mention the engine, so I'll assume an O-360.

With a constant speed prop, at 2700 rpm at 7000 ft, on a standard day, I would expect about about 145 hp from the engine. With a fixed pitch prop, I'm guessing a climb rpm of 2300, which would give about 132 hp from the engine (numbers from Lycoming's power charts, assuming 22.2 inches of MP). I'll assume prop efficiency of 80%, for lack of any better number.

The difference in rate of climb (ft/min) = (33,000/weight) x ((hp1 x prop_eff1) - (hp2 x prop_eff2)).

With a weight of 1400 lb, as an example, the difference in rate of climb would be about (33,000/1400) x ((145 x 0.8) - (132 x 0.8)) = 245 ft/mn.

For small differences in weight, if everything else remains the same, the rate of climb would be inversely proportional to the weight. E.g. if the fixed pitch prop was 30 lb lighter, and the rate of climb at 1400 lb was 1000 ft/mn (I pulled this number out of my butt, as I have no idea what it would be at 7000 ft with a fixed pitch prop), the rate of climb at 1370 lb would be = 1000 x 1400 / 1370 = 1022 ft/mn.

So, we can say that a change from a FP prop to a CS prop, would give an improvement in climb performance at 7,000 ft on the order of 245 ft/mn at 1400 lb. If we assume a 30 lb weight increase, we lose about 22 ft/mn there, for a net increase of around 220 ft/mn (in round numbers).