Rotax 912 cylinder temps

[Brett]

I’ve read the Rotax manual and know the max temps and so on but I’m chasing actual in field CHT temps.  At what point would you start to think the engine is getting on the warm side? 

 

I’ve just installed a 912 and only have 2.1 flight hours on the (second hand) engine.  I’ve only ever flown Rotax’s before mainly in Tecnam’s where they run nice and cool. 

 

What temps do people usually see in flight and climb? 

 

Thanks ? 

 

 

[M61A1]

Mine sit around 100-110 in cruise. On a hot day on continuous climb the highest I’ve seen is 130.  

 

Zenith 601HD

 

 

[Brett]

Well that’s somewhat reassuring,,, I was thinking mine were overly high.  Still very early days in my test flights though.  

 

Problem is if I ground run even for relatively short times it just continues to rise rather rapidly,,, in the air though temps come down. 

 

 

 

[skippydiesel]

Hi Brett,

 

I confess to being an "old fart" not used to interpreting electronic read outs so hopefully no offence will be taken.

 

I know the Sonex has the reputation of being a "hot" little ship specially when motivated by a 912 (80-100 hp ??) but  the numbers on the screen you have posted seem to be "all over the shop";

 

• Air Speed 134 knots - okay if speedy reputation actually true.
   
  
• Fuel flow 19.8 lph - seems very high for rpm - I only get this sort of consumption in climb out and about 18 lph in high speed cruise @ 5400 rpm (ground adjustable prop set to advantage climb) 912 ULS
   
  
• RPM 4927 - seems low for  the fuel burn, are you using a CS prop?
   
  

 

"Problem is if I ground run even for relatively short times it just continues to rise rather rapidly,,, in the air though temps come down." - sounds like an insufficient air flow issue, probably related to the shape/proximity of the prop where its "sweeps" past the cowl inlets, not generating enough air flow  - in the air, the forward movement generates enough air flow to keep engine cool.

 

 

[Brett]

Well picked up with the fuel flow per rpm ? 

 

-I have yet to fit a return flow sender so fuel flow is not reading correctly

 

-The current (amp) sensor is also not fitted so defaults to that number. 

 

-Running the recommended Sensenich fixed pitched prop,, (has too much pitch)

 

-Airspeed indicator was calibrated during my initial 40 hours test period and deemed to be within tolerances and also by flying the square and recording numbers and so on at all different heights etc etc.  I have only just started to do the new testing required for phase 1 ,,, I will revisit it but as nothing has been changed in regard to instruments or airframe besides engine so I doubt it’s reading incorrectly.  It’s definitely a bit faster with this engine over the Aerovee turbo but not by lots. 

 

What I’m  mostly worried about is the temp rising when waiting on the hold point or something like that... I need some form of better airflow on the ground. 

 

This is why why we have the test period to get the bugs sorted. 

 

 

[skippydiesel]

You might like to check out the blade shape on the Fiti props - I have one. They look (can be deceiving) like they are pumping air into the inlets.

 

I dont have any experience with other props but can say I am very happy with my Fiti two blade ground adjustable - it just seems to deliver the goods.

 

 

[Brett]

Ok I will,,   I really wanted to keep it simple though. Fixed fitch and standard spinner to avoid unnecessary cowling work. 

 

 

[skippydiesel]

Ok I will,,   I really wanted to keep it simple though. Fixed fitch and standard spinner to avoid unnecessary cowling work. 

 

There is a Fiti CS but my prop is a ground adjustable, which gives you the chance to "fine tune" the prop pitch to your application (its still a fixed pitch in flight)  - there should be no change to spinner or cowling. Where the Fiti design differs from many other props seems to be in the "twist" and cord width (broad blade). This may just "pump" a bit more air through your cowling when on the ground.

 

 

 

 

[skippydiesel]

Just thought I might show you a pic of a similar aircraft to yours ATEC Solo,  fitted with a 2 blade Fiti (I am the agent for ATEC but not Fiti)

 

 

 

[Downunder]

My oil temp is generally equal to or slightly higher than cht. Cht's generally in the 90's and I do push the rpm's.

 

Rarely do I reach 100 c cht but do go above in 100 c oil temps in climb....

 

I personally wouldn't like cht's much above (if at all) 100 c. 

 

If you are getting that now, summer is going to be difficult.

 

Course pitching is know to give higher temps. Especially oil.

 

 

[M61A1]

Just thought I might show you a pic of a similar aircraft to yours ATEC Solo,  fitted with a 2 blade Fiti (I am the agent for ATEC but not Fiti)

 

[ATTACH]39656[/ATTACH]

 

The Sensenich props are a similar profile to the prop you show there.

 

http://www.sensenich.com/shop/aircraft/2-blade-rotax-ground-adjustable-propeller/

 

 

[M61A1]

Well picked up with the fuel flow per rpm ? 

 

-I have yet to fit a return flow sender so fuel flow is not reading correctly

 

-The current (amp) sensor is also not fitted so defaults to that number. 

 

-Running the recommended Sensenich fixed pitched prop,, (has too much pitch)

 

-Airspeed indicator was calibrated during my initial 40 hours test period and deemed to be within tolerances and also by flying the square and recording numbers and so on at all different heights etc etc.  I have only just started to do the new testing required for phase 1 ,,, I will revisit it but as nothing has been changed in regard to instruments or airframe besides engine so I doubt it’s reading incorrectly.  It’s definitely a bit faster with this engine over the Aerovee turbo but not by lots. 

 

What I’m  mostly worried about is the temp rising when waiting on the hold point or something like that... I need some form of better airflow on the ground. 

 

This is why why we have the test period to get the bugs sorted. 

 

Your prop needs to be pitched to get 5500-5600 at wide open throttle straight and level at your preferred cruise altitude. Being over pitched will raise temps. Are your heads the late model ones that have the temp probes in the coolant or the earlier ones that actually read cht?

 

Do you have a coolant temp readout?

 

What sort of coolant do you run? 

 

I am currently using Evans waterless, which will result it higher temps, than 50/50 water/concentrate.

 

if you only have the earlier cht only probes, Rotax require you to have a coolant temp display also unless using waterless.

 

 

[skippydiesel]

The Sensenich props are a similar profile to the prop you show there.

 

http://www.sensenich.com/shop/aircraft/2-blade-rotax-ground-adjustable-propeller/

 

I dooont think so - check out the unmounted photos. Fiti 2 blades are particularly wide, especially near the root - can be seen well in the side shot of my cowling - hence my suggestion that they may be pumping a bit more air into the cowling openings when aircraft on the ground.

 

 

[skippydiesel]

Your prop needs to be pitched to get 5500-5600 at wide open throttle straight and level at your preferred cruise altitude. - is this a Rotax recommendation??

 

Being over pitched will raise temps. - agreed that this is possible but hardly likely when idling/taxing on the ground

 

What sort of coolant do you run? - ugh! as long as the coolant meets the Rotax standards it should have no bearing on the ground temps.

 

I am currently using Evans waterless, which will result it higher temps, than 50/50 water/concentrate. - he is looking for lower ground temps.

 

if you only have the earlier cht only probes, Rotax require you to have a coolant temp display also unless using waterless - relevant?.

 

 

 

[M61A1]

1. Yes Skip, that is straight from the Rotax Owners site. 5500-5600 WOT

 

2. The reason I asked about the coolant type is that  Evans Waterless runs up to 30° hotter, which works for some people but not others. If the OP is using Evans,  it would be advantageous to switch to a water/glycol mix.

 

3. I asked about coolant temp display because it is useful to know if coolant temp is high up there with your CHT. It can help determine whether you radiator is not cooling or cowling air is not cooling.

 

4. The outlets on you cowl will have far more effect than the inlets of your cowl.

 

5. As for the Sensenich. Here is a photo at a similar angle to your  Fiti. I recently installed a second hand one. the chord near the root is quite broad with a good helix.

 

Edit:

 

 I since reread the OP and he has a fixed pitch one. So I guess that not really relevant.

 

 

 

[Brett]

Thanks for the reply

 

Yes I also have a coolant temp sensor,,, sits at about 100 on climb and 85 in cruise

 

I use normal 50/50 coolant

 

Its and older model engine with CHT sensors

 

I do realise the prop needs work but that certainly won’t happen overnight being a Sensenich prop

 

The cruise temps aren’t that bad, I mostly have issues on ground runs. If I muck around too much it gets quite high CHT,,, but also the oil isn’t warm enough sometimes for takeoff ,,,, not good... I would hate to have an extended delay before take off... not very optimal.  Somehow I need better airflow on Taxi.  

 

The prop is like a canoe paddle,,, I’m actually quite surprised it doesn’t blast the air through.... 

 

 

[Downunder]

Fitting an oil thermostat may help to bring the oil temp up faster.

 

https://www.aircraftspruce.com/catalog/eppages/xcomthermostat.php

 

 

[corvairkr]

G’day Brett,

 

I’ll lay money on your problem being your ducting to your radiator not allowing enough airflow.

 

i’ve recently had to mount my radiator vertically on the engine mount near the Firewall on my 601 HDS and had the same problem. I originally had a Plenum over the radiator fed  by various inlets but eventually  had to make a NACA duct feeding directly to the face of the radiator to allow effective ground cooling. 

 

It now runs waaay to cool but that’s a simple fix with some Aluminium tape. 

 

Jason 

 

 

 

[M61A1]

I’ll lay money on your problem being your ducting to your radiator not allowing enough airflow

 

 

 

Based on post #16 I would have to agree Jason.  I put my radiator underneath and while my CHT is relatively high (100-110) my coolant temps never get above 80.

 

My radiator inlet is quite small, but I worked on the 500% rule ( outlet should be 500% of inlet)

 

I suspect my my CHT is as high as it is because the cowl doesn't allow enough air out. It is quite pressurised in flight and even on ground runs it will blow the inspection door open (at idle) if it isn't fastened. But my temps are well within limits so I haven't bothered to fiddle with it.

 

In the pic below the whole rear of the cooling unit is gilled while the radiator lays almost flat.

 

I like how yours is done, I considered doing it that way, but I didn't feel like doing any more work on the cowl. 

 

[skippydiesel]

1. Yes Skip, that is straight from the Rotax Owners site. 5500-5600 WOT - Sooo how do you balance that with a recommended static minimum rpm of  2200 and same advice for climb out.

 

2. The reason I asked about the coolant type is that  Evans Waterless runs up to 30° hotter, which works for some people but not others. If the OP is using Evans,  it would be advantageous to switch to a water/glycol mix. - Agreed !

 

3. I asked about coolant temp display because it is useful to know if coolant temp is high up there with your CHT. It can help determine whether you radiator is not cooling or cowling air is not cooling. - May need to expand on this idea.

 

4. The outlets on you cowl will have far more effect than the inlets of your cowl. - True but you still have to get that air moving through in the first instance (chicken & egg). Insufficient air movement insufficient cooling. Inlet size/shape & proximity to fan should be studied carefully. Internal restrictions to flow must not be overlooked and then there is the exit air vents. My guess is that  the cowl being originally designed for an air cooled donk needs considerable design work to accommodate a radiator.

 

5. As for the Sensenich. Here is a photo at a similar angle to your  Fiti. I recently installed a second hand one. the chord near the root is quite broad with a good helix. - Looks the goods/ very pretty. Metal ?? The Fiti is composite.

 

Edit:

 

 I since reread the OP and he has a fixed pitch one. So I guess that not really relevant.

 

[ATTACH]39682[/ATTACH]

 

 

 

[facthunter]

 There's a fair chance the airflow near the spinner is stalled at no forward speed. I've always wondered whether angling the dual intakes to take advantage of the rotating nature of the airflow under those conditions, would aid cooling on the ground.

 

 Regarding head temps of 130 C   (without using Evans)  isn't there a distinct possibility of hot spots and excess pressurizing. and consequent coolant loss.. I've had the "green stuff" on the cowl and window a few times ..Nev

 

 

[M61A1]

Thanks for the reply

 

Yes I also have a coolant temp sensor,,, sits at about 100 on climb and 85 in cruise

 

I use normal 50/50 coolant

 

Its and older model engine with CHT sensors

 

I do realise the prop needs work but that certainly won’t happen overnight being a Sensenich prop

 

The cruise temps aren’t that bad, I mostly have issues on ground runs. If I muck around too much it gets quite high CHT,,, but also the oil isn’t warm enough sometimes for takeoff ,,,, not good... I would hate to have an extended delay before take off... not very optimal.  Somehow I need better airflow on Taxi.  

 

The prop is like a canoe paddle,,, I’m actually quite surprised it doesn’t blast the air through.... 

 

[ATTACH]39683[/ATTACH]

 

[ATTACH]39684[/ATTACH]

 

What does the cooling exit look like? I can't see much there. you may benefit from a lip on the exit to aid creating a low pressure and draw out the cowling air. The inlets look plenty big enough.

 

 

[M61A1]

Looks the goods/ very pretty. Metal ?? The Fiti is composite.

 

Skip, the Sensenich ground adjustable are hollow composite blades. It weights a little less than the Warp Drive I removed but most of the weight is in the hub, so it has much less inertia than the Warp Drive. 

 

I probably would have just bought a Bolly but this one came along at an affordable price. I replaced the Warp Drive after seeing some pics of a Sling in the desert with a departed blade and then finding out that this was more common than I'd like.

 

 

[Brett]

The cooling exit has been increased and I have a significant lip on the outlet... The Aerovee turbo always had nice cool temps with the same outlet but even so I increased it substantially bit by bit.  

 

I will try and make a better more streamlined intake plenum and if that doesn't help I will go with the naca duct idea. Another builder has done that with success so maybe the path to take. I just don't want to chop huge holes in the cowls if I dont need to. 

 

Thanks for all the replies... 

 

 

[skippydiesel]

1. Yes Skip, that is straight from the Rotax Owners site. 5500-5600 WOT - Sooo how do you balance that with a recommended static minimum rpm of  2200 and same advice for climb out.

 

I am surprised I was taken to task on this - should have been 5200 rpm - my craven apologies.