Prop tip speed on my (Spitfire) Maule
Thread Starter
Join Date: Aug 2005
Location: Glens o' Angus by way of LA
Age: 60
Posts: 1,975
Likes: 0
Received 0 Likes
on
0 Posts
Prop tip speed on my (Spitfire) Maule
Got a call today from the mechanic friend at one of the local fields I visit who suggested I look at my prop pitch settings as when I was taking off the other day he said it sounded like "a spitfire doing a low pass" with that distinctive twang. Which got me thinking when I was flying out of a busy US airfield last year I received a few warning letters from the field management that my plane had busted the acceptable thresholds on their sound monitors.
The prop is an STC field modification approved MTV-9-B/200-53 (3 blade) which replaced the original 76" 2 blade Hartzell.
The mechanic suggested that spinning at an excessively high a rate is inefficient and could damage the prop and as much as I like the idea of it twanging like a Spit on take off I want to make sure I'm getting the most out of it.
Do any of you engineering types know how to calculate what RPM would give the most efficient tip speed on a 200cm prop?
The prop is an STC field modification approved MTV-9-B/200-53 (3 blade) which replaced the original 76" 2 blade Hartzell.
The mechanic suggested that spinning at an excessively high a rate is inefficient and could damage the prop and as much as I like the idea of it twanging like a Spit on take off I want to make sure I'm getting the most out of it.
Do any of you engineering types know how to calculate what RPM would give the most efficient tip speed on a 200cm prop?
Last edited by piperboy84; 29th Nov 2017 at 20:21.
There are a handful of pilots who fly over my house with prop tips going near-supersonic. It makes a dreadful racket, much louder than other aircraft. There are times when I'm tempted to get the handheld and tell them to slow the &&(*$ prop down.
Most planes spin the prop at 2400 or 2500 rpm. The noisy ones spin at 2700. The difference is small but enough to make a big difference to the shock waves coming off the tips.
If your Maule spins at 2700, I'd suggest using 2500 unless you absolutely need every last HP - which in a Maule I imagine would mean if you want to take off in less than the length of the aircraft.
Anyway it's easy enough to figure out the tip speed - πrΩ and all that.
Most planes spin the prop at 2400 or 2500 rpm. The noisy ones spin at 2700. The difference is small but enough to make a big difference to the shock waves coming off the tips.
If your Maule spins at 2700, I'd suggest using 2500 unless you absolutely need every last HP - which in a Maule I imagine would mean if you want to take off in less than the length of the aircraft.
Anyway it's easy enough to figure out the tip speed - πrΩ and all that.
a) Take prop Inches diameter divided by 12 for feet diameter.
b) Multiply by Pi (i.e. 3.142) for circle distance feet per rev.
c) Multiply by RPM (for tip's distance moved per minute)
d) divide by 60 to get tip speed in feet per second !
The speed of sound at sea level is 1100 ft/second -- Compare !
Hope that is O.K.
mike hallam.
b) Multiply by Pi (i.e. 3.142) for circle distance feet per rev.
c) Multiply by RPM (for tip's distance moved per minute)
d) divide by 60 to get tip speed in feet per second !
The speed of sound at sea level is 1100 ft/second -- Compare !
Hope that is O.K.
mike hallam.
Thread Starter
Join Date: Aug 2005
Location: Glens o' Angus by way of LA
Age: 60
Posts: 1,975
Likes: 0
Received 0 Likes
on
0 Posts
Thanks Mike,
How does this look?
Prop Length cm 200
In inches Inch 6.5616
X P 20.6165472
RPM 2700 55664.67744
Tip speed@ RPM 927.744624
How does this look?
Prop Length cm 200
In inches Inch 6.5616
X P 20.6165472
RPM 2700 55664.67744
Tip speed@ RPM 927.744624
That looks O.K. for calculation but represents 85% of the speed of sound.
Somewhere there should be a guide % for a/c prop tip speeds, as approaching closer to 1100 ft/sec could allow some fractions of the air to go supersonic.
But on that score I'm definitely NOT a guru. In any case there may be other factors which make a fast prop noisier than desired.
mike hallam.
Somewhere there should be a guide % for a/c prop tip speeds, as approaching closer to 1100 ft/sec could allow some fractions of the air to go supersonic.
But on that score I'm definitely NOT a guru. In any case there may be other factors which make a fast prop noisier than desired.
mike hallam.
Join Date: Feb 2007
Location: Amsterdam
Posts: 4,598
Likes: 0
Received 0 Likes
on
0 Posts
a) Take prop Inches diameter divided by 12 for feet diameter.
b) Multiply by Pi (i.e. 3.142) for circle distance feet per rev.
c) Multiply by RPM (for tip's distance moved per minute)
d) divide by 60 to get tip speed in feet per second !
The speed of sound at sea level is 1100 ft/second -- Compare !
Hope that is O.K.
mike hallam.
b) Multiply by Pi (i.e. 3.142) for circle distance feet per rev.
c) Multiply by RPM (for tip's distance moved per minute)
d) divide by 60 to get tip speed in feet per second !
The speed of sound at sea level is 1100 ft/second -- Compare !
Hope that is O.K.
mike hallam.
You can simply do a bit of pythagoras: Take the prop speed against the airframe (formula above), squared, and add the airspeed, squared. Take the square root of that to get the prop tip against airspeed. (Easiest if you do this all in metric numbers...)
Furthermore, I don't know the exact number but you will want your prop tip to remain below about 80% of the speed of sound. The prop itself is, after all, a wing (aerodynamically speaking). There will therefore be a local acceleration of the air. So even though the prop itself is subsonic, air may still be accelerated locally to supersonic speeds. You therefore need a buffer of about 15-20%.
Anyway, if your static prop speed is already at Mach 0.85, I can well imagine that your aircraft is LOUD.
I fly a GA8, which has a big prop and a 300 HP IO-540. Our POH, and noise certificate based on the POH, specifies a maximum RPM of 2500. 2700 is selectable but after passing a detent, and "in emergency only". It makes a huge difference.
Some similar 3-bladed MT propellers were certified in Germany with a restriction to 2500 rpm continuous, 2700 rpm for takeoff. Discussion with the manufacturer indicates that this was for noise only, not for another technical reason. From a propeller life and reliability perspective this restriction is irrelevant, but it does indicate that some of the MT propellers are substantially louder at 2700 rpm. I have one, electrically controlled which allows me to preselect rpm. Mine is 175 cm diameter, smaller than the OPs, but I typically set 2600 or 2650 rpm for takeoff. I've been told mine is loud at 2700 rpm and I can imagine the 200 cm prop would be louder still.
Last edited by Silvaire1; 30th Nov 2017 at 15:06.
Join Date: Aug 2000
Location: S Warwickshire
Posts: 1,214
Likes: 0
Received 0 Likes
on
0 Posts
Your mechanic probably thinks it is overspeeding because he's used to the sound of 2 blade props. A 3 blade prop will have a pitch at blade passing frequency of 135 HZ at 2700RPM against 90 Hz for a 2 blade.
Assuming you are using 2700 RPM for take off, the tip relative Mach no. will be about .85. Noise is very non-linear with tip speed as you approach transonic. Just pulling the RPM back to 2500-2600 once you are clear of obstacles will make a big difference.
Efficiency is obtained by using the lowest RPM and highest MP for a given power setting and setting best mixture. For take off performance you will want to use the full rated power obtained by full throttle and max RPM.
If you want to be a good neighbour, reduce RPM as soon as possible consistent with safe operation and engine limitations. The reduced climb rate will be more than compensated by the reduced noise levels.
Assuming you are using 2700 RPM for take off, the tip relative Mach no. will be about .85. Noise is very non-linear with tip speed as you approach transonic. Just pulling the RPM back to 2500-2600 once you are clear of obstacles will make a big difference.
Efficiency is obtained by using the lowest RPM and highest MP for a given power setting and setting best mixture. For take off performance you will want to use the full rated power obtained by full throttle and max RPM.
If you want to be a good neighbour, reduce RPM as soon as possible consistent with safe operation and engine limitations. The reduced climb rate will be more than compensated by the reduced noise levels.
Moderator
Operate your MTV-9 at 2500 RPM rather than 2700. I have done the EASA approved noise test on the MTV-9 on a 182 amphibian. 2500 RPM met their requirement for quiet which the 2700 certainly does not. The climb performance difference between 2500 and 2700 is not significant. The measurement of noise of the aircraft as a whole was made at the arequired point, 2 km from brake release, with a specified climb after liftoff. What I noticed, as my altitude was also carefully measured, was that the difference in altitude achieved at that point was 60' more at 2700 than 2500. 60' is negligible for your performance, particularly if your noise output is no longer offending your neighbours, 'cause you reduced RPM.
On my MTV-15 on my Lycoming O-360, I find that my best performance is achieved with the engine 2" oversquare. The MT 3 blade props have a greater "activity factor" so a slower RPM can be beneficial.
On a related observation, consider your power off glide with a windmilling MTV prop, you may find a more steep glide. Consider selecting full course pitch, if a glide is required. Go up high, and experiment.
On my MTV-15 on my Lycoming O-360, I find that my best performance is achieved with the engine 2" oversquare. The MT 3 blade props have a greater "activity factor" so a slower RPM can be beneficial.
On a related observation, consider your power off glide with a windmilling MTV prop, you may find a more steep glide. Consider selecting full course pitch, if a glide is required. Go up high, and experiment.
Thread Starter
Join Date: Aug 2005
Location: Glens o' Angus by way of LA
Age: 60
Posts: 1,975
Likes: 0
Received 0 Likes
on
0 Posts
On my MTV-15 on my Lycoming O-360, I find that my best performance is achieved with the engine 2" oversquare. The MT 3 blade props have a greater "activity factor" so a slower RPM can be beneficial.
Also what's "activity factor" ? Do you mean those composite props slap around a bit more than the factory recommended metal Hartzell on the Lyc.360?
Moderator
The activity factor is the ratio of the collective area of the propeller blades to the area of the propeller disc.
I cruise the Teal 24" 2100RPM, and is seems very happy. I can't say that that is an authoritative power setting, but my plane is non certified. I have done detonation testing of an engine with the same compression ratio, and the data from that testing makes me comfortable with this power setting. I do have an engine scanner so I can watch temps very carefully.
Wood blades do tend to be more quiet, as they lack the twang of the metal blades.
I cruise the Teal 24" 2100RPM, and is seems very happy. I can't say that that is an authoritative power setting, but my plane is non certified. I have done detonation testing of an engine with the same compression ratio, and the data from that testing makes me comfortable with this power setting. I do have an engine scanner so I can watch temps very carefully.
Wood blades do tend to be more quiet, as they lack the twang of the metal blades.
24" @2200RPM
The activity factor is the ratio of the collective area of the propeller blades to the area of the propeller disc
solidity (total activity factor) = activity factor X number of blades
Join Date: Nov 2016
Location: 57 North
Posts: 78
Likes: 0
Received 0 Likes
on
0 Posts
pb84,
I found this rather nice, practical explanation of prop tip speed and how to calculate it:
Propeller Performance: An introduction, by EPI Inc.
I entered the data into an Excel spreadsheet, because your question reminded me that I had been meaning to do the same calculations for the 265 HP C182 towplane that I fly. At 2700 rpm, it sounds like a C185 floatplane taking off. For noise abatement, I reduce RPM by at least 400 after becoming safely airborne.
The attached chart shows the tip speed Mach number for your old 72" prop and the new 200 cm / 79" prop. Your new prop is bound to be noisier at any given RPM, because it has 7" more diameter.
Looking at the chart, and assuming Sea Level on a Standard Day and a TAS of 60 kt, your old prop had a tip speed of M 0.76 at 2700 rpm, whereas the new one has a tip speed of M 0.83. To have a tip speed of M 0.76, you would have to reduce the RPM to about 2550.
I found this rather nice, practical explanation of prop tip speed and how to calculate it:
Propeller Performance: An introduction, by EPI Inc.
I entered the data into an Excel spreadsheet, because your question reminded me that I had been meaning to do the same calculations for the 265 HP C182 towplane that I fly. At 2700 rpm, it sounds like a C185 floatplane taking off. For noise abatement, I reduce RPM by at least 400 after becoming safely airborne.
The attached chart shows the tip speed Mach number for your old 72" prop and the new 200 cm / 79" prop. Your new prop is bound to be noisier at any given RPM, because it has 7" more diameter.
Looking at the chart, and assuming Sea Level on a Standard Day and a TAS of 60 kt, your old prop had a tip speed of M 0.76 at 2700 rpm, whereas the new one has a tip speed of M 0.83. To have a tip speed of M 0.76, you would have to reduce the RPM to about 2550.
PB84,
That 1452 departure this afternoon was a little noisy (you flew past the office), but only transient ... when the observer is near the plane of the prop, kind of like the "neeeeeooooooowwwww" sound kids make when pretending to fly...
Were you using a lower RPM that time?
Russell
Helimed76
That 1452 departure this afternoon was a little noisy (you flew past the office), but only transient ... when the observer is near the plane of the prop, kind of like the "neeeeeooooooowwwww" sound kids make when pretending to fly...
Were you using a lower RPM that time?
Russell
Helimed76