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Best Prop Tip Speed

TXFlyGuy

Well Known Member
Thanks to the advice given here, Catto Propeller was contacted.

This is news all of the RV community can use, and good to know.

The answer was any tip speed below .72 Mach will work for a high speed, efficient cruise.

This is a bit faster than what we originally thought.
 
Prop tip speed

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Default Best Prop Tip Speed?
Having friends with RV's (mostly 8's), the discussion comes up about propeller design, and the efficiency in certain rpm settings.

A former prop engineer for a major company has stated that they always designed the props for best efficiency in cruise at 750 fpm tip speed. This would be close to Mach .65.

What is your experience in the RV world with various propellers? Is the .65 tip speed pure BS, or is it a real world number for most efficient cruise?

Yes, there are many factors involved, but just trying to simplify the subject so I can wrap my brain around it!

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For a tip speed of 750 fpm, running at 2700 rpm, you would need to run a 47.7 inch prop....... Is it my math, or ?????
 
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Tip speed

Some excellent information on tip speeds &% of mach along with maximum efficiency projections, can be found on "pponk.com" thence from there select "props" and type in your RPM & prop length into the calculator .
Sorry I don't know how to post a link :confused:
 
From the website.....

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.
 
So...is the .72 Mach tip speed a universal rule-of-thumb? That info came from Catto.
How about Whirlwind Propeller? What do they say? I know there are lots of Whirlwind props flying on RV's.

Somewhere I was told that magic "sweet spot" for the most efficient tip speed was .65 Mach (+/-).

A couple days ago, I sent an email to Whirlwind Propeller, in hopes of getting a response. Note that Catto Propeller responded literally within minutes.
 
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The two main competing factors which dictate a certain tip speed for maximum efficiency are: tip compressibility drag rise, and mass flow through the propeller disk.

We are generally stuck with a given propeller shaft speed, say 2700 RPM.
That means diameter + a little adder for forward speed, times a constant = mach. Greater diameter produces a higher mach number. So, smaller is better.
However, thrust is produced by grabbing air and pushing it rearward. The bigger the diameter, the greater the mass, the less slippage and the higher the efficiency. So, bigger is better.

These two factors compete directly. So, one can rock these factors back and forth on a computer and find the sweet spot for efficiency and maybe feed in other practical requirements as well.

As I was reading Craig Catto's autobio a couple of years ago. I felt a kinship and above all an admiration for his background, knowledge and ability. I'm sure he is a couple of steps beyond the basics I have outlined.

Data point: Cessna 182, 230 HP 2600 RPM 82" prop 948 FPS ~.86 Mach tip speed.

Ron
 
Prop tip speed (and other prop parameters)

Many years ago, Ed Dokus wrote a program to design propellers, sort of a spreadsheet to "what if" a propeller. Enter your parameters, click "Calculate" and see results to include thrust and efficiency.

You may download PropDesign.exe at this URL. (http://dubner.us/PropDesign/index.html). I just verified that it will still run on Windows-7. (If you use Windows-10, well ... you have my sympathy.)

I ported PropDesign to Windows -- that was my contribution and that's about all I know about propellers (so don't ask me any propeller questions <g>).

Go easy on the server; it's an old PC running 24/7 in my hangar and won't stand up to the Slashdot effect.

--
Joe
 
I just verified that it will still run on Windows-7. (If you use Windows-10, well ... you have my sympathy.)

--
Joe

FWIW - While some older programs won't run on Windows 7, I have yet to run across one that WILL run on Windows 7 which WON'T run on Windows 10 (you may have to run it in compatibility mode).
 
Max thrust or max efficiency?

Well I guess at fixed RPM they amount to the same thing since power input would be the same.

To some extent the best tip speed depends on the prop design.
The Hartzell BA prop has a wider chord and thinner airfoil than a whirlwind or Catto prop so higher drag rise Mach.

So they answer depends on the prop design. Somewhere between 0.75 and 0.8 is a good area.

As a side note I designed a prop for flight on mars and the helical tip Mach no. was 1.3.

BTW 750 ft/s is M=0.68. (Not .65)
 
BTW 750 ft/s is M=0.68. (Not .65)

Maybe he lives in Death Valley, in the summer.
Serious question: Do any props have cold temperature RPM limits (to keep from getting close to, or exceeding, Mach 1, where their efficiency presumably goes down)?
 
BTW 750 ft/s is M=0.68. (Not .65)

A propeller engineer stated 750 FPS is what they always targeted for in cruise tip speed.

Another friend (engineer type / airline pilot / Reno Race winner) advised me that .65 Mach was the proverbial sweet spot for tips at cruise.

I did hear from Whirlwind. My prop is not typical, so we won't have any real good data until we do actual flight testing.
 
Tip Mach Number and Efficiency

A few things to clear up real quick: there are two separate questions that are really being asked here and in a short reply I can only answer one: what tip speed should propellers be LIMITED to to avoid massive loss of efficiency?

All the 'rules of thumb' are aiming to prevent ANY portion of the air from hitting Mach 1.0, the reason we limit the tip speeds to '0.72' as quoted in this thread is because the dynamic pressure over the width of the propeller's tip can increase that Tip Mach number from 0.72 toward a Local Mach number of 1.0 DEPENDING on the geometry of the tip (airfoil shape/size). At Whirl Wind Propellers we typically aim for tip mach numbers closer to 0.8 depending on the tip airfoil and expected operating conditions. Remember speed of sound changes with temperature and pressure, but your max engine RPM is a fixed number. Temperature and pressure will change locally across the airfoil of the propeller at highly compressible speeds so you have to consider that as well as the density altitude you operate at.

The other question being implied is: what is the most efficient speed for a propeller to operate at?
This questions should be broken into a design question and a usage question, but I'm not going into it now anyway. Mainly just wanted to point out the obvious: by operating your propeller at it's max tip speed, you are not gauranteeing that it is operating at it's max efficiency.
 
The other question being implied is: what is the most efficient speed for a propeller to operate at?
This questions should be broken into a design question and a usage question, but I'm not going into it now anyway. Mainly just wanted to point out the obvious: by operating your propeller at it's max tip speed, you are not gauranteeing that it is operating at it's max efficiency.

I guess this is my actual question...how fast can we spin the prop, .65, .72, or 750 FPS, and not lose any efficiency?

And where is the prop at it's most efficient operating RPM in terms of thrust produced?
 
1) It's hard to say without a detailed analysis and information on exactly which prop you have and its tip geometry. Each prop has its ideal design point so lets assume yours is designed for 0.72 tip mach. As you spin it faster, it will lose efficiency gradually until it hits a local mach number of 1.0 which may be a tip mach number around 0.76 to 0.88 (http://www.aerospaceweb.org/question/airfoils/q0003.shtml). When it hits that magic number efficiency will plummet and the prop could fail structurally. 750 fps is just an attempt to approximate mach 0.7, but equates to 0.66 mach at sea level or 0.72 mach at 18000 ft.

2) Thrust of a propeller decreases with speed. Think about a constant speed propeller: as you go faster the pitch will increase until it is pointing almost straight in the direction of flight and producing almost zero thrust, but as it reduces thrust it also reduces drag. At it's most efficient rpm, the propeller is producing thrust exactly equal to the drag of the airframe it was designed for at the design speed.

Hope this all makes sense; if I'm not quite answering your question you can private message me so we don't clutter up the thread too much.
 
Forward speed

Don't forget the plane is also moving through the air. Oversimplified, if the air is moving through the prop at 200kcas, that's roughly M0.30 (ISA at SL). If the tip speed is already M0.90, do the trigonometry with the static prop tip speed, and the speed of the air past the prop tip is M0.95. I haven't the foggiest how fast a plane will be going if the prop wash is going 200kcas, I'm sure significantly slower

Also, the prop is an airfoil, so accelerates air on the top of the camber, increasing the local mach number even more. I'm sure a prop tip going that fast will be developing shock waves around the tip despite the actual prop staying slightly subsonic. Portions of the airflow will probably be supersonic.
 
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