[rebel-builders] More on propeller mach speed

Ron Shannon · Post by **Ron Shannon** » Sun Feb 19, 2012 12:20 pm

FYI - One of many web-based tip speed calculators is at:

http://www.pponk.com/HTML%20PAGES/propcalc.html

FWIW, the site's proprietor adheres to the convention/opinion that
optimal tip speed is "between .88M and .92M". Personally, I appreciate
Ken's analysis along with Mike's spam can data points, both of which
indicate lower M numbers are preferred.

In any case, the calculator is fun. :-)

Ron
254R

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Mike Kimball · Post by **Mike Kimball** » Sun Feb 19, 2012 12:20 pm

I'm about ready to change the "good range" limits in my spreadsheet. But
I'm going to wait to hear back from the professor I emailed in the aviation
department at my old alma mater. Hopefully, the result of this discussion
will be that I don't have to change a thing on my plane! Disregarding all
the math and theory for a moment, it still just seems like my prop speeds
are just too low based on all the other planes I have ever flown since I
started flying in 1980. I know the M14 prop speeds are pretty low too, but
that means finding a giant prop to swap out for what I've currently got.

Mike
044SR

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Ron
Shannon
Sent: Sunday, September 02, 2007 12:00 AM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] More on propeller mach speed

FYI - One of many web-based tip speed calculators is at:

http://www.pponk.com/HTML%20PAGES/propcalc.html

FWIW, the site's proprietor adheres to the convention/opinion that
optimal tip speed is "between .88M and .92M". Personally, I appreciate
Ken's analysis along with Mike's spam can data points, both of which
indicate lower M numbers are preferred.

In any case, the calculator is fun. :-)

Ron
254R

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Ken · Post by **Ken** » Sun Feb 19, 2012 12:20 pm

The main reason for a high tip speed is for putting more power into a
prop when you can't increase the diameter due to ground clearance, float
clearance, picking up too much water spray, or torsional vibration
concerns with the engine. Full vibration surveys are expensive so just
spin the prop faster and the customer gets better performance but at the
expense of noise and fuel consumption. Customer doesn't care about
either if fuel is cheap and/or he only turns the high rpm at takeoff.

Reynolds numbers and such come into play but there is a reason that no
commercial jets fly at .9 M

Joe - haven't had a chance to look at that website yet.

Ken

Mike Kimball wrote:

I'm about ready to change the "good range" limits in my spreadsheet. But
I'm going to wait to hear back from the professor I emailed in the aviation
department at my old alma mater. Hopefully, the result of this discussion
will be that I don't have to change a thing on my plane! Disregarding all
the math and theory for a moment, it still just seems like my prop speeds
are just too low based on all the other planes I have ever flown since I
started flying in 1980. I know the M14 prop speeds are pretty low too, but
that means finding a giant prop to swap out for what I've currently got.

Mike
044SR

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Ken · Post by **Ken** » Sun Feb 19, 2012 12:20 pm

Note that you can't get near these quoted rpm's during takeoff unless
you have a variable pitch prop so the Cessna's for sure are running even
lower tip speed than this. Well except when at 8000 feet at wide open
throttle making 75% of rated power which is an attractive way to run for
them but just not where most spend a lot of time.

Joe - A quick look at the UL260 makes me consider it to be a just barely
80 hp engine for a Rebel. I doubt that it will turn very many fixed
pitch 72" props at 2800rpm for takeoff and still give satisfactoy cruise
but that's just my wild guess. If true though it really isn't even an
80 hp engine on a Rebel. Tom Inglis gets good performance from his 72" 2
blade Warp prop (no blade taper) on his 80 hp. Rotax 912. A good
comparison might be what his prop rpm is at takeoff and cruise and
whether his engine is turning close to max hp. rpm.

Also we were impressed this summer by Jim Cole's 100 hp rotax 912S.
Good performance with a 72" 3 blade Warp (no blade taper) so maybe he
can also tell us the prop rpm and any thoughts about his prop. That is
the same prop that I run. It is cheap, ground adjustable, and has a
good reputation but it doesn't come larger than 72" which means the
larger engines must take the penalty of 3 blades to absorb the power.
Ground adjustable is a huge advantage for a unique engine and blades
that can be shortened is another ace in the hole. Don't know if he would
recommend it for that engine though. Even at my low tip speed it could
easilly handle more power than I have which it will need to do if I bump
the power on floats. The thing is I run about .66M at takeoff and
normally cruise at .62M tip speed according to the calculator. Quiet
and efficient IMO. Walter said that prop performed better for him at
2600+ rpm but my fuel consumption almost doubles if run that hard. I'm
pretty confident that I make about 120 hp during takeoff and climb which
is in the same ballpark as a fixed pitch 0-320 and I seem to have about
the same performance. I cruise slower than those engines though and
seem to enjoy similar fuel consumption and cruise speed as Tom and Jim.
I'm heavier so I do burn a bit more than them but I'd say that the
lower tip speed is working out satisfactorilly for me.

The UL260 engine looks interesting. I don't agree with some of his
statements including much of what he says about diesels though. Anyway
if the engine achieves several hundred hours on a couple of airframes
with a prop that you like then it might be very attractive if the price
is reasonable. Until it has proven reliability and performance though,
personally I'd still favor the rotax. Unless it is certified or
vibration tested with the prop you want, it is still very much an
experiment until it proves itself in service. Interestingly we are
starting to hear about rotax 912's that have been retro fitted with EFI ;)

Ken

Mike Kimball wrote:

I got curious with recent research and discussion and decided to take a look
at some typical, modern (term used loosely except for the Lancair) aircraft.
I found the following:

Piper Archer - Takeoff RPM 2700, propeller diameter 76 inches, mach speed
0.815
Cessna 172 - Same as the Archer
Cessna 152 - Takeoff RPM 2750, propeller diameter 69 inches, mach speed
0.756
Cessna 182 - Using a BlackMac 3-blade, scimitar blade design conversion,
takeoff RPM 2600, propeller diameter 80 inches, mach speed 0.826
Lancair 4P - Cont. TSIO-550, Takeoff RPM 2600, 3 or 4 blade, all approved
props are 76 inch diameter, mach speed 0.786

Clearly, the spam-can manufacturers adhear to Ken's "around 0.8 mach"
philosophy. Slow planes versus fast planes doesn't seem to matter with the
Lancair in the same boat. Lot's more thinking to do before I decide whether
or not to make a change.

Mike
044SR

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Joe Ronco · Post by **Joe Ronco** » Sun Feb 19, 2012 12:20 pm

Thanks Ken.

Joe

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of
Ken
Sent: Sunday, September 02, 2007 2:53 PM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] More on propeller mach speed

Note that you can't get near these quoted rpm's during takeoff unless
you have a variable pitch prop so the Cessna's for sure are running even

lower tip speed than this. Well except when at 8000 feet at wide open
throttle making 75% of rated power which is an attractive way to run for

them but just not where most spend a lot of time.

Joe - A quick look at the UL260 makes me consider it to be a just barely

80 hp engine for a Rebel. I doubt that it will turn very many fixed
pitch 72" props at 2800rpm for takeoff and still give satisfactoy cruise

but that's just my wild guess. If true though it really isn't even an
80 hp engine on a Rebel. Tom Inglis gets good performance from his 72" 2

blade Warp prop (no blade taper) on his 80 hp. Rotax 912. A good
comparison might be what his prop rpm is at takeoff and cruise and
whether his engine is turning close to max hp. rpm.

Also we were impressed this summer by Jim Cole's 100 hp rotax 912S.
Good performance with a 72" 3 blade Warp (no blade taper) so maybe he
can also tell us the prop rpm and any thoughts about his prop. That is
the same prop that I run. It is cheap, ground adjustable, and has a
good reputation but it doesn't come larger than 72" which means the
larger engines must take the penalty of 3 blades to absorb the power.
Ground adjustable is a huge advantage for a unique engine and blades
that can be shortened is another ace in the hole. Don't know if he would

recommend it for that engine though. Even at my low tip speed it could
easilly handle more power than I have which it will need to do if I bump

the power on floats. The thing is I run about .66M at takeoff and
normally cruise at .62M tip speed according to the calculator. Quiet
and efficient IMO. Walter said that prop performed better for him at
2600+ rpm but my fuel consumption almost doubles if run that hard. I'm
pretty confident that I make about 120 hp during takeoff and climb which

is in the same ballpark as a fixed pitch 0-320 and I seem to have about
the same performance. I cruise slower than those engines though and
seem to enjoy similar fuel consumption and cruise speed as Tom and Jim.
I'm heavier so I do burn a bit more than them but I'd say that the
lower tip speed is working out satisfactorilly for me.

The UL260 engine looks interesting. I don't agree with some of his
statements including much of what he says about diesels though. Anyway
if the engine achieves several hundred hours on a couple of airframes
with a prop that you like then it might be very attractive if the price
is reasonable. Until it has proven reliability and performance though,
personally I'd still favor the rotax. Unless it is certified or
vibration tested with the prop you want, it is still very much an
experiment until it proves itself in service. Interestingly we are
starting to hear about rotax 912's that have been retro fitted with EFI
;)

Ken

Mike Kimball wrote:

I got curious with recent research and discussion and decided to take a

look at some typical, modern (term used loosely except for the Lancair)

aircraft. I found the following:

Piper Archer - Takeoff RPM 2700, propeller diameter 76 inches, mach
speed 0.815 Cessna 172 - Same as the Archer
Cessna 152 - Takeoff RPM 2750, propeller diameter 69 inches, mach speed
0.756
Cessna 182 - Using a BlackMac 3-blade, scimitar blade design

conversion,

takeoff RPM 2600, propeller diameter 80 inches, mach speed 0.826
Lancair 4P - Cont. TSIO-550, Takeoff RPM 2600, 3 or 4 blade, all

approved

props are 76 inch diameter, mach speed 0.786

Clearly, the spam-can manufacturers adhear to Ken's "around 0.8 mach"
philosophy. Slow planes versus fast planes doesn't seem to matter with

the Lancair in the same boat. Lot's more thinking to do before I
decide whether or not to make a change.

Mike
044SR

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Walter Klatt · Post by **Walter Klatt** » Sun Feb 19, 2012 12:20 pm

Ken, in reference to my old Warp at 2600, that's when it would really bite
and use up the torque, so it does make sense that you would use more fuel at
that RPM, too. It's not that it wasn't efficient at lower RPM.

Walter

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Ken
Sent: Sunday, September 02, 2007 1:53 PM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] More on propeller mach speed

Note that you can't get near these quoted rpm's during takeoff unless
you have a variable pitch prop so the Cessna's for sure are running even
lower tip speed than this. Well except when at 8000 feet at wide open
throttle making 75% of rated power which is an attractive way to run for
them but just not where most spend a lot of time.

Joe - A quick look at the UL260 makes me consider it to be a just barely
80 hp engine for a Rebel. I doubt that it will turn very many fixed
pitch 72" props at 2800rpm for takeoff and still give satisfactoy cruise
but that's just my wild guess. If true though it really isn't even an
80 hp engine on a Rebel. Tom Inglis gets good performance from his 72" 2
blade Warp prop (no blade taper) on his 80 hp. Rotax 912. A good
comparison might be what his prop rpm is at takeoff and cruise and
whether his engine is turning close to max hp. rpm.

Also we were impressed this summer by Jim Cole's 100 hp rotax 912S.
Good performance with a 72" 3 blade Warp (no blade taper) so maybe he
can also tell us the prop rpm and any thoughts about his prop. That is
the same prop that I run. It is cheap, ground adjustable, and has a
good reputation but it doesn't come larger than 72" which means the
larger engines must take the penalty of 3 blades to absorb the power.
Ground adjustable is a huge advantage for a unique engine and blades
that can be shortened is another ace in the hole. Don't know if he would
recommend it for that engine though. Even at my low tip speed it could
easilly handle more power than I have which it will need to do if I bump
the power on floats. The thing is I run about .66M at takeoff and
normally cruise at .62M tip speed according to the calculator. Quiet
and efficient IMO. Walter said that prop performed better for him at
2600+ rpm but my fuel consumption almost doubles if run that hard. I'm
pretty confident that I make about 120 hp during takeoff and climb which
is in the same ballpark as a fixed pitch 0-320 and I seem to have about
the same performance. I cruise slower than those engines though and
seem to enjoy similar fuel consumption and cruise speed as Tom and Jim.
I'm heavier so I do burn a bit more than them but I'd say that the
lower tip speed is working out satisfactorilly for me.

The UL260 engine looks interesting. I don't agree with some of his
statements including much of what he says about diesels though. Anyway
if the engine achieves several hundred hours on a couple of airframes
with a prop that you like then it might be very attractive if the price
is reasonable. Until it has proven reliability and performance though,
personally I'd still favor the rotax. Unless it is certified or
vibration tested with the prop you want, it is still very much an
experiment until it proves itself in service. Interestingly we are
starting to hear about rotax 912's that have been retro fitted with EFI ;)

Ken

Mike Kimball wrote:

I got curious with recent research and discussion and decided to take a

look

at some typical, modern (term used loosely except for the Lancair)

aircraft.

I found the following:

Piper Archer - Takeoff RPM 2700, propeller diameter 76 inches, mach speed
0.815
Cessna 172 - Same as the Archer
Cessna 152 - Takeoff RPM 2750, propeller diameter 69 inches, mach speed
0.756
Cessna 182 - Using a BlackMac 3-blade, scimitar blade design conversion,
takeoff RPM 2600, propeller diameter 80 inches, mach speed 0.826
Lancair 4P - Cont. TSIO-550, Takeoff RPM 2600, 3 or 4 blade, all approved
props are 76 inch diameter, mach speed 0.786

Clearly, the spam-can manufacturers adhear to Ken's "around 0.8 mach"
philosophy. Slow planes versus fast planes doesn't seem to matter with the
Lancair in the same boat. Lot's more thinking to do before I decide

whether

or not to make a change.

Mike
044SR

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Ken · Post by **Ken** » Sun Feb 19, 2012 12:20 pm

Yes indeed Walter. The extra speed just doesn't seem worth all that
gas to me so I generally go slow and enjoy the ramble/ trip ;)

Ken

Walter Klatt wrote:

Ken, in reference to my old Warp at 2600, that's when it would really bite
and use up the torque, so it does make sense that you would use more fuel at
that RPM, too. It's not that it wasn't efficient at lower RPM.

Walter

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Walter Klatt · Post by **Walter Klatt** » Sun Feb 19, 2012 12:20 pm

Yeah, but it sure made for good climb out... If I go back on wheels some
time, I should try the Warp again and see what it can do. It's been a long
time since I have seen 1600 fpm.

Walter

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Ken
Sent: Sunday, September 02, 2007 6:23 PM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] More on propeller mach speed

Yes indeed Walter. The extra speed just doesn't seem worth all that
gas to me so I generally go slow and enjoy the ramble/ trip ;)

Ken

Walter Klatt wrote:

Ken, in reference to my old Warp at 2600, that's when it would really bite
and use up the torque, so it does make sense that you would use more fuel

at

that RPM, too. It's not that it wasn't efficient at lower RPM.

Walter

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