Rpm on takeoff.

[PT20J]

  On 9/5/2021 at 5:58 AM, Raymond J said:

It seems to me that the clubs this is just for the cooling flow in the old facilities with mechanical froude brake. Today, the installation is often an electric (generator) or hydraulic froude brake, the cooling installation is specific and there is no longer a club.

Expand  

Well, I'm not an engine builder, but I think this equipment is pretty expensive. I can see the engineering department wanting a variable load perhaps, but for production testing all that is necessary it to ascertain that the engine can make rated power. I've been to a couple of local shops that rebuild radial engines from R-985s up to R-2800s and they both used test clubs. Here's what the Lycoming factory uses:

 

[PT20J]

  On 9/5/2021 at 2:45 PM, PilotCoyote said:

That is also my recollection from flying the TH-55!

Expand  

I think Hughes used an HIO-360 —- different engine. 

[A64Pilot]

  On 9/5/2021 at 6:59 PM, PT20J said:

I think Hughes used an HIO-360 —- different engine. 

Expand  

That would be why I said it was a different engine.

In truth it’s different, but not very different.

[Will.iam]

Just get a hobby prop tachometer. It works from in the cabin through the windscreen so at anytime in flight or takeoff you can see what speed the prop is turning. Has settings for 2 to 9 blades and showed my digital tach to be 10 rpm off at 2700. 
 

Hangar 9 Micro Digital Tachometer, HAN156 https://www.amazon.com/dp/B0006N72U2/ref=cm_sw_r_sms_api_glt_fabc_Q1R1AFT4CH1JPFY5895J

[A64Pilot]

  On 9/8/2021 at 11:28 PM, Will.iam said:

Just get a hobby prop tachometer. It works from in the cabin through the windscreen so at anytime in flight or takeoff you can see what speed the prop is turning. Has settings for 2 to 9 blades and showed my digital tach to be 10 rpm off at 2700. 
 

Hangar 9 Micro Digital Tachometer, HAN156 https://www.amazon.com/dp/B0006N72U2/ref=cm_sw_r_sms_api_glt_fabc_Q1R1AFT4CH1JPFY5895J

Expand  

The funny thing is that you can check calibration of those things easily, select 2 blade prop and point it at a fluorescent light and it will indicate 3600 RPM.

‘They are amazingly accurate.

 

[Will.iam]

  On 9/9/2021 at 4:31 PM, A64Pilot said:

The funny thing is that you can check calibration of those things easily, select 2 blade prop and point it at a fluorescent light and it will indicate 3600 RPM.

‘They are amazingly accurate.

 

Expand  

If the light is flickering 60 times a second and you had a 1 blade prop that would be 3600rpm but 2 blade setting the tachometer will count 2 flashes as one  revolution so that would be 1800rpm and 3 blade setting would show 1200rpm right?  Damn now I’m  going to have to go get the tachometer out of the hanger and test this out when i get back home. 

[EricJ]

  On 9/11/2021 at 4:42 AM, Will.iam said:

If the light is flickering 60 times a second and you had a 1 blade prop that would be 3600rpm but 2 blade setting the tachometer will count 2 flashes as one  revolution so that would be 1800rpm and 3 blade setting would show 1200rpm right?  Damn now I’m  going to have to go get the tachometer out of the hanger and test this out when i get back home. 

Expand  

At 60 Hz a light flickers 120 times a second.   There are two zero crossings where the wave goes to zero and the light goes out twice every period.

[A64Pilot]

I’m not smart enough to know why, but try it and see.

It will not work with a regular bulb though, and last time I did it was looong before LED’s, so I have no idea about them, but suspect they may work

[PT20J]

It doesn’t work with an incandescent bulb because, as the name implies, the light is emitted by the resistive filament being heated to incandescence and the light output is constant.

As @EricJ stated, a fluorescent light (including many street lamps) flickers at 120 Hz because the light is generated by the alternating current flow exciting a mercury vapor within the bulb. The bulb extinguishes every time the current reverses. The human eye senses light by chemical reactions in the retina which take longer to respond than the time the light is extinguished and so most people cannot perceive the flicker. It’s also why old CRT televisions appear to display a complete image even though it’s actually created by a single dot moving rapidly across the phosphor-coated screen. 

Some LEDs are modulated for dimming. Every manufacturer has it’s own method. When LED taxiway lights were installed at KMRY, I noticed that I could see a trail of dots if I moved my head at just the right speed. I’ve also noticed this effect on some newer autos with LED tail lights. 

Skip

[David Lloyd]

At the old airport there was a light near the runup area.  1800 RPM on the tach, the prop blades would appear stationary.  A few RPM one way or other, the blades would appear to be rotating.  Stationary appeared in multiples of 60 (Hz).  Some older digital cameras had the same thing going on.  Don't remember what the numbers were for that.

[EricJ]

  On 9/11/2021 at 3:24 PM, David Lloyd said:

At the old airport there was a light near the runup area.  1800 RPM on the tach, the prop blades would appear stationary.  A few RPM one way or other, the blades would appear to be rotating.  Stationary appeared in multiples of 60 (Hz).  Some older digital cameras had the same thing going on.  Don't remember what the numbers were for that.

Expand  

FWIW, I did an instructional video years ago showing how that kind of "aliasing" works using a modified LED strobe lamp and a model helicopter.   The audio is crap, but if you're a math or tech nerd it might be interesting.
 

 

[carusoam]

Way cool Eric!

Thanks for sharing the details.

Go MS!

Best regards,

-a-

[brian1570]

Don't want to hi jack the thread but I have a similar issue. My 64 E has a hartzell 3 blade and I don't see anywhere close to 2700 rpm. I just figured they set the rpm based on the prop? But after reading this thread I'm thinking that's not correct. I will need to go confirm for sure but IIRC I see ~2600 on take off. (I live at sea level btw) Also as soon as I back off the throttle(at all) from wide open the rpm's will immediately settle at ~2500 or so. 

[Hank]

@brian1570, I have a Hartzell 3-blade on my C, and it gets pretty close to 2700 on takeoff, always has. I've owned her since '07, the prop was installed already. I'd be worried if it was noticeably less.

Also, my MP increases when I reduce RPM, generally an inch or so. I do not reduce anything until reaching my cruise altitude, so I can't offer any advice about the "after takeoff power reduction" other than "don’t!"

Since I reduce throttle then RPM then mixture, I also don't know what RPM does when reducing throttle (it's at redline on takeoff, and Idon't watch it that closely when slowing near my destination), but I don't recall seeing RPM change when advancing throttle (pushing forward is mixture first, then RPM then throttle; sometimes I'll hold prop and throttle together, crooked, so that prop advances further and reaches full well before throttle,  but still . . . .

[PT20J]

When you don’t get rated rpm during the takeoff roll just before liftoff, there are only a few possibilities:

1. The tach is reading low

2. The governor is set too low

3. The engine isn’t putting out rated power

4. The low pitch stops in the prop are set incorrectly

Skip

[carusoam]

Welcome aboard brian!

You came to the right place…

You asked the right question…

You got some great responses…

 

What’s next…

Some people have measured the difference in T/O length between 2500rpm and 2700rpm…   1,200’ vs. 800’… (kind of serious…)

Some people check their tach reading… old mechanical techs can be lazy…. There is an app for that… or light reflecting sensor for that…

Some people check their climb rate performance at full throttle and compare to book numbers…

Some people check the STC for their specific prop to see what RPM should be available for T/O power at sea level…  RPM will be stated clearly for your prop/engine combination…

 

Sooo….  There are a couple of things you can do yourself before going back to your mechanic with what you know…

If everything is working to book performance numbers, and your prop is actually spinning at 2700rpm, but your tach says something else…

Or everything was good last week, now the engine sounds funny, and I don’t want to fly it because the rpms aren’t where they should be… on my 1800’ runway…

Wonder why the prop isn’t set up properly any longer…? Check the logs to see the install notes… who did  what when…

 

There is a good plan for each challenge… 

PP thoughts only, not a mechanic…

Best regards,

-a-

 

[0TreeLemur]

Reading this thread, if MS had a FAQ section, this would be an excellent entry.  "My prop is not making 2700 rpm on takeoff, what should I do?"

Answer: what @PT20J typed above-

1. The tach is reading low

2. The governor is set too low

3. The engine isn’t putting out rated power

4. The low pitch stops in the prop are set incorrectly

Followed by:

This is a safety of flight issue.  You want your engine to produce maximum power at takeoff.  An engine not producing maximum power at takeoff reduces your options in the event of an engine failure during the takeoff climb.  Reducing power in the takeoff climb is not necessary.   Airspeed controls CHT much more effectively than power, and power reductions again limit options in the case of a loss of power.

If your engine is not making 2700 rpm on takeoff, diagnose and repair/adjust/replace so that it does.

[MBDiagMan]

Mine gives 2660 with everything forward on takeoff and climbout.  I worried about it at first but finally realized that if I still had a tachometer instead of the JPI, I would be reading it as 2700.

[brian1570]

  On 9/24/2021 at 1:44 AM, Hank said:

@brian1570, I have a Hartzell 3-blade on my C, and it gets pretty close to 2700 on takeoff, always has. I've owned her since '07, the prop was installed already. I'd be worried if it was noticeably less.

Also, my MP increases when I reduce RPM, generally an inch or so. I do not reduce anything until reaching my cruise altitude, so I can't offer any advice about the "after takeoff power reduction" other than "don’t!"

Since I reduce throttle then RPM then mixture, I also don't know what RPM does when reducing throttle (it's at redline on takeoff, and Idon't watch it that closely when slowing near my destination), but I don't recall seeing RPM change when advancing throttle (pushing forward is mixture first, then RPM then throttle; sometimes I'll hold prop and throttle together, crooked, so that prop advances further and reaches full well before throttle,  but still . . . .

Expand  

The POH for my E says to reduce power after take off(to 2550-2600 IIRC). I typically climb out at 25/25 which gets me around 700 fpm @ 120 or so mph. If I leave it firewalled all the way up initial climb is 1100 fpm or so till I start to loose MP to altitude. I usually don't like to run it that hard though. 

[brian1570]

  On 9/24/2021 at 4:00 PM, 0TreeLemur said:

Reading this thread, if MS had a FAQ section, this would be an excellent entry.  "My prop is not making 2700 rpm on takeoff, what should I do?"

Answer: what @PT20J typed above-

1. The tach is reading low

2. The governor is set too low

3. The engine isn’t putting out rated power

4. The low pitch stops in the prop are set incorrectly

Followed by:

This is a safety of flight issue.  You want your engine to produce maximum power at takeoff.  An engine not producing maximum power at takeoff reduces your options in the event of an engine failure during the takeoff climb.  Reducing power in the takeoff climb is not necessary.   Airspeed controls CHT much more effectively than power, and power reductions again limit options in the case of a loss of power.

If your engine is not making 2700 rpm on takeoff, diagnose and repair/adjust/replace so that it does.

Expand  

So since my last post I have paid a little closer attention to my rpm's and I'm actually making somewhere around 2650 rpm, which I think for now is acceptable. Especially since, as I stated above, my initial WOT climb out is at 1100 fpm.  I do however want to swap back to a two blade though. I don't like the harmonics of this three blade. On long flights it's pretty annoying. Something else I noticed was that my factory tach reads about 100 rpm lower than my JPI. So maybe I got it in my head at one point basing it off my factory tach and not my JPI. 

[Hank]

  On 9/28/2021 at 7:14 PM, brian1570 said:

The POH for my E says to reduce power after take off(to 2550-2600 IIRC). I typically climb out at 25/25 which gets me around 700 fpm @ 120 or so mph. If I leave it firewalled all the way up initial climb is 1100 fpm or so till I start to loose MP to altitude. I usually don't like to run it that hard though. 

Expand  

Do two climbs to altitude in similar weather, one WOT/2700 and one pulled back. Bet the reduced-power climb has hotter CHT and OilT.

Climbing with full power will push more cooling air through the cowling for cylinders, heads and oil. The higher climb rate gets you to altitude sooner, and up to cruise speed faster. So your engine sees climb stress for much less time. To me, that's kinder and gentler.

[brian1570]

  On 9/28/2021 at 7:46 PM, Hank said:

Do two climbs to altitude in similar weather, one WOT/2700 and one pulled back. Bet the reduced-power climb has hotter CHT and OilT.

Climbing with full power will push more cooling air through the cowling for cylinders, heads and oil. The higher climb rate gets you to altitude sooner, and up to cruise speed faster. So your engine sees climb stress for much less time. To me, that's kinder and gentler.

Expand  

Good point. I don't get the chance to try this often as my homefield is under the houston bravo. The next xc I do I'll try this out.

[A64Pilot]

  On 9/28/2021 at 7:46 PM, Hank said:

Do two climbs to altitude in similar weather, one WOT/2700 and one pulled back. Bet the reduced-power climb has hotter CHT and OilT.

Climbing with full power will push more cooling air through the cowling for cylinders, heads and oil. The higher climb rate gets you to altitude sooner, and up to cruise speed faster. So your engine sees climb stress for much less time. To me, that's kinder and gentler.

Expand  

Every piston engine I’ve experience with whether carburetor or fuel injected also goes into “power enrichment” at some point very close to full throttle, which means that it’s running extra rich.

‘I agree with your observation that at full throttle an engine will run cooler, but I believe it’s largely due to it being extra rich, that is what power enrichment is for after all, and or reduce detonation.

Even my 20 HP Suzuki outboard in my dinghy had power enrichment as I believe even modern cars do.