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Cruise at 8500'

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David-aviator

Well Known Member
Now that things are settled down it is time to get some performance numbers before blasting off to OSH. One needs to know how much fuel it will take to get there and how much loiter fuel will remain. You never know about that place during the convention. :)

Don't ask me why, but I like to cruise the 0360 180hp at 8 gallons an hour and take whatever speed it produces. A previous airplane yielded 160 knots but it was much different being a canard. I know this machine won't do that well but that's the trade off with the total performance concept invented by Van.

I took it up to 8500 as the base line and spent a few minutes on the auto pilot getting the thing stabilized. EGT was leaned to peak and then to 50 ROP, which I believe is best power (but it probably isn't with a fixed pitch prop running at partial throttle).

The GPS settled down and three runs resulted in 171, 173 and 134 which turned out to be 151 KTAS. Not bad and that's at about 52.26% power based on fuel flow at a BSFC of .51. (174 mph yielding 21.75 mpg in still air)

Tomorrow I will crank it up to 60% and 65% power (9.2 and 10gph) and see what that yields. No sense being stuck in a rut at 8 gph. The Lycoming sure can't complain about cruising at 60 or 65% and maybe the mpg won't fall off all that much. We will see.
 
Yes, you can..

..cruise at 160 kts on 8 gph.

Mine does 160 kts (3-way NTPS GPS verified) at 8 gph at 8,000 D.Alt. That is 50 degF LOP, but you have roughly the same engine and prop.

At 50 ROP I'm getting an SFC of about .48. At 50 LOP, same 8 gph, I'm getting about .40 SFC. That's a mpg benefit of 20%, cooler CHT, etc.

These numbers are for Superior IO-360+. Lycoming numbers are similar but not quite as good. But yours is not an ordinary Lycoming. You have different sump and FI but arguably better.

In the summertime, 8500 DAlt comes at lower MSL, so you might do even better at 8500 MSL or higher.
 
I took the flying machine up to 8500' MSL again today and this time noted the density altitude, Dynon said it was 10,480'. I checked that with an online DA calculator and it is close - OAT was 53F (12C) and SL pressure was 29.78". I did 3 runs.

52% power (8 gph) yielded 153 KTAS this time or 176 mph = 22 mpg.

60% power (9.2 gph) yielded 160 KTAS or 184 mph = 20 mpg.

65% power (10 gph) yielded 165 KTAS or 190 mph = 19 mpg.

I think I have enough information to get to anywhere I care to go to and know how much fuel it will take. As always, the slower you go, the better the mileage. A 20% increase in fuel burn going from 52% to 65% power yields only an 8% increase in speed. That's probably consistent with theories about drag going up by some root factor with increase in speed. (I am no engineer)

I do like the way the Lycoming is running - magnetos and all. :)

All this info is subject to verification. The speed numbers are in Van's ball park but as always, do some testing of your own airplane to confirm what it will do. I don't want anyone running out of fuel on account of what is reported here.
 
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Two little issues

1. You can run LOP and get much better mileage, easier on the engine, etc.
2. The wind makes at least as much difference as power settings, often more.
 
Carb vs FI for LOP...

Carbed engines do not distribute fuel as efficiently as fuel injected engines. Running LOP with a carb is a riskier proposition since your cylinders could have varying fuel/air mixture supply.

Be careful.
 
If you don't have EGT/CHT sensors on each cylinder to catch it, you can have one (or more) cylinders running richer than others, in the peak-EGT range, resulting in higher CHT's and possibly damaging the cylinder or valves. Fuel injection gives a more consistent mixture distribution than a carb and lessens this problem.
 
And.....

Great answer Greg,

And, to pile on, because of the inefficiencies of the carb manifold system, you could be monitoring accurately your EGT/CHT gauges on your multi probe aircraft and then BANG! they change because there is no way fuel distribution can be consistantly uniform in such a system. I say BANG because you can easily cook a cylinder. The range from LOP and TOO **** HOT is too close for my engines comfort!

Even in an FI engine, unless the jets are properly tuned, even then you can get un-uniform flow rates that will hurt significantly your ability to safely LOP. Most of the LOP horror stories are in these situations.

Before anyone thinks I am a LOP basher, far from it. In my RV-6 I have a Monty Barrett IO-360 and it is sweetly tuned (as all MB engines are!!!) and enjoy rediculusly low fuel flows for the great performance attained. However, to save the cost and complexity of an FI system, I employed a carb on my new O-360 powered RV-4 and, while I have achieved closer tolerances with my EGT/CHT temps than I thought I could, I believe LOP is mostly unatainable in my -4 and will be happy with the $$$$ savings in parts for the carb system used vice FI and stay ROP.

FYI, a local cylinder guru LOVES us airplane owners running LOP because he has really been able to increase his repair business with worn out cylinders and valve guides. LOP ops is GREAT if applied correctly, actually better than not, but can be quickly disasterous if entered with a small clue bag!

Scary thought: Some pilots are running LOP with just ONE EGT/CHT probe...YIKES!

Just my opnions of course:rolleyes:
 
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airguy said:
If you don't have EGT/CHT sensors on each cylinder to catch it, you can have one (or more) cylinders running richer than others, in the peak-EGT range, resulting in higher CHT's and possibly damaging the cylinder or valves. Fuel injection gives a more consistent mixture distribution than a carb and lessens this problem.
Click to expand...

True, but everything I have read indicates operating at peak EGT is only an issue at high power settings. From memory, Lycoming says that provided your CHT temps are OK you can do whatever you want with the mixture knob and you can't hurt your engine even if you run it at peak EGT provided you are below 75% power. I think Superior was a little more conservative and used 65%. If I recall correctly the Advanced Pilot gurus say something similar. The upshot is that for RVs with carb engines that typically cruise around 50 to 65% power you can lean the engine as much as you want without worrying if you are in the peak EGT range or not provided your CHTs are ok. Higher, non standard compression ratios and EI can complicate the issue.
I am no guru so feel free to correct me if I am wrong.

Fin
 
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