As always, the Cessna operations and flight manual, and the advice of a certified flight instructor, should be your primary source of information regarding the safe operation of your aircraft.
Contents:
Where to cruise?
Low altitude cruise
Middle altitude cruise
High altitude cruise
What profile to fly?
Where (what altitude) should I expect to Cruise at?
There is much personal choice and opinion on this subject, so we put this question before the CFO membership.The results are as shown here:
What is the *highest* cruise altitude you use (in MSL?)
| RG | Votes | Approximate % | |
| RG: 8,000 or less | 51 | 25% |
|
| RG: 10,000 | 74 | 36% |
|
| RG: 12,500 | 55 | 26% |
|
| RG: 14,000 | 15 | 7% |
|
| RG: 16:000 | 13 | 6% |
|
| FG | Votes | Approximate % | |
| FG: 8,000 or less | 61 | 30% |
|
| FG: 10,000 | 71 | 35% |
|
| FG: 12,000 | 53 | 26% |
|
| FG: 14,000 | 15 | 7% |
|
| FG: 16,000 | 1 | 0% | |
To summarize the conclusion, they are remarkably similar. I was surprised that I should clearly have added an altitude below 8,000 feet.. evidently some folks just don't climb, even on a long trip with a tailwind.
One would expect the RG to have a much higher normal cruise, but the shift seemed to be about 6%.. that is, the lower altitude was 6% lower for the RG, and that 6% translated through to show up at the top of the RG altitude.
Also note that only the RG was normally turbocharged (other than the two exceptions.) That may almost account for the 6% in our sample, which based on the comments seemed to include a number of Turbo owners.
So it appears that the average Cardinal owner would expect to use 10,000 feet as a possible cruise altitude if their trip and conditions suggested that they fly as high as possible.
So what should one expect at each cruise level, up to that maximum?
Low Altitude Cruise
Anything below about 5000 feet can be thought of as low altitude cruise. In these altitudes you will be using some throttle to reduce the manifold pressure to a selected level.Most Cardinal pilots pick a manifold pressure and RPM combination that simply feels right for their airplane. Usually that starts with RPM. Most Cardinals (or other airplanes, for that matter) will have a 'sweet spot', an RPM where the engine is simply happier.
When you get an overhaul or a prop balance this can change. If you don't like where yours is 'sweet' it could be worth a try.
Once the RPM is picked, just look up the power setting you want in the operations manual and set your manifold pressure from that. The book is pretty accurate in the Cardinal.
The book is also pretty accurate about cruise numbers. (Note that I refer to post-1970 books... I've heard bad rumors and don't have personal experience with the earlier books.)
There is a power computer available that does the same thing, but it's results are really the same as the tables. It's a nice tool if you are short of other instrumentation or like such gadgets.
Most people cruise at 65% to 75% power, unless you are trying to resolve engine compression or blowby problems. I need to write a page on that process...
As always, the best measure of power (until someone invents a % power gauge, as many are trying to do) is read directly from fuel flow. If you have a direct reading fuel flow system, once the system is leaned you can set the power with hi accuracy from that indicator. Check out this page for details.
Middle Altitude Cruise
Without applying too much science to this choice, I'd call the middle altitudes those in which you are able to run 'normal' power settings with full throttle.I've heard, and it makes sense to me, that full throttle operation is the most efficient. No turbulence around the throttle plate, a smoother flow through the distribution box under the oil pan... I try for full throttle as soon as I can.
At this point you only have one way to set power: by changing RPM. Higher RPM will result in higher power, although it's not quite linear. Again the book or power computer will help you sort out where you are.
And again, the best measure of power is read directly from fuel flow. Check out this page for details.
Mid altitude cruise will give you the highest airspeeds, short of a turbocharger. Click Here for the page on Turbo cruise.
Keep in mind that the RG fuel flow gauge is actually a pressure gauge calibrated in Gallons Per Hour. If you have dirty injectors or other non-standard flow, it will not be accurate. This is not a common problem.
It has been said by some Cardinal owners that the ideal cruise settings are full throttle and an RPM setting at the bottom of the green. It's suggested that the lower number of turns of the engine results in less friction and loss.
I'm not sure that science supports that argument, but if your engine runs smoothly at low RPM you might find some remarkable efficiencies at those settings.
High Altitude Cruise
As you climb higher you will start to lose power, but the fuel flow will go down as well. And your fuel efficiency will be higher as you are flying in thinner (lower drag) air.Of course your prop will also be less efficient, but their is a net gain in fuel efficiency.
For most high altitude flights, you will want full throttle and top of the green RPM. As the power drops you may feel better about operations lean of peak, more about that on This page.
As an example of a high altitude flight, our trip back from Colorado Springs to the Chicago area last summer was done at 12,500 in our stock '76 RG. True airspeed was around 136, and the fuel flow, lean of peak, was around 8.2 GPH. We flew home, with some headwinds, in 6:20 and landed with 17 gallons on board.
I have detailed data from other flights, but not here. In the future I'll add to this section with that information.
What Profile to fly?
There has been a lot of speculation about the benefits of flying higher: is the improved efficiency worth the extra time and fuel to climb?People who have set out to prove this one way or the other usually come up with a dead heat: no real benefit either way.
My personal experience is that high altitudes are good when you are not in a hurry an have a long way to go.
Of course the overriding influence is the wind. I usually go for a slow cruise climb and level off every so often to get an actual wind impact reading. When I find a peak, I either go back to the peak or stop where I am.
There is also a difficult to quantify benefit to going just slightly downhill for as long as possible for arrival. I find that when I'm not staying high for smooth air, I can gain an extra 10 knots with just a slight descent. Some day someone will do the math to prove or disprove this as an efficiency trick.