Need to build 1,000 hrs. Cruise @ 4,200 RPM?

[gogogo888]

Hi,

My CTSW prop is set at 5,500 RPM WOT.

Various forum said that Rotax 912 should be cruising at 5,100 RPM to prevent crankshaft failure.

I need to build 1,000 hours toward my ATP,  where fuel saving is my main concern.

The POH said the normal cruising RPM is 4,200 - 5,200.

My question is, is cruising at 4,200 RPM safe for the engine?

[Roger Lee]

The answer basic answer is no. The manuals are written for 3 different engines (912UL, 912ULS and the 914) and for worldwide use. Not just for the US and or fixed pitch props. A lot of the world gets to use in flight adjustable props. Remember too that the compression ratio also affects this answer. The 912UL and 914 is a 9: 1 and the ULS is a 10.8: 1 (just call it 11:1). The higher compression will generate more pulsation and or vibration through the engine.

The 912 engine was designed to run in the 5K's for normal cruise times. It was never designed to run all the time in the 4K's. It runs leaner in the mid 4K's and has more internal vibration of which you can not feel. You can run in the 4's with no issue like when setting up approaches or other short duration needs.

Fixed pitch props. Your number above at 5800 rpm is great for a climb prop or when using a variable pitch prop during take off, but if left there you loose speed and fuel economy.

As an example with your numbers above. The 912ULS should never be run down at 1400 rpm. It will beat the gearbox up. The 912UL isn't as bad, but even that said it should not be idled down that low you any time.

Then the normal climb of 4800 is contradicted in SB's and Rotax technically would like to see 5200 rpm in climb, but that's more aimed at adjustable pitch. If you were able to climb out at 5200 rpm then your WOT in level flight would be at or above 5800 rpm.

So the bottom line is we have to interoperate some numbers and apply them to not only the correct engine, but our uses from engine to engine, plane to plane and prop setup and even other countries allowance i.e. in flight adjustable props.

If you want to cruise at a low rpm ALL the time try using 5000. Rotax and Bing have the carbs set up to run richer the higher in rpm you go to protect the engine.

 

[gogogo888]

16 minutes ago, Roger Lee said:

The answer basic answer is no. The manuals are written for 3 different engines (912UL, 912ULS and the 914) and for worldwide use. Not just for the US and or fixed pitch props. A lot of the world gets to use in flight adjustable props. Remember too that the compression ratio also affects this answer. The 912UL and 914 is a 9: 1 and the ULS is a 10.8: 1 (just call it 11:1). The higher compression will generate more pulsation and or vibration through the engine.

The 912 engine was designed to run in the 5K's for normal cruise times. It was never designed to run all the time in the 4K's. It runs leaner in the mid 4K's and has more internal vibration of which you can not feel. You can run in the 4's with no issue like when setting up approaches or other short duration needs.

Fixed pitch props. Your number above at 5800 rpm is great for a climb prop or when using a variable pitch prop during take off, but if left there you loose speed and fuel economy.

As an example with your numbers above. The 912ULS should never be run down at 1400 rpm. It will beat the gearbox up. The 912UL isn't as bad, but even that said it should not be idled down that low you any time.

Then the normal climb of 4800 is contradicted in SB's and Rotax technically would like to see 5200 rpm in climb, but that's more aimed at adjustable pitch. If you were able to climb out at 5200 rpm then your WOT in level flight would be at or above 5800 rpm.

So the bottom line is we have to interoperate some numbers and apply them to not only the correct engine, but our uses from engine to engine, plane to plane and prop setup and even other countries allowance i.e. in flight adjustable props.

If you want to cruise at a low rpm ALL the time try using 5000. Rotax and Bing have the carbs set up to run richer the higher in rpm you go to protect the engine.

 

 

 

 

Thank you Roger, I will keep my RPM running at 5000.

[GrassStripFlyBoy]

You picked a good plane and engine to rack up some hours, the other factor in shaving costs is what fuel to run.  If I was burning through it as quick as you will be, I'd consider buying 93 unleaded and mixing with 89 at a 50/50 split to land on the 91 oct mark, certainly in the cooler months.  I use straight 93 these days, but this might pull a dollar to buck fifty off each hour compared to all 93, and far more if you're buying rec fuel.  100ll and the 25 hour oil changes is the last option here.  Obviously you don't have to "mix" the gas in containers, just keep track of what the ratio is becoming in the tanks, and keep each tank above 91 min.

[gogogo888]

14 minutes ago, GrassStripFlyBoy said:

You picked a good plane and engine to rack up some hours, the other factor in shaving costs is what fuel to run.  If I was burning through it as quick as you will be, I'd consider buying 93 unleaded and mixing with 89 at a 50/50 split to land on the 91 oct mark, certainly in the cooler months.  I use straight 93 these days, but this might pull a dollar to buck fifty off each hour compared to all 93, and far more if you're buying rec fuel.  100ll and the 25 hour oil changes is the last option here.  Obviously you don't have to "mix" the gas in containers, just keep track of what the ratio is becoming in the tanks, and keep each tank above 91 min.

Never thought of mixing the 89 and 93, I've been using straight 93 for the past 200 hours.

I've done the saving calculation between running at 4200 rpm and 5000 rpm, running at 4200 rpm will save $3,000 for 1,000 hours.

But if I mix the 89 and 93 fuel and run at 5000 rpm, I will save about $1,500 flying 1,000 hours.

Seems like this is a good compromise!

 

[Roger Lee]

If you mess up your engine or just the gearbox alone it will cost $3K. I just had to repair a poorly maintained gearbox. It was exactly $3K. So in that case nothing was saved. Just be careful not to cause a long term issue to save a few bucks. It could cost more in the long run.

Stay at above 5K rpm in normal cruise, use the right oil, maintain  the engine properly and you'll have all the hours you want with no issues.

[gogogo888]

13 minutes ago, Roger Lee said:

If you mess up your engine or just the gearbox alone it will cost $3K. I just had to repair a poorly maintained gearbox. It was exactly $3K. So in that case nothing was saved. Just be careful not to cause a long term issue to save a few bucks. It could cost more in the long run.

Stay at above 5K rpm in normal cruise, use the right oil, maintain  the engine properly and you'll have all the hours you want with no issues.

I've been running 4200 rpm the past 200 hours. I hope no damage is done. Will start running at 5,100 rpm and mix 89 and 93 unleaded 50/50.

This seems like the best way to save money.

 

 

[FlyingMonkey]

I run 5000rpm when just puttering around (let's face it, 80% of my flying time).  You're only burning 4gph or so there, that's about as economical as an airplane gets.  And at 5000rpm you can actually go places, I get about 106-110kt with that rpm, plenty fast to go fun places while you're building your hours.

[gogogo888]

3 hours ago, FlyingMonkey said:

I run 5000rpm when just puttering around (let's face it, 80% of my flying time).  You're only burning 4gph or so there, that's about as economical as an airplane gets.  And at 5000rpm you can actually go places, I get about 106-110kt with that rpm, plenty fast to go fun places while you're building your hours.

 

Back when I was running at 4,200 RPM @ 85kt, I was burning around 3.5 gph. Since Roger said running less than 5,000 rpm is bad for the engine, I'm running at 5,000 rpm now,  I burn around 5gph, I get about 105kt.

If I go full power, RPM goes up to 5,500 and I can get around 120kt-125kt, gas burn is around 6.5gph.

I agree that burning 4-5gph at 5,000 RPM is very economical.

 

[Roger Lee]

5 gpm at 5K rpm is a little high. I would confirm that manually with dipping the tank over 4-5 flights. My bet is you'll need to adjust the "K" factor in the Dynon D120. 

[Tom Baker]

Also if you can only turn 5500 full throttle you could stand a little less prop pitch. That would also lower your fuel burn at 5000 RPM.

[gogogo888]

So far i have done about 2 long flights @ 5k rpm, it seems like I’m buying gas more often. I will test a few more flight to confirm it is indeed burning 5gph.

 

To add more info. I’m using e-props, my take off ground roll gets up to 5,700 rpm and climb at 5,400 as soon as I lift off.

[gogogo888]

50 minutes ago, Tom Baker said:

Also if you can only turn 5500 full throttle you could stand a little less prop pitch. That would also lower your fuel burn at 5000 RPM.

Will take out 100-200 rpm if the plane is confirmed burning 5gph. Hopefully it will burn less gas.

[Roger Lee]

14 hours ago, gogogo888 said:

So far i have done about 2 long flights @ 5k rpm, it seems like I’m buying gas more often. I will test a few more flight to confirm it is indeed burning 5gph.

 

To add more info. I’m using e-props, my take off ground roll gets up to 5,700 rpm and climb at 5,400 as soon as I lift off.

You need more prop pitch. You have a great climb prop,  but a not so good cruise prop. The pitch on this is way too flat. This climb prop pitch is costing you cruise speed and fuel economy. Most of us get around 4950 - 5050 in climb. WOT in level flight is well balanced at 5600-5650. With your numbers in climb I'd love to know what WOT is in level flight. Don't let it go over 5800 WOT. If you can see 5400 WOT in climb my bet is you'll be over the 5800. If so you may be even loosing climb performance.

[gogogo888]

51 minutes ago, Roger Lee said:

You need more prop pitch. You have a great climb prop,  but a not so good cruise prop. The pitch on this is way too flat. This climb prop pitch is costing you cruise speed and fuel economy. Most of us get around 4950 - 5050 in climb. WOT in level flight is well balanced at 5600-5650. With your numbers in climb I'd love to know what WOT is in level flight. Don't let it go over 5800 WOT. If you can see 5400 WOT in climb my bet is you'll be over the 5800. If so you may be even loosing climb performance.

Yes, my WOT level flight setting is 5,500 rpm, again, I’m using e-props with the recommended setting. 

[gogogo888]

I also reached out to e-props and asked them for recommendations on RPM settings. Here is the reply from the CEO. What does everyone think?

 

 

 

 

In order to consume as little as possible, your engine should not be running at 5000 rpm.
I'm sorry to tell you this, but it's not true: Rotax engines should not be running at 5000 rpm to extend their life.

Read the Rotax documentation again. For example this document : https://aircraft.e-props.fr/DOCS/PA_RPM_912s.png

Rotax states that you can fly for example at 4300 rpm with an intake pressure of 24 in.HG, for very long periods of time and without any worries. 
Since it is Rotax data, it is obvious that it should not damage the engine.

Are you sure about what your mechanic told you? Because I don't know of any pilot or school that requires you to fly at a minimum of 5000 rpm on a Rotax 912 series.

To consume little and keep a good cruising speed, we advise you to adopt 4.800 rpm(65% of the power), with an intake pressure of 26 in.HG. These are the parameters of all our customers who make long journeys with their VL3, CT, SHARK, SAVANNAH, etc.

If you do not have an intake pressure gauge, the ideal setting of 5500 rpm full throttle in horizontal flight for your E-Props propeller ensures that you have 26 in.HG when you have 4800 rpm. It is calculated and tested.
And you'll have excellent speed performance without consuming too much fuel.

I hope I have been helpful, and I wish you very nice and long flights !


 

Have a nice day ! / Bonne journée ! 
Best regards / Bien cordialement, 

Anne LAVRAND 
Présidente / Direction Commerciale 
CEO / Commercial Manager

[Roger Lee]

" I’m using e-props, my take off ground roll gets up to 5,700 rpm and climb at 5,400 as soon as I lift off."

These take off rpm's aren't in line with a prop that only gets 5500 WOT in level flight. One side of this or the other is off unless you take off super flat and no flaps.

[Madhatter]

I've been using e- prop for while now, climb is 5200 and wot is 5500. Anything over or under 5500 wot, it underperforms. 

[gogogo888]

4 hours ago, Roger Lee said:

" I’m using e-props, my take off ground roll gets up to 5,700 rpm and climb at 5,400 as soon as I lift off."

These take off rpm's aren't in line with a prop that only gets 5500 WOT in level flight. One side of this or the other is off unless you take off super flat and no flaps.

I've done more testing today, my WOT in level flight is 5500, but can slowing go up to 5600, I pull the power back once it slowly gets up to 5550, never tried to hit 5600, but I assume it can very slowly get up to 5600.

My climb with 15 or 0 flaps below 80 kts is 5350 rpm, when I get to -6 flaps, climbing is around 5200 rpm.

 

[gogogo888]

4 hours ago, Madhatter said:

I've been using e- prop for while now, climb is 5200 and wot is 5500. Anything over or under 5500 wot, it underperforms. 

What is your fuel burn at 4300, 4800 rpm and 5000 rpm?

[207WF]

looks like from the dynon shot that your pitch is in the ball park.

My CTSW would do about what you are at that rpm in cruise, but at a lower fuel flow.  Your higher flow suggests your prop is just a little bit too coarse.

I like 5200 rpm where I get 112 knot true and 4 2/3 gph.  Suggest you try just a tiny bit flatter on the prop.

WF

[gogogo888]

5 minutes ago, 207WF said:

looks like from the dynon shot that your pitch is in the ball park.

My CTSW would do about what you are at that rpm in cruise, but at a lower fuel flow.  Your higher flow suggests your prop is just a little bit too coarse.

I like 5200 rpm where I get 112 knot true and 4 2/3 gph.  Suggest you try just a tiny bit flatter on the prop.

WF

Agreed, I think I need to take 100 rpm off and see if it helps with the fuel burn.

[GlennM]

There is an interesting video by Mike Busch about flying on $7 aviation fuel.  He brings up the Carson speed, which is the 1.319 or something over best L/D. This was developed by the US Navy for getting someplace with the most efficiency,  not the longest endurance.

 

I takes best L/D to be glide speed because best range speed is not published in the CTSW.  Therefore, it is about 83 kts, which is close to your 4200rpm.

 

If Rotax publishes rpm range of 4200-5500 continuous,  you are in that range and should be ok. 

 

 

[gogogo888]

On 8/26/2020 at 2:47 PM, GlennM said:

There is an interesting video by Mike Busch about flying on $7 aviation fuel.  He brings up the Carson speed, which is the 1.319 or something over best L/D. This was developed by the US Navy for getting someplace with the most efficiency,  not the longest endurance.

 

I takes best L/D to be glide speed because best range speed is not published in the CTSW.  Therefore, it is about 83 kts, which is close to your 4200rpm.

 

If Rotax publishes rpm range of 4200-5500 continuous,  you are in that range and should be ok. 

 

 

Thanks for the Carson speed 83kt tip, it is very close to the speed I'm flying now.

I'm using the e-props CEO recommendation of 4,300 RPM, he got the data from Rotax, since it is Rotax' own data, it should be safe to use for the engine and the gearbox.

I get that many people are recommending 5,000 RPM, but looking at Rotax' own data, I am willing to take the risk and run the engine at 4,300 RPM, my gear box will need to be inspected in about 200 hours, we will see if the 4,300 RPM works.

[GlennM]

I would be surprised if you had a problem. In my thinking, if there was a big issue at a given rpm, it would be listed and placarded against going there.  Some planes have a range you cannot sit in that is less than red line.  You can transition through, of course, but not stay there. I can't remember an example. I think Mooney M20, but cannot be certain. Since Rotax publishes 4200-5500 rpm cruise, I would not worry.  Phil Lockwood said the biggest issue he sees with limiting the life of the 912 is the exhaust valves and that does not seem to be rpm related at first glance.  The rest is pretty robust.