Fuel consumption

A Place to Share Experiences and Discuss Aviation Stories

Moderators: 6643, 6183, V529

Forum rules
You must be a member of the Cessna 120-140 Association in order to post new topics, reply to existing topics, or search for information on this forum. Use the "Join" link in the red menu bar.
Post Reply
8424
Posts: 214
Joined: Sun Mar 17, 2019 11:43 am
Location: OK74
Name: Steve A H
Aircraft Type: 1950 C140A
Occupation-Interests: Retired
Contact:

Fuel consumption

Post by 8424 »

OK, here's everyone's chance to get in your 2 cents worth!

This is a full throttle take-off and climb out to 1,000 ft on a standard day at sea level.

According to the Overhaul Manual, the O-200 maximum RPM is 2750-2800 (Test Operation Limits).
The fuel consumption at full throttle is 51.55 lbs per hour or 8.59 GPH (Test Operating LImits).
(these two are NOT tied together in the Test Operating Limits chart: i.e., fuel consumption at 2750-2800 is 51.55 lbs per hour)

If the propeller restricts RPM to 2600 RPM at full throttle, is the fuel consumption still 8.59 GPH, or is it reduced relative to the lower RPM?

In other words, does the throttle position or the propeller control fuel consumption?

Since an engine is basically a pump, does it have to turn faster to pump more fuel regardless of the fuel available to pump?

Steve A H
User avatar
6643
Posts: 2326
Joined: Tue May 01, 2018 7:00 am
Location: KLCI
Name: John C
Aircraft Type: 1946 C140/C90
Occupation-Interests: A&P, semi-retired
Contact:

Re: Fuel consumption

Post by 6643 »

8424 wrote: Fri Aug 27, 2021 6:53 amIf the propeller restricts RPM to 2600 RPM at full throttle, is the fuel consumption still 8.59 GPH, or is it reduced relative to the lower RPM?
There should be two fuel curves. One is "propeller load" and the other is "cruise". You will burn more fuel at 2600 in a climb at full throttle than you will at 2600 in level flight at partial throttle.
In other words, does the throttle position or the propeller control fuel consumption?
Yes ;)
Since an engine is basically a pump, does it have to turn faster to pump more fuel regardless of the fuel available to pump?
Two factors determine fuel consumption: manifold pressure and RPM. For a given RPM, the throttle controls manifold pressure. The carburetor delivers fuel based on how much air flows through it, which is a function of the manifold pressure.

So, you can have the same fuel flow at lower RPM and high manifold pressure (climb), and again at high RPM and low manifold pressure (dive).
8424
Posts: 214
Joined: Sun Mar 17, 2019 11:43 am
Location: OK74
Name: Steve A H
Aircraft Type: 1950 C140A
Occupation-Interests: Retired
Contact:

Re: Fuel consumption

Post by 8424 »

Now John, that's a great lesson on RPM/Manifold pressure/throttle position/fuel flow!

We all know fuel consumption is higher at full throttle climb out than in cruise for a given RPM. I just was wondering if it maxed out the fuel at 8.59 GPH. The climb/fuel consumption charts for a 1976 150 show a climb from 1,000 to 6000 uses fuel at the rate of 9.33 GPH (9 minute climb using 1.4 Gallons) with no RPM given. It's interesting that Cessna managed to exceed the Overhaul Test Limits full throttle number of 8.59 GPH. Maybe it's the difference between real world testing and crunching numbers.

While folks with constant speed props deal with this each flight, the rest of us just push in the throttle and go with what ever it works out to be.

It really needs to get cooler outside so I can go do something instead of look up stuff!

Steve A H
User avatar
6898
Posts: 307
Joined: Tue May 01, 2018 7:00 am
Location: IS65 Sandwich IL.
Name: Jeff T
Aircraft Type: 48 140 C90
Occupation-Interests: Auto,Marine&Aviation Repair
Contact:

Re: Fuel consumption

Post by 6898 »

Interesting question and a great response Thank You . That's why this site is of such value, REAL help answers. I will add 11.5 hrs of HOT STICKY flying ( At Gross ) from northern IL. to convention in GA. and back we used the most fuel ever. 1948 C90 with marvel 95% power 6.64 gal per Hr. average. Did I mention HOT?!!!
Jeff T 1948 C-140 NC3600V
Past President 120 140 Assoc. 2019-2023
Post Reply

Who is online

Users browsing this forum: No registered users and 7 guests