setting cruise power with IAS

infotango

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rob!
So I have always been told to cruise a 172 at 2300 RPM which seems like a crummy idea, since that power will result in a varying fuel flow with altitude and a different true airspeed with altitude.
I have noticed that when I set the factory recommended RPM setting for altitude at a power setting of 65 percent power I almost always notice 105 indicated. So I'm thinking that flying with whatever cruise power nets me 105 indicated in level flight will always result in a power setting of roughly 65 percent power, (and the resulting predictable fuel flow). Using IAS as an airspeed setting also seems to make flight planning much easier, I now simply use that speed and convert to TAS on the whiz wheel.
My question is, is this flawed logic? Will I always, in fact, be running the engine on the 172 at 65% if I follow this manner of setting cruise power in level flight?
 
65% power with the 180 hp Penn Yan O-360 conversion is about 2400 rpm. I find that to be a good cruising power setting in our club's C-172N and when everything is "right" I get about 114 KIAS at reasonable altitudes. Just another data point.
 
That is probably good enough. But I doubt it's really accurate.

I never really put much thought into crusing a 172. I go somewhere inbetween 2300 to 2500 RPM..and lean a bit. I plan 8 GPH..and a KTAS of somewhere around 110 knots. It's proven to be plenty accurate. I for sure don't bust out the POH everytime I fly and try to determine the proper setting for the temperature, altitude, etc.

Honestly. I put very little planning into most of my XC's. I'll draw a line on a sectional..if that...look at the page in the AF/D for the airport... pickup a briefing from FSS on my drive to the airport..throw the identifer into the GPS..and follow along with my finger during the flight on the sectional...Sometimes for extra credit I'll use the VOR to double check my position.
 
Whoever told you 2300 is trying to save a bit of gas during training, or rack up your hobbs time. Go by whatever the book says for the power you want. I'll usually start looking around 2500-2550 rpm for around 70-75% in the '78 Skyhawk I fly.
 
infotango said:
I have noticed that when I set the factory recommended RPM setting for altitude at a power setting of 65 percent power I almost always notice 105 indicated. So I'm thinking that flying with whatever cruise power nets me 105 indicated in level flight will always result in a power setting of roughly 65 percent power, (and the resulting predictable fuel flow). Using IAS as an airspeed setting also seems to make flight planning much easier, I now simply use that speed and convert to TAS on the whiz wheel.
A given horsepower will always give the same indicated airspeed within a fairly small margin (until you get to speeds with drag divergences, etc.)...fuel flow for a given horsepower (when properly leaned) will be the same, as well...

That being said, in piston airplanes most of us just pick a power setting that we like, as long as it's within limits...I've seen and used anywhere from 2200 RPM to full throttle with a fixed pitch prop. To figure out the proper rpm for, say, 65% power at various altitudes is more work than I personally feel is necessary in these airplanes.

Airplanes with constant-speed props, on the other hand, use both MP and RPM to set power, and so a given combination will always yield the same percent horsepower (again, when properly leaned). Thus, if "23 square" is 65%, it will be 65% at any altitude, and the fuel flow and IAS will be the same. The only difference will be the TAS obtained for the particular altitude and temperature.

So, to make a short story long, you are correct...you can use IAS to set the desired power, and get the predicted fuel flow for the airplane.

Fly safe!

David
 
MauleSkinner said:
So, to make a short story long, you are correct...you can use IAS to set the desired power, and get the predicted fuel flow for the airplane.

Fly safe!

David

Not to be picky, but this doesn't seem to be right. (Although I most certainly could be wrong). Wouldn't the IAS change with the loading of the aircraft? In theory, couldn't % power change with how the engine is leaned? ( If you're not leaned properly you'd be operating at less than that % level, or if leaned LOP)

On the most simple level, I could agree, but it seems like the falacy of confirming the consequent.
 
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MauleSkinner said:
That being said, in piston airplanes most of us just pick a power setting that we like, as long as it's within limits...I've seen and used anywhere from 2200 RPM to full throttle with a fixed pitch prop. To figure out the proper rpm for, say, 65% power at various altitudes is more work than I personally feel is necessary in these airplanes.

My totally non scientific method for the fixed prop planes:

Below 6000 and not in a hurry: 2400
Below 6000 and in a hurry: 2500

Above 6000? Add 100 to the above numbers.

Of course leaned as necessary.
 
Joe Williams said:
Whoever told you 2300 is trying to save a bit of gas during training, or rack up your hobbs time. .

DING DING DING WE HAVE A WINNER:yes:
 
AirBaker said:
You mean you weren't flying at BRS?

Best rental speed?

I try to treat rentals with proper respect, but I would like to get there sooner or later!! LOL. In Skyhawks and other planes with O-320s and O-360s, I've looked a bunch of POHs and PIMs over and 2500-2550 doesn't seem abusive to me. I got interested after flying a Tiger 325 nm at 2300 rpm. I got curious after we refueled and found I'd only burned something like 7.5 gph. Figured my TAS, about 108 IIRC and realized I'd perhaps been a little to easy on the throttle. We had good tailwinds on the way there, and were covering ground at a decent clip (better than 130 kts) I didn't think we were poking along... return trip into the winds was at 2550 :)
 
Aint no point to saving gas in a rental if they are charging you wet. I noticed in training on the Beech, we plodded along at 60-65% power. As far as setting your power with IAS, you may want to rethink that approach. You ever try pushing something with a rope?
 
MauleSkinner said:
A given horsepower will always give the same indicated airspeed within a fairly small margin (until you get to speeds with drag divergences, etc.)...fuel flow for a given horsepower (when properly leaned) will be the same, as well...

So, to make a short story long, you are correct...you can use IAS to set the desired power, and get the predicted fuel flow for the airplane.

Fly safe!

David


David,

I humbly disagree. My airplane has a 8 - 10 kt window depending upon loading. I'm also assuming that you meant to add that your statement is only true if the density altitude is constant. IOW, I can set 65% power at 3000ft and 65% power at 11,000ft, I guarantee that my IAS will be substantially different.

James Dean
 
MauleSkinner said:
Airplanes with constant-speed props, on the other hand, use both MP and RPM to set power, and so a given combination will always yield the same percent horsepower (again, when properly leaned). Thus, if "23 square" is 65%, it will be 65% at any altitude, and the fuel flow and IAS will be the same.
This is not consistent with the power charts for my IO-470-N. For a given rpm, the amount of MP needed to produce a particular % of rated power decreases in an almost linear fashion as altitude increases. For example, 21.8"/2300 rpm produces 55% at sea level, but at 10,000' 19"/2300 produces the same power and fuel flow.

I plotted the graphs below from the TCM charts as a "crutch" to help me set power when I first got the airplane.

-- Pilawt
 

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FF will be constant for a given power setting, regardless of altitude, or rpm, assuming you lean consistently. So 65% is the same fuel burn whether you get it at 3000 feet with 20" MP/2300 RPM , or at 3000 feet at 19" MP/2400 RPM, or at 7000 feet with 17" MP and 2500 RPM. The only difference a turbocharger makes is that you can maintain available power higher. The noticeable difference as you climb is the amount of throttle required to maintain your power setting. But as we've seen, more throttle does not mean higher fuel flow.

I always select the lowest available RPM (for comfort) and adjust the MP for the desired power.

In a rental 172, I cruise at max available power all the time - I'm usually at 5000 or higher so even with full throttle I'm only hoping for 75%. I just watch the RPM to be sure I don't overspeed the engine in a descent.
 
My experience is that with a FP prop, you get just about the same IAS at all altitudes for the same % power. CS props seem to change the situation. Never taken the time to figure out the theoretical "why?"
 
Pilawt said:
This is not consistent with the power charts for my IO-470-N. For a given rpm, the amount of MP needed to produce a particular % of rated power decreases in an almost linear fashion as altitude increases. For example, 21.8"/2300 rpm produces 55% at sea level, but at 10,000' 19"/2300 produces the same power and fuel flow.

I plotted the graphs below from the TCM charts as a "crutch" to help me set power when I first got the airplane.

-- Pilawt

There's a reason for this. In thinner air, the volumetric efficiency goes up for a fixed MP because there is less "back pressure" to impede the flow of exhaust gasses.
 
lancefisher said:
There's a reason for this. In thinner air, the volumetric efficiency goes up for a fixed MP because there is less "back pressure" to impede the flow of exhaust gasses.
Fixed pitch: Set power with TAS, not IAS. You'll get book power if the a/c is in rig. If it's not in rig, who knows what you've got.
 
ya but you can use your indicated airspeed to find your true airspeed easy enough. use the ring if installed or whip out your e6b ( i know you want to jesse) or what is it 2%/1000 feet approximately?
 
OK...I'm gonna claim stupidity ;)

My reply was based on observation in what is probably too limited a set of circumstances...I tend to fly the same altitude ranges, and CG really doesn't vary all that much in my day-to-day flying. Obviously I made some assumptions that I shouldn't have.

I apologize.

Fly safe!

David
 
MauleSkinner said:
OK...I'm gonna claim stupidity ;)

My reply was based on observation in what is probably too limited a set of circumstances...I tend to fly the same altitude ranges, and CG really doesn't vary all that much in my day-to-day flying. Obviously I made some assumptions that I shouldn't have.

I apologize.

Fly safe!

David

It's all one big equation. So hold everything else constant, and you should see the same results. It would be better to use such an observation as a 'double check' that the rest of the variables are all OK.
 
infotango said:
So I'm thinking that flying with whatever cruise power nets me 105 indicated in level flight will always result in a power setting of roughly 65 percent power, (and the resulting predictable fuel flow). Using IAS as an airspeed setting also seems to make flight planning much easier, I now simply use that speed and convert to TAS on the whiz wheel.

The added benefit of this method is that you'll be able to spot carb ice much easier. Is your airspeed down to 102 with the same power setting and conditions? Time to give that carb heat a yank and watch what happens!
 
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