Running at High RPM

DrBenny

New Member
Assuming you are not a renter, what arguments can be made for running at high RPM in something like a 'Hawk? The only benefit I can find is speed. But one pays in range, noise, and stress on the airframe and engine (though that is design stress).

For cruise, I usually choose a 2100-2200 rpm setting for the 'Hawk at 2000-5000' MSL. These settings are at the bottom of the green line, but I like them the best. Anyway, here are some numbers for your perusal from the POH:

At standard temperature at 4000 feet we get:

2100 rpm - 46% pwr - 93 KTAS - 5.5 GPH
2200 rpm - 51% pwr - 99 KTAS - 5.9 GPH
2300 rpm - 57% pwr - 105 KTAS - 6.4 GPH
2400 rpm - 64% pwr - 110 KTAS - 7.1 GPH
2500 rpm - 71% pwr - 115 KTAS - 8.0 GPH
2550 rpm - 75% pwr - 118 KTAS - 8.4 GPH

IOW, If I went to full power, which at this altitude would be 2550 rpm and 75%, I would only get 118 KTAS and I would be burning 8.4 GPH. I would only be gaining 25 KTAS over the 2100 rpm setting. Put another way, I get a 27% increase in speed, but I pay the price of a 53% increase in fuel consumption. I am probably also lowering my time to overhaul since the engine is under more stress.

Opinions?
 

Alchemy

Partner, Ally, Friend
I just did a 500+ NM trip in a 172 the yesterday and ran it between 2400 and 2450 RPM the whole time, cruising at 6500 and 7500 feet MSL.

Basically, I'm not paying for gas but I am paying for time, so I wanted to expedite the trip as much as practical (plus flying for 3 hours in a straight line by yourself over the desert can get pretty boring). I always kept an eye on the oil temperature gauge and made sure I leaned the mixture as reccomended in the POH. No problems whatsoever, I was even getting groundspeeds in 125 knot range on the way back (17 knot tailwind) The 172 should run fine at these power settings.

That said, I probably would not want to ever run the engine over 2500 RPM for an extended period, and usually 2450 is the highest RPM I'll run at continuously during cruise.
 

Alchemy

Partner, Ally, Friend
How ethnocentric of me. I just assumed he was talking about skyhawks since they are so ubiquitous. That and the figures in my '77 172N POH exactly match the ones he listed.
 

Eagle

New Member
I run at max rpm and max power 90% of the time.

the difference in fuel burn at altitude is less than a 2 gal per hour..

Better for the engine to run wide open too.
 

DrBenny

New Member
[ QUOTE ]
Sparrow Hawk?
Sky Hawk?
Hawykeye?
Tomahawk?
TRAUMAhawk?


[/ QUOTE ]

Haha!

Good point. I was talking about the Cessna 172.
 

DrBenny

New Member
[ QUOTE ]
I run at max rpm and max power 90% of the time.

the difference in fuel burn at altitude is less than a 2 gal per hour..

Better for the engine to run wide open too.

[/ QUOTE ]
Eagle, what aircraft are you talking about? There is some controversy among the CFIs and A&Ps I know about whether or not it is "good" for the engine to run at max power. Some say yes, but only for the first 50 hours (to set the rings, I think). Some say no, it shortens time to overhaul, and I've also heard that it doesn't matter as much as remembering to lean, use carb heat at the right times, etc.

Any A&Ps on this webboard?
 

IrishSheepdog

Sitting in the median
When I've flown PA-28-181's and -201's on long haul trips, I will usually run at 75% power. This is because our trip times are scheduled for 65% power, but don't account for more than .2 on the ground. At many airports we fly to (DSM, STL, MDW, SBN, FWA, EVV, etc.) we can find ground times of .3 or .4. So I push the power up to make it within limits.

At 75% power, RPM settings will often be around 2480 or 2500 RPM, with max being 2700. I've also flown at 2600 RPM for "best speed" at times, with that 100 RPM buffer just to make sure I didn't go over accidentally.

If it's in the green arc, then it's fine! If it's yellow, well it hasn't broken yet...
J/k.
 

davetheflyer

New Member
In GA airplanes, I usually rent so I go for speed over fuel economy and use 75% power. If I'm in a big hurry, I use the red line on the tach.

When I'm at work our power setting is usually limited by one of two things. First, we have a Vne speed that we can easily exceed at low altitudes.

Second, there are engine limitations. At higher altitudes we are usually limited by EGT. At lower altitudes, torque is normally the limiting factor.
 

pilot602

If specified, this will replace the title that
We generally cruise at 65% which gives us a fuel burn of just over 16 gph. We used to cruise at 55% but the few extra RPMs helps to stabilize the aircraft in bumps (so does leaving fuel in the tip tanks) and puts us closer to our 160mph TAS mark (old Pipers use MPH, not knots).

At 65% we get about 5 hours with an IFR reserve or about a 750 nm range. If we bring that back to 55% we can bump that up close to a 900nm w/reserve.

The book says at 45% we get a 1,200nm range but I think that is without a reserve.

But because we pay our own gas (not renting) and will have to foot TWO overhaul bills we try to run the engines with a little care and want to get maximum use out of the gas in the tanks.
 

E_Dawg

Moderator
Hmmmm... I pay for time but not for gas... I usually run it right up to red line in the 172 (or full throttle); never had a problem. The POH does not say it's a problem, and assuming you can get the RPM that high in the first place it is still well within it's design limits.
 

E_Dawg

Moderator
The 152 is a bit of a different story... the one I fly has a tendancy to overrev so I usually pull the power way back in cruise which gives around 85 - 95kts IAS (95 - 100ish GS). That thing is such a kite though, half the time I'll be at full power at 85 maintaining altitude and the other half I'll be at 50% throttle with the nose down maintaining altitude at 105 IAS. Go figure....

The thing is pretty economical though... Flew it once for 2.2 ish and only had to put 12.5gal in it (5.6gal / hr though I always use 7gal/hr with an hour reserve in flight planning).
 

I_Money

Moderator
If the engine instruments are in the green I do not see a problem with the power setting you choose.
 
There's two power settings in skyhawks, ON and OFF !!! I almost always keep the engine at 2500 and sometimes more at altitude. Some very experienced A&Ps, including my examiner and the guy that owns the airplanes I fly agree with this although I've still to come across a really good explanation as to why it's good for the engine, don't we have any A&Ps on the forum ?

A question, do you drive at 55 MPH in your car all the time? neither do I ...
 

ananoman

New Member
There should be no adverse affect on engine life at high power settings as long as no limitations are exceeded. The reason we fly is to go fast. Aircraft engines are designed to cruise at high power settings. Everything in them is optimized for this. If you fly at lower throttle settings you are not getting optimum efficiency. Cam timing and ignition timing are all set for 'max power'. The only reason to fly at a lower setting is endurance or range.

You will note that at higher settings there is a diminishing return of fuel flow to speed. It can sometimes be to your advantage to choose 65% over 75% in the big engines to save several gph and only loose a few knots. I do not see any real reason to run less than this unless you are only going a very short distance. I also do not see an advantage of flying at higher than 75% power in most airplanes. Fuel flow usually goes way up for a very small increase in speed.

Things like changing your oil often, using a quality oil and flying freqently will have much more of an influence on your engines life. Also leaning properly to keep deposits out of the combustion chambers and off the spark plugs and valves will have a positive effect on engine life and fuel use. Even if you think that using very low power settings results in long engine life ask yourself this: If you fly at 45% power and your engine last 20% longer did you really gain anything if it took you 20% more time to get there?
 

pilot602

If specified, this will replace the title that
[ QUOTE ]
If you fly at 45% power and your engine last 20% longer did you really gain anything if it took you 20% more time to get there?


[/ QUOTE ]

Ahh but Confucious asks: if we're out here buzzing around trying to build hours what's the hurry in getting there faster? Who here didn't wish they had 20% more time in their logbooks?
 

DrBenny

New Member
Nice post. The only comment I had was on this:

[ QUOTE ]
The reason we fly is to go fast.

[/ QUOTE ]
The reason I fly is because I enjoy flying, so I'm not in any hurry.
 

Kingairer

'Tiger Team' Member
I met a guy while i was stuck in an airport last October. He had a Cessna 140 and was flying it across the country. I couldnt help but think how much that would suck and get old. (I guess this happens after you have CFIed for awhile). I always assumed everybody loved to go faster rather then slower. I have always liked and gotten my thrills from flying faster equipment. This guy blew me away b/c he really liked going slower, so he could enjoy the scenery better. More power to him, but i aint going with him!
Different strokes for different folks,
KA
 
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