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

[Bill3558]

How do we protect our exhaust valves? 

[BravoFoxtrot]

The only reference that I have found is from the AeroTrek Inspection Checklist. They use the same 912ULS as the CT.

"If one or more of the compressions are low -- and in the case of Rotax engines low means less than 82/87 -- then
listen carefully at the exhaust while carry out the leak down check on that cylinder. On engines that have used
significant amounts of leaded fuel (avgas), a build up of lead deposits on the exhaust valve stem will eventually hold
the exhaust valve open. This is heard as a hiss within the exhaust muffler during the check. If this symptom is found,
then early remedial action will save the valve and seat, if not then a more expensive repair will be necessary later.
Remove the head and lap the valve in as detailed in the Rotax maintenance manuals."

Would seem from the above that if you use unleaded gas it would not be a problem. Hopefully others with more knowledge on this will contribute.

[GlennM]

https://www.youtube.com/watch?v=jOsCBrQMzWY

Discussion of TBO from Phil's experience starting at 37 minutes in.

 

I think running unleaded gasoline is the only thing you can do.  It seems to me valves and guides are replacement parts over time, like cylinders on air-cooled engines.

[AGLyme]

Originally, my goal was to support my airport and buy Avgas and add Decalin.  I changed my mind after reviewing posts on this board from veteran Rotax owners, and, asking the question at the Rotax maintenance course.  I only use 100LL if I absolutely have to while traveling. 

[okent]

Someone correct me if I'm wrong but I think I had read where if you're burning 100LL the rpm shouldn't be run continuously under 4700.

[FlyingMonkey]

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. 

 

 

I think your math is off.  Your 83kt number only works if you are using the best L/D number of 62-62kt, which is the number at 15° flaps.  I doubt you are cruising at 15° flaps all day.  The best L/D number at -6° flaps is around 78kt, so that makes your Carson speed ~102kt.

[FlyingMonkey]

As for what rpm to run the engine, my answer would be "the rpm to meet the current needs of the flight".  I have flown at 5500rpm for hours straight on long cross-country flights keeping up with faster airplanes, and also at 3800rpm for pretty long periods keeping formation with slower airplanes.  I try to keep most of my flying around 5000rpm unless I'm trying to go long distances, then it's 5300-ish rpm.

It all kind of balances out, and I do try to occasionally vary the rpm so it's not sitting at the high end or low end of the rpm range without change for hours.  If how I operate the engine means I might need to put some money into additional maintenance later I'm cool with that, if that means that I get to do the flying I want to do.

My airplane currently has 800 hours and runs fantastically well.  In another 200hrs I will pull the gearbox for overhaul, and when that happens I'll let you all know if my "reckless behavior" causes any problems.  I'm betting it will be fine.

[GlennM]

9 hours ago, FlyingMonkey said:

I think your math is off.  Your 83kt number only works if you are using the best L/D number of 62-62kt, which is the number at 15° flaps.  I doubt you are cruising at 15° flaps all day.  The best L/D number at -6° flaps is around 78kt, so that makes your Carson speed ~102kt.

Here is a quote from Wikipedia of lift to drag ratio:

 

 As it turns out, the glide ratio, which is the ratio of an (unpowered) aircraft's forward motion to its descent, is (when flown at constant speed) numerically equal to the aircraft's L/D.  That is what I used for a CTSW.

[Tom Baker]

On 8/19/2020 at 6:37 PM, 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.

I was just reading back through the thread. If you are making long flights and starting with full fuel you could be losing some overboard through the vents. That could increase your fuel usage.

[Tom Baker]

10 hours ago, GlennM said:

Here is a quote from Wikipedia of lift to drag ratio:

 

 As it turns out, the glide ratio, which is the ratio of an (unpowered) aircraft's forward motion to its descent, is (when flown at constant speed) numerically equal to the aircraft's L/D.  That is what I used for a CTSW.

I remember that when I started with the CT's in 2007 they advertised that you could fly 1000 miles in a CT. The speed to achieve that was right around 100 knots. The fuel flow at 100 knots does seem to indicate that you could indeed fly 1000 miles with full fuel. From watching the numbers any slower or faster would reduce the range. I agree with Andy that you picked the numbers from the book for 15° flaps, and for -6° flaps the speed should be higher.

[FlyingMonkey]

1 minute ago, Tom Baker said:

I remember that when I started with the CT's in 2007 they advertised that you could fly 1000 miles in a CT. The speed to achieve that was right around 100 knots. The fuel flow at 100 knots does seem to indicate that you could indeed fly 1000 miles with full fuel.

But who would want to?!?  My old back and bladder won't stand up to that level of abuse! 

I will admit to successfully using an empty Gatorade bottle as a "range extender" on one particular long flight back from out west, but it definitely required the autopilot and a fair degree of engineering and contortion...  

[Tom Baker]

1 minute ago, FlyingMonkey said:

But who would want to?!?  My old back and bladder won't stand up to that level of abuse! 

I will admit to successfully using an empty Gatorade bottle as a "range extender" on one particular long flight back from out west, but it definitely required the autopilot and a fair degree of engineering and contortion...  

Not me, but I have a friend that flies like that. He has crossed the country several times in a T6, he goes high, pulls the power way back, leans it out, and just hangs out. He says he can get the fuel burn under 25 gallons per hour. 

[FlyingMonkey]

5 minutes ago, Tom Baker said:

He says he can get the fuel burn under 25 gallons per hour. 

And who says aviation is expensive?!?  

[gogogo888]

17 minutes ago, Tom Baker said:

I was just reading back through the thread. If you are making long flights and starting with full fuel you could be losing some overboard through the vents. That could increase your fuel usage.

You are right!

I did see some fuel leak when I was fueling it up to 17 gallon, I'm only fueling it up to 16 gallon now.

I buy 24 gallon of gas every 6.5 hour, that is about 3.7 GPH cruising at 4300 RPM and 2 touch downs per 3 hour.