Does anyone here understand prop aerodynamics?

[Russ McCutcheon]

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

[Kevin Horton]

Originally Posted by Steve Sampson

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?

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.

[garnt.piper]

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!

[David-aviator]

Originally Posted by Russ McCutcheon

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.......

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

[rv6ejguy]

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.

[Steve Sampson]

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

[Beer30?]

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?

[Kevin Horton]

Originally Posted by Beer30?

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

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).