How low do you fly for cross country?

[Antoine]

We recently made a trip - yes, to my island -
Weather was once again challenging enough that I had to file... VFR. We had nasty icing layers and I had no option but to stay low (and split the trip on two days).

I realized that I actually almost never cruise this low and had quite some difficulty setting up the engine.
Normally I will seek a cruise altitude at or above 8000 ft and just operate WOT around 9 GPH. This gives a nice cruise speed of 140-145 KTAS depending upon nose wheel karma and the engine sounds happy like an old dog snoring near a log fire.

Flying between 2000 and 5000 ft was another game altogether. After some experiments, I ended up running the engine at 2450 RPM and throttled back to 25MP to keep it sounding happy. I felt the engine telling me "I really don't like too much MP with this RPM".

I then tested with 2600 RPM and about 10 GPH - WOT and easily got 150 KTAS.
Fuel flow at 12 GPH got us even faster but the data is unreliable because it was bumpy.

This was with full fuel and maybe 200 lbs under gross weight.

Apart from the fact that my GF could practice her "Château-watch" talent, I did NOT like cruising low.
I feel my IO-360 is only really happy when up above 7000 ft and running WOT/peak EGT. What is your experience?

[41000]

I used to run 45% 1800/20, 110k TAS on 5.4 gph, truly gives you vfr easily gives you 6hrs range, not for everyone and don't drink any coffe before,

[Steve]

Like 41000, I like to get fuel efficiency (at the cost of a little speed). My airplane is 5-7 knots slower than the newer DA40s anyway. I will fly 65% or 55% (depending on altitude). Almost always at 2200 RPM, MP by the tables. On spring break, we had a leg from Fort Stockton, TX to Cochise County, AZ (366 NM) and burned only 22 gallons.

Steve

[JimP]

I too typically fly above 7,000 cross country but when I am below 6,000 I limit MP to 2450 and RPM to 2500 (usually 2430) because I feel like the engine could hurt itself with the greater power at lower altitudes. This is typically just over 10 GPH. I keep the EGT at or below 1500 because I am worried about burning (or fatiguing valves). Do you have any concerns at peak EGT? Am I being overly cautious?

[rwtucker]

When I'm flying long point A to point B distances, I want to get there as fast as I can with as few stops as I can manage. (Usually it is a body stop where I take on fuel as a secondary consideration.) My preferred plan is 11,000-13,000 MSL, 2450 RPM, WOT, 1,350-1,400 EGT. The gods willing, those settings result in the highest ground speed I can get at around 9 GPH.

[Chris B]

I keep the EGT at or below 1500 because I am worried about burning (or fatiguing valves). Do you have any concerns at peak EGT? Am I being overly cautious?

Hi Jim -

Assuming that you are not flying in the arctic (or Minnesota in the winter ) the limiting factor for a NA engine is CHT. As long as this stays below 380F in cruise you should be fine. Slightly higher might be OK, but most of the gurus recommend <380F. Anything higher than ~415F *definitely* increases wear.

IMO, EGT is mainly useful as a quick-response leading indicator for where CHT is headed. EGT values vary between aircraft of the same type, based on probe placement. So the absolute numbers are generally meaningless. Early engine monitors did not even label EGT values (later added in marketing one-upmanship). However, CHT is absolute.

Chris

[rwtucker]

However, CHT is absolute.

I agree with Chris that CHT is the best indicator of engine stress. As Chris suggested, I do my best to keep it under 400 and preferably lower than 380. In contrast, there are only a few situations in which high EGT will burn a valve in a normally aspirated power-plant. Lycoming tells us to fly our engine at any EGT we please so long as power is 75% or less. Mostly, you want to worry about burning valves in turbocharged engines. (Don't ask me how I know.)

However, I think I disagree with Chris a little on the usefulness of the EGT display as an active engine management indicator. Older aircraft lacked a few indicators that we now take for granted. EGT is one of the most useful to me.

When it comes to setting HP (speed), CHT is at best a fuzzy and sluggish lagging indicator. Up to a point, lower CHTs mean longer engine life and a much lower probability of launching a barrel through your cowling. However, within the acceptable CHT ranges (say, 250-380), colder means less HP for a given configuration. I can fly a smooth and quiet LOP giving me relatively high EGTs and low CHTs in the 290 range. I'm also slogging along watching C172s overtake me.

The reason I suggested 1,350 EGT is that this temp generally reflects maximum HP for your chosen RPM. This temp/HP relationship is more-or-less independent of altitude or density altitude, and it reflects the best you can do under the circumstances at the moment. For example, I actively manage short field take-off EGTs for 1,350, adjusting mixture on the T/O roll and initial climb to safety to maintain the target. Valves will not burn and 1,350 and HP will be max for the altitude. CHTs are pretty much useless for the short period of time that take-off and initial climb occurs. On cruise, I follow the same EGT guidelines. What are my CHTs at 2,450/WOT/1,350? It depends on OAT. Other things being equal, a given CHT will not reflect my HP; a given EGT will.

One other factor that needs to be mentioned is oil temp. Typically, LOP CHTs around 300 cause oil temps to drop to levels that do not dissipate water. No big deal in the short run but a factor on long and/or frequent runs in this configuration.

All of this is probably controversial. Here's hoping someone will offer data more precise than the guidance out there for engines of our general type. I have not seen data specific to an IO-360M1A but on take-off, it is pretty easy to feel the change in HP as you depart from 1,350.

[RC7]

I'm with Robert on this one. In my opinion, the airplane is slow enough as is. That being said, my engine management was pretty simple - if we were high enough that WOT is <= 24.5", I'd leave it wide open and set the RPM at 2450. If WOT gave me more, I'd reduce power until 24.5".

As far as leaning, I would simply target ~1450 EGT. All parameters were always well within limits and performance was decent. I'm too lazy to sit there and constantly tweak the mixture/prop/throttle setting to save 0.1 GPH.

[Antoine]

I'm too lazy to sit there and constantly tweak the mixture/prop/throttle setting to save 0.1 GPH.

Funny - you're my exact opposite: I won't stop tweaking until we land! And I found out it's more like 1 GPH.
Try dropping RPM in cruise (but not low alt)

I am glad I'm not the only one who feels the need to throttle back down low... It's like engine talk: don't give me too much MP!

[rwtucker]

Sometimes I play with RPM/MP/EGT and the G1000 range indicator but the combination creates a large family of curves. It would be nice to nail one variable down, maybe two when you reach an altitude where WOT is 75% or less. Aside from the XLS tuned exhaust's resonance at 2,450, it would seem that ideal prop speed for max cruise would vary with DA. Has anyone seen any charts on that for our props? Does anyone have a sense of the FADEC algorithms for engines similar to ours? I remember the now long retired generation of pilots saying that 1,800 RPM was close to the theoretical ideal for efficiency with many props. The compromises were made to accommodate engines that needed to turn a little faster to get HP.