[thilokind]

Hi everyone,

I was planning to put a fix pitched prop on my RV 8A, but I just came accross a MT prop, 4 blade, adjustable pitch (electric). The prop is 174 cm / 68'' and 43 lbs. Since it is electric adjustable pitch I won't need a governour. Questions:
- is the diameter and the weight okay for an RV 8A (battery is in the front)?
- is a 4 blade prop with an IO 360 (180 HP) okay on an RV 8A?

Thanks

 

[Norman CYYJ]

It will not be as efficient as a two bladed prop.

 

[pierre smith]

Not necessarily.

The late Paul Lipps put a 4 blade fixed pitch prop of his own design, on a Reno racing biplane and it went faster than with any previous prop.

Best,

 

[FORANE]

43 lbs.
- is the diameter and the weight okay for an RV 8A (battery is in the front)?
- is a 4 blade prop with an IO 360 (180 HP) okay on an RV 8A?

Thanks

Don't forget to consider in your weight and balance calculation you must add about 3 lbs to the 43 lb figure for the spinner.

I have the MTV 7 (3 blade) on another experimental (non-rv) with o-320. When I installed it on that plane I had to add a battery behind the baggage area to compensate for balance. I have been generally happy with the prop. Have had a few electrical failures with it resulting in a fixed pitch where it happened to be at time of failure. The older MTV models have 2 power wires soldered to the very front of part number 223 (motor) of the diagram here:
http://www.mt-propeller.com/pdf/datsheet/mtv-10.pdf
The newer models have threaded screws and nutplates there and is a stronger design.

Check your ground clearance and consider the type of strip you intend to operate from. I only have 8 inches ground clearance on my plane.

Consider spinner diameter; mine increased an inch when I went to the MTV.

Consider spinner to cowling gap; mine increased about 3/4 inch when I went to the MTV.

 

[Snowflake]

The late Paul Lipps put a 4 blade fixed pitch prop of his own design, on a Reno racing biplane and it went faster than with any previous prop.

That just means he went from a bunch of sub-optimal two-bladed props to a purpose-built 4-bladed prop tailored for a specific application. If a two-bladed prop were built tailored for the specific application, it would be more efficient.

You cannae change the laws of physics...

 

[humptybump]

I'm sure there is a way to make it work. That said, a 43 lbs propeller will complicate you CG. A 2-blade Catto with nickel edges weighs 12.5 lbs.

You said your battery is forward. You might want to do some calculations with moving your battery aft. Alternately, an LiFePO4 could save you 10-12 lbs.

 

[RVjim]

4 blade MT propeller

Hi everyone,

I was planning to put a fix pitched prop on my RV 8A, but I just came accross a MT prop, 4 blade, adjustable pitch (electric). The prop is 174 cm / 68'' and 43 lbs. Since it is electric adjustable pitch I won't need a governour. Questions:
- is the diameter and the weight okay for an RV 8A (battery is in the front)?
- is a 4 blade prop with an IO 360 (180 HP) okay on an RV 8A?

Thanks

If you provide the full propeller number, I can tell you the aircraft that uses the MT Propeller.

MT Propeller has always refused to design a 4 blade propeller for the Lyc. 360 engine on a RV aircraft.

What you are very likely to have is a MTV-10-B/174-08, or -12, propeller. This propeller was designed for the Robin DR300 aircraft certified in Europe. The MT Propellers designed for aircraft in Europe are designed for minimum noise.

Jim Ayers
MT Propeller distributor

 

[SvingenB]

That just means he went from a bunch of sub-optimal two-bladed props to a purpose-built 4-bladed prop tailored for a specific application. If a two-bladed prop were built tailored for the specific application, it would be more efficient.

You cannae change the laws of physics...

We have a 4 blade on our Pawnee (235 HP) used for glider towing. Lots of power, cool sound

I have never really read or seen that two blades are more efficient than 3 or more on a propeller in a general sense from an engineering point of view. What IS more efficient is to accelerate a large amount of air a little bit, than accelerating a small amount of air a whole lot. Based on that principle a large propeller is more efficient than a small propeller given the same HP (two blades and large diameter is better than 5 blades and small diameter). But that's only one part of the story. When taking into account other factors such as relative tip speed, cord thickness, friction, mach number etc. you cannot any longer say that two blades are more efficient. It is more efficient for some applications, but unefficient - even unusable for others. The laws of physics and/or practicalities makes it unusable.

I guess the Pawnee would in fact be slightly more efficient with two or three blades due to the low operational speeds, but it would also make much more noise, and maybe even go supersonic at the tip, reducing efficiency. Maybe three blades are optimal and two blades are only good for low RPM cruise? Maybe 4 blades is the optimal for glider towing? We want best efficiency at high RPM, at high power.

 

[RVjim]

The 2 blade myth again?

That just means he went from a bunch of sub-optimal two-bladed props to a purpose-built 4-bladed prop tailored for a specific application. If a two-bladed prop were built tailored for the specific application, it would be more efficient.

You cannae change the laws of physics...

There is a performance chart that uses the power coefficient and J factor to determine the number of blades required for maximum efficiency. This is the basis for not using a 4 blade propeller on a Lyc. 360 engine, and for using a 4 blade propeller on a Lyc. 540.

This calculation for the Laws of Physics supports the 2 blade propeller as being the most efficient for low horsepower and low altitude.

Higher horsepower and higher altitude equals more blades.

The 2 blade propeller is great for a 65 hp. Luscome.
It is also great for the RV series at 500 MSL.

I have found that the 3 blade propeller is more efficient for the RV series aircraft at about 7,500'.

I have a 4 blade propeller on my modified Harmon Rocket 2 with a Lyc. IO-540 engine.

The 4 blade propeller is 4 knots faster than the 3 blade propeller at 12,500'.
The 4 blade propeller is 2 knots slower than the 3 blade propeller at 4,000'.
Just as the propeller efficiency chart predicts.

Jim Ayers

 

[Kahuna]

I have a 4 blade propeller on my modified Harmon Rocket 2 with a Lyc. IO-540 engine.

The 4 blade propeller is 4 knots faster than the 3 blade propeller at 12,500'.
The 4 blade propeller is 2 knots slower than the 3 blade propeller at 4,000'.
Just as the propeller efficiency chart predicts.

Jim Ayers

I notice Jim that you compare the 4 and 3 blade to each other but you do not compare the performance against a 2 blade at any altitude or at all. You mention in your message title 'the 2 blade myth again' but there is no 2 blade data here against the 3 or 4 blade. Perhaps you could give us those performance numbers 2 blade against the 3 & 4 blade too?

 

[SHIPCHIEF]

I wish Paul Lipps were still with us. He had a great way of going directly to the meaningful data.
I recall he was in favor of 3 and 4 blade props with certain provisions, like less prop diameter. Tip drag is a wasteful power consumer, and torque is directly effected by 'length of arm' = blade length.
So he had a formula that shows the cross over point for drag on 2, 3, 4 blade props. He also worked to reduce tip vortex losses, so a conventional diameter, broad tip blade, 4 blade prop might give fantastic static thrust, but the benefit might diminish to a negative as speed increases?
Would be a fairly expensive experiment if it doesn't work out.

 

[SvingenB]

Test report 2 vs 4 blade

Test report of 2 blade metal vs 4 blade Hoffman wood/comp prop on the Pawnee 235 HP 540 (MS Word file):
http://www.milehighgliding.com/common/test_report.doc

CONCLUSION
In almost every respect, the Hoffmann four-blade propeller is superior to the OEM McCauley metal propeller, especially for a glider tow operation. Much less noise, equal climb, faster descent, better ground and hangar door clearance. The Hoffmann can be supplied new with a one month order lead time. All glider operators should have them

 

[RV8RIVETER]

I wish Paul Lipps were still with us. He had a great way of going directly to the meaningful data.
\.

I can't agree more, the experimental community lost a bright mind and helpful fellow. Here are 2 old threads with comments by Paul about the blade number argument. There is no magic blade number.

http://www.vansairforce.com/community/showthread.php?t=457&highlight=elippse+blade

http://www.vansairforce.com/community/showthread.php?t=69971&highlight=elippse+blade

 

[rocketbob]

I used to talk to Paul often as we got to be pretty good friends. Miss that dude. This was a subject we talked about. There is a significant drag penalty for blade roots both in cooling drag and drag of the prop itself. Traditionally this is why adding more blades reduced efficiency. However, with Paul's prop designs, this was not the case. More blades meant that the tip diameter can be reduced thus lessening tip losses to the mach shockwave. One of his Reno racing designs used a 4-bladed prop. He also had prop flying on an automotive conversion without a gearbox, and it had a 7-blade prop turning somewhere in the neighborhood of 5000rpm at the prop. Not sure how that ended up but they were working thru some issues before he died.

 

[RVjim]

2 & 3 blade comparrison

I notice Jim that you compare the 4 and 3 blade to each other but you do not compare the performance against a 2 blade at any altitude or at all. You mention in your message title 'the 2 blade myth again' but there is no 2 blade data here against the 3 or 4 blade. Perhaps you could give us those performance numbers 2 blade against the 3 & 4 blade too?

In choosing a propeller for a HR2 with a 260 hp racing at Reno, a summary of the data I collected follows:

Hartzell (2 blade) with "D" twist blades was 2 knots slower than the 3 blade MT propeller. With both of these propellers, the maximum airspeed was at 2600 RPM. The airspeed decreased above 2600 RPM. (Propeller drag increasing faster than engine horsepower increase.)

Hartzell (2 blade) with "J" twist was the same as the 3 blade MT propeller at 2600 RPM or less. With the 260 hp engine, the airspeed kept increasing up to at least 2800 RPM. In another persons testing with a 330 hp engine with the "J" twist blade, the airspeed decreased above 2600 RPM.

Sorry. Too many parameters to have a simple answer.

There is a simple question, though. What is your aircraft mission?
Fly for fun? Cross country medium altitude economy cruise? Low level maximum speed cruise? Bragging rights fastest speed?

Each question has a different propeller as an answer.

Back to the original question. If the aircraft in question is for "fun" flying, then the MTV-10 propeller will fit that need.

Jim Ayers