Best Prop Tip Speed?

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autopilot

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Jan gave you some good numbers to work with. The usual reasons for keeping prop speeds down to ~800 ft/sec is to avoid the tips going supersonic. Supersonic props are a different animal to regular props, which is why they are rare. If props go supersonic, efficiency drops, with much of the lost power is converted to noise. The shock waves also need a much more robust prop (metal, maybe composite, forget wood) and different blade design. If you don't mind losing power, wearing earplugs and spending lots of money, you can go supersonic. But you won't make many friends like that...
I will pay all due attention to Jan as he seems to know what he is talking about. Most of the confusion around suitable tip-speed seems to come from air temp at take off (max power) and air temp at cruise altitude and cruise speed. I will keep my tips below mach and within practical limits.
.
 

Jan Carlsson

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When you want to know the Power or RPM at Cruise from a fixed set of RPM / Power at max, you use the 3th Power (if that is correct in English)
RPM goes down to % of max (3th Rooth %P) ( fixed pitch) (SL)

When throttle back!

85% Power is at 94,7% RPM
75% Power is at 90,85% RPM
65% Power is at 86,62% RPM
55% Power is at 81,93% RPM
 
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autopilot

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When you want to know the Power or RPM at Cruise from a fixed set of RPM / Power at max, you use the 3th Power (if that is correct in English)
RPM goes down to % of max (3th Rooth %P) ( fixed pitch) (SL)

When throttle back!

85% Power is at 94,7% RPM
75% Power is at 90,85% RPM
65% Power is at 86,62% RPM
55% Power is at 81,93% RPM
If I understand you correctly Jan, 75% of power from 160Hp isn't 120Hp., it's 90.85% of RPM at full power engine RPm's (8600)????? so 90.85% of 8600 = 75% power, or is it 90.85% of prop rev's @ full pwr??
 

Jan Carlsson

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75% is Always 75%

you get this effect when you THROTTLE back on a fixed pitch propeller.

it is not like when you pull down the RPM with the blue knob and keep the black full forward.

When throttle back, it is the propeller demand that set the Power and not the full throttle brake curve.
We also see in the graph that if we want to increase RPM, speed, (propeller, boat, plane or car) the Power demand is (V1/V)^3

Propeller demand.jpg

and with your stated Engine data i found in your post, here max is the 195 HP at 10750.
the different between the red FULL THROTTLE Engine curve and the blue propeller demand curve make it possible to increase RPM untill it match at 100 %

Propeller Demand - ENGINE.jpg
 
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TXFlyGuy

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The general rule is go to the largest diameter that's geometrically feasible.

This is a typical design cycle:
  • Propeller diameter is usually restricted by aircraft geometry, so this is determined outside of the design of the propeller itself
  • Tip speed will be set to a range, based on limiting tip Mach number and required thrust and power absorption
  • Number of blades will be set by the required blade area. Since induced losses are proportional to the square of the lift on each blade (diameter fixed), more blades is better but....
  • Blade root thickness and chord are set by structural criteria; for large props, these are vibratory loads during takeoff, when there are large areas of separated flow on the blades and ....
  • More blades tend to be more costly, but ....
  • The loads on the pitch change mechanism scale by blade chord squared, so ....
  • those on a 4-bladed prop are lower than one with two, but...
  • Increasing the number of blades increases the parts count in the pitch change mechanism, and the initial and operating costs.

Unless a two-engine airplane is being designed, don't go to two props: it's nothing but an unnecessary mechanical complication.

If, as is likely, Autopilot is looking for a fixed-pitch prop, he may want to start by setting the tip speed based at cruise, say 700 fps, and back up from this to the prop's rpm and the ratio for his reduction gearing to have his engine running at or just below the engine's RPM for peak torque at full throttle, where piston engines tend to have their best efficiency. NACA has a lot of references regarding propeller design and selection, e.g., Gilman, Jean, "Propeller-performance charts for transport airplanes", NACA Technical Note 2966, National Advisory Committee for Aeronautics. Langley Aeronautical Lab.; Langley Field, VA, United States: 1953, <https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930084064.pdf> and Biermann, David, "Propeller charts for the determination of the rotational speed for the maximum ratio of the propulsive efficiency to the specific fuel consumption," NACA Technical Report 749, National Advisory Committee for Aeronautics. Langley Aeronautical Lab.; Langley Field, VA, United States: 1942, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091827.pdf
Our design is set. 94 1/2", four blade, hydraulic constant speed prop. By Whirlwind Propeller Company. The max design rpm is 2600 for the prop. But we will never go above 2040 prop rpm (840 fps). That is at 4500 engine rpm, which equals about 412 hp at sea level.

Our high speed cruise setting will be 3800 engine rpm (330 hp @ sea level, 230 hp at 10,000'), or 1720 prop rpm. That is 708 fps.
 

Pete Plumb

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So, what is the best, or max prop tip speed for high speed cruise?

Is it 475 mph, as stated by Warp Drive? They claim that anything beyond that only increases noise, decreases efficiency, and causes a reduction in IAS.

What say you?

I'd like to chime in on this one. Admittedly, I haven't read through the entire thread and knowing this group, much smarter guys than me have already answered this but here is my 2 cents. You know me, I love numbers and hate guessing so for what it's worth:

Take a look at the following charts from Theory of Wing Sections' [very limited] data on subsonic airfoils and you'll see some interesting characteristics when the tip speed reaches mach .72 or so. Camber and thickness have a tremendous effect on what starts happening. The charts below show the lift and drag characteristics plotted against Mach number and Alpha (AOA). I've only included polars for zero camber airfoils and 2 percent camber airfoils because you'll be able to "see" what is happening with increased camber and thickness. The lift and drag curves of prop tip airfoils over 12 percent thick and over 2 percent camber get REAL UGLY at high Mach numbers! Remember these curves next time you look at the tip section of the next prop you look at. Most guys use flat-bottomed (meaning super-high camber) airfoils all the way out to the tip - which is, by the way, placing the highest drag/least efficient airfoils on the longest moment-arm that your engine HP is trying to overcome. I cringe every time I see a 15 percent, flat bottomed, sharp L.E. airfoil on the tip of a prop! That thing plowing through the air at almost 800 f/s is just horrible! Might as well cut off about 4"! I design my props with these airfoil characteristics in mind which means thin (meaning small thickness ratio), zero camber airfoils at the tip sections over M-.7.

Okay, I'll get off my soapbox and let's go through your original question.

In your example, 475 mph is about 700 f/s which is about M-.63 at sea level. As you can see from the charts, at .63, nothing is starting to happen to the curves except at high Alphas where we are getting an INCREASE of CL and CD. Now, you said "high cruise speed" so the ADVANCING tip speed is 700/cos pitch which is going to be the true airspeed of the tip airfoil at cruise. I'd limit the diameter so the true airspeed of your tips is BELOW the Mach number where your prop's tip airfoil starts going to s***.

Some useful, simple prop formulas:

Tip Speed, f/s= circumference x RPM/720 NOTE: inches
Pitch, (inches)= V x 720/RPM NOTE: V is f/s of aircraft
Pitch Angle, in degrees = Pitch/Circum (tan-1)
Advance Vsub1= V/cos pitch

Mach number CD 0008 sml.jpgMach number CD 0012 sml.jpgMach number CD 2312 and 2315 sml.jpgMach number CL 0012 and 2306 sml.jpgMach number CL 2309 and 2312 sml.jpg
 

Jan Carlsson

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Thank you Pete.

it is a Little bit different in real World with 3D airstream, that seems to delay the influence of the speed of sound, from what these 2D flow show.
 

Pete Plumb

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Thank you Pete.

it is a Little bit different in real World with 3D airstream, that seems to delay the influence of the speed of sound, from what these 2D flow show.
Thanks Jan. True. NACA tested and published polars for infinite aspect ratio didn't they! Theory Of Wing Sections has a very limited data base for airfoils at mid to critical mach numbers but it is all I have to go from. Jan, do you know of any books that have good data on airfoils at the higher mach numbers? I guess if I had tons of time I could eventually get good prop airfoil data through testing empirically.
 

Swampyankee

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Thanks Jan. True. NACA tested and published polars for infinite aspect ratio didn't they! Theory Of Wing Sections has a very limited data base for airfoils at mid to critical mach numbers but it is all I have to go from. Jan, do you know of any books that have good data on airfoils at the higher mach numbers? I guess if I had tons of time I could eventually get good prop airfoil data through testing empirically.
Well, there is a great deal of data on transonic flow over finite wings. Ackeret, Buseman, Prandtl, Glauert, RT Jones, Shapiro, Antony Jameson, Frances Bauer, Paul Garabedian, and many others have written on the subject.
http://heli-air.net/2016/02/18/swept-wings-in-transonic-flow-2/
http://www.dept.aoe.vt.edu/~mason/Mason_f/TransonicWings.pdf
http://www.dept.aoe.vt.edu/~mason/Mason_f/ConfigAeroTransonics.pdf
 

Pete Plumb

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Now that's some good stuff! Thanks for the links! Especially the third one. Can't wait to read through it carefully. I have a Complete Works of RT Jones. I'll look through it as well. RT gave it to me personally when I had my shop in Tehachapi in the 80s. He was a brilliant guy.
 

TXFlyGuy

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Is it Ok to resurrect an old thread...3 years later?

90" prop, paddle style 4 blade.
Max Cruise rpm = 4100 (engine) / 1855 (prop)
The engine will put out 350 hp (sea level)
The tip speed will be right at .65 Mach / 730 fps

Are these good numbers, or just pure BS? I realize there is more to it, but trying to keep it simple so I can wrap my brain around it!

Yes, we have emailed the manufacturer about this also. Just curious what the experts here have to add in regards to the most efficient tip speed for fast (high speed) cruise.
5AD58526-5BCA-4A61-80F8-AA3450DFFEEA-002.jpg
 

Norman

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An engineer who used to be a propeller designer told me "the sum of the local helical Mach number plus the local airfoil thickness should be comfortably less than about 0.9" so if the helical speed at the tip is 0.8M the prop tip should be less than 10% thick.
 

lr27

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HI Jan, I have just today done a little re-engineering and may be able to enlarge the prop to 1.6 or even 1.65. AND, you have just made me realize (although I know this) if I am splitting my trq, I am also splitting my Hp! Jan, can you tell me what is 800ft/sec in Mach?
Just divide the speed by the speed of sound under the conditions you expect to fly in. At sea level in standard conditions it's 1116 fps according to the standard atmosphere calculator I found on line. It will go down with altitude and up with temperature. About Mach 0.72 . If that's,not quite what you were asking for, my apologies.

Blane:
Any non-obvious reasons why it fell out of favor? Seems like in the case of unusual configurations, it might make sense. Particularly if the rpm needs to be reduced. Of course, in the case of two props, the rpm's don't have to be reduced as much.
 

blane.c

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Referencing the Wright brothers two propeller one engine Wright Flyer I assume.

I think just the obvious ones.
 

TXFlyGuy

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To produce maximum thrust at full power your tip speed should fall between .88 and .92 mach. To move between .88 and .92 mach usually takes a change of about 110 to 120 RPM. This of course varies depending on your particular propeller and the temperature.


If your tip speed is less than .88 mach you should increase RPM to achieve maximum thrust. If your tip speed is greater than .92 mach you should reduce RPM to achieve maximum thrust. Do not exceed the published operating limitations of your engine or propeller.

Over .92 mach the airflow begins to detach from the propeller which decreases efficiency and dramatically increases noise. To improve performance and public relations you should consider reducing RPM so as to fall within the .88 to .92 mach range. Your propeller will be producing maximum thrust which is good for you, and less noise which is good for all of us.
 

blane.c

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To produce maximum thrust at full power your tip speed should fall between .88 and .92 mach. To move between .88 and .92 mach usually takes a change of about 110 to 120 RPM. This of course varies depending on your particular propeller and the temperature.


If your tip speed is less than .88 mach you should increase RPM to achieve maximum thrust. If your tip speed is greater than .92 mach you should reduce RPM to achieve maximum thrust. Do not exceed the published operating limitations of your engine or propeller.

Over .92 mach the airflow begins to detach from the propeller which decreases efficiency and dramatically increases noise. To improve performance and public relations you should consider reducing RPM so as to fall within the .88 to .92 mach range. Your propeller will be producing maximum thrust which is good for you, and less noise which is good for all of us.
Wouldn't this start involving the aircraft speed or is that already being accounted for?
 

Dan Thomas

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Don't forget that the prop is an airfoil, and the flow over the cambered side is faster than the blade at that point. With high mach tip speeds you could be developing a power-robbing shock wave at the tip.
 

Norman

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Wouldn't this start involving the aircraft speed or is that already being accounted for?
That's what the helical velocity (Vh) is. Helical speed is the vector sum of the tangential speed (Vt) due to rotation and the forward speed

Tangential speed is rate of rotation times circumference at that location. In MPH, that works out to:
Vt (mph) = .00298*RPM*diameter (in inches)

Square the forward speed (Vf), square the tangential speed (Vt), add them together and take the square root of the result to find the helical velocity:
Vh = SQRT( Vf^2 + Vt^2 )
 
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lr27

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TCFlyguy: Having trouble understanding your comment. It seems to me that it must be based on a number of assumptions, including that you have more power than you can effectively use.
 
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