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Power setting at low altitudes

mchargmg

mchargmg

Well Known Member
Hi all,
I fly a 180HP with CS prop 6A. I have always flown at high density altitudes. Our field elevation is 6800 ft, and we routinely see 10,000 ft DA in summer. So I have never really worried when I wanted to fly lean of peak up here. I just dial my mixture down and find the peak EGT and then go lean of that. I normally fly WOT at about 2350 RPM. At my altitudes I normally get about 21 inches of MP.

Recently I have been flying some pilots and paws flights (very fun!). These have been to Oklahoma and back, where the field elevation is about 1000 ft. I am seeing Manifold Pressures of 25 and 26, unheard of! I must be making 75% power or more.

I am a little worried about dialing the mixture down to look for the peak temps. Not sure if it is safe, even for a few seconds. So I just pulled the mixture to what I knew was lean, and flew at about the same speed I have before.

Any ideas? Is it ok to work through the EGT peak to find it? Any suggestions are appreciated.

Geoff
 
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In a standard Lycoming with standard fuel you can lean without fear anytime the power is set to 75% or less. No need to rush. You should have MP/RPM tables for your engine so you know what power you?re running.
 
When LOP, fuel flow is a direct measure of % power. In a 180hp engine, 9.1gph is 75%. 8.5gph is 70% 7.9 is 65%, 7.3gph is 60%. Rune WOT and dial in the fuel flow corresponding to the % power you want to fly.
 
Oklahoma the home of GAMI and the APS class (which I teach).

Simply, WOT for throttle. RPM....pick something smooth, 2350-2600, the appropriately LOP.

So how far you ask? Work out roughly what you would have when ROP. Take your example of 25-26" range. Call that 85% of available throttle from sea level. Take 2450 RPM which is about 90% RPM.

0.85 x 0.9 = 0.765 or 76.5% power when ROP. If you go appropriately LOP from there you will be at 10% less power, and say 7% points off that is about 70% power when LOP.

Refer to the table below and you will see that no more than 25 or so dF LOP is needed.

80%-100% use 60-80dF LOP
75% use about 40dF LOP
70% use about 25dF LOP
65% or less use 10-15dF LOP

:)
 
mchargmg said:
Any suggestions are appreciated.
Click to expand...

KIS approach...you have a manifold pressure (21" at WOT) and RPM setting (2350) with which you are comfortable running LOP at altitude. So use the throttle; set the same 21" when flying low in the flatlands and lean as you please.
 
BobTurner said:
In a standard Lycoming with standard fuel you can lean without fear anytime the power is set to 75% or less. No need to rush. You should have MP/RPM tables for your engine so you know what power you?re running.
Click to expand...

As Bob said, anything under 75%, lean as you like. You can use the rule of 48 rule of thumb to estimate 75% power. Add the manifold pressure and the rpm in hundreds to reach 48. For example,

21?/2700 rpm
22?/2600 rpm
23?/2500 rpm
24?/2400 rpm (the magical 24 squared)
25?/2300 rpm (yes over square is fine as long as there aren?t any rpm restrictions on the TCDS, Type Certificate Data Sheet, for your engine & prop combo).

Use 49 at warmer temps/lower altitude and 47 at colder temps/high altitude. Note, 24 squared on cool day at low altitude (i.e. 10 C @ 1000?) is the same power setting as 24 squared on a warm day (i.e. 10 C @ 5000?).
 
I?ve stated this a few times before on VAF, if using an iPad, go to the App Store and search for Jim Petty?s excellent FREE app called Aircraftpower
Select your engine type, then input MP/RPM/PA/OAT, and you read the % of power you are using, including the diagram!, great stuff :)
 
DeeCee 57 said:
I?ve stated this a few times before on VAF, if using an iPad, go to the App Store and search for Jim Petty?s excellent FREE app called Aircraftpower
Select your engine type, then input MP/RPM/PA/OAT, and you read the % of power you are using, including the diagram!, great stuff :)
Click to expand...

If those are all the variables then this app only works at some unstated mixture setting. And certainly not LOP.
 
BobTurner said:
If those are all the variables then this app only works at some unstated mixture setting. And certainly not LOP.
Click to expand...

I downloaded the app. It's pretty cool. The app appears to mimic the charts in the Lycoming Operator's Manual which are based on Maximum Power Mixture, approximately 100-150 deg F ROP.

P.S. I just read through the instructions in the app. It states that the engine is assumed to be operating at best power. It also states that the charts are modeled from the charts in the Lycoming Operator's Manuals and are within 0.5% accuracy.
 
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I came up with estimates based on rules of thumb, things I?ve read in the past, and some from Lycoming power charts. The SFC charts were difficult to interpret because the lines and graphs were not fine enough, but I got some SFC info from what I?ve read here or on other forums to come up with estimates. They seem to be pretty accurate. I spoke with a Lycoming engineer a few years ago that told me, if I was running my engine at 65% or lower, I couldn?t hurt my engine with the mixture control. If I got it too lean it would just run rough.


LYCOMING O-360-A1A / 180 HP
POWER CALCULATION ESTIMATES


Approximate % power estimate:

MP + RPM/100 = N
N = 47 = 75% 75% = 135 hp
N = 44 = 65% 65% = 117 hp
N = 42 = 55% 55% = 99 hp

Fuel Flow Calculations:

Lycoming Leaning Power Definitions -
POWER = 100-150 degrees rich of peak EGT
ECONOMY = Peak EGT
Lean Of Peak = All 4 cylinders lean of peak EGT

Fuel Flow calculation by rule of thumb:

POWER HP / 12.3 = FF


ECON HP / 13.8 = FF


LOP HP / 14.9 = FF


Fuel Flow calculation by SFC:

SFC = FF (LB/HR) / HP

POWER = .488 SFC
ECON = .435 SFC
LOP = .402 SFC



O-360-A1A / 180 HP Cruise Fuel Flow:

GPH

LOP ECON POWER
75% / 135 HP 9.0 9.7 11.0
70% / 126 HP 8.5 9.1 10.2
65% / 117 HP 7.8 8.5 9.5
60% / 108 HP 7.2 7.8 8.8
55% / 99 HP 6.6 7.2 8.0
 
Looks like I didn?t format this correctly, so columns don?t line up, but I?m sure you can figure that out if you?re interested.....
 
If your O-320 is 160 HP then::

75% = 120 HP
65% = 104 HP
55% = 88 HP

If the SFC is the same as the O-360, and I would assume it?s pretty close, then you can calculate fuel flow:

SFC = FF (lbs/hr) / HP

75% - .488 = FF / 120
.488 X 120 = FF (lbs/hr)
58.56 = FF in lbs per hour
58.56 / 6 = 9.76 gal per hour

You could also use that other rule of thumb:

Power (100-150 rich of peak):
75% - 120HP / 12.3 = 9.756 gal per hour

LOP (all four cylinders lean of peak)

75% - 120 HP / 14.9 = 8.05 gal per hour

You can figure out different % power settings doing the math. But this is assuming the specific fuel consumption and rules of thumb are similar for an O-360 and an O-320. And I believe they are very close for a normally aspirated engine.
 
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