Doesn't the 'full throttle' curve take this into account already? It's showing different values for BHP and lbs of fuel consumed at different RPMs at full throttle. As the propeller load allows the RPMs to rise, wouldn't you just move along the 'full throttle' curve from left to right?
Listen to a Constant Speed prop change the RPM's during run up. The throttle is fixed and the prop load is changed. The RPMs change per the prop load
DWood wrote:
Also the fuel flow is based on engine RPM and not throttle position.
That seems counter intuitive to me. 2500 at full throttle in a climb vs 2500 in a glide at idle wouldn't have the same fuel burn, right? When you're at full throttle (or any other given throttle setting, provided it doesn't change), then I believe that fuel burn is a function of RPM.
These are normally aspirated engines using a carb that bases the amount of fuel pulled into the engine on airflow through the carb venturi. The amount of air which results on the amount of fuel is based on engine RPM. The fuel flow is richest at full throttle so the engine isn't too lean resulting in bad things. In cruise when you pull back the throttle, it is a little leaner. That might be causing some confusion. However, this graph is full throttle which is probably conservative to straight and level.
I think another part that is confusing is the Specific Fuel Consumption as it is decreasing as RPM increase. That is only part of the formula as the BHP is increasing. The formula results with increasing fuel consumption.
We're dealing with a fixed pitch prop on the O-300 so wouldn't that not apply? At full throttle, prop load determines RPM (I start my takeoff roll with 2300 RPM and as the plane starts moving it will get to 2450 as I climb out at 70), but when you're at full throttle the BHP produced is solely a function of what RPM the fixed pitch prop is able to turn at.
Example: at full throttle, sea level, if you are level and let the engine spin up to 2700, you have 145 HP. if you have the same configuration (full throttle, sea level) but you've just lifted off, the engine may only turn at 2400 RPM. The engine only knows that it's at full throttle and can only manage 2400.
Agreed, the engine can only produce 2400 based on the prop load and at 2400 RPM, the fuel burn is less than 2700. Maybe Richard or someone with fuel flow can independently validate this.
The manifold pressure will be the same in both cases, but the 2700 rpm scenario makes more horse power than the 2400 rpm scenario because of the RPM, which changes as a result of the propellor load independent of the fuel flow. All the engine knows is that manifold pressure is maxed out and the RPM it can turn. A fixed pitch prop has a given propeller load for any given throttle setting and RPM.
Based on this graph the RPM is a function of Propeller loading. Your manifold pressure isn't adjustable and is a result of RPM which is a result of Prop Loading. Please go back to your takeoff situation where your RPM gets higher as you go down the runway. Your speed is increasing and thus decreases the propeller load. The same thing happens when you level off after climb. As you level off your speed increases resulting in the prop load decreasing and results in your RPMs increasing.
On take off the prop load is increased when you start pointing the nose up and the RPM's will drop as a result until you level off or are pointed down. Think of a take off (full power) stall.
Just to throw a wrench into this, did you notice that manifold pressure on both graphs are IN HG. However, the O-300 is between 20 and 30 IN HG and C145-2H is 135 to 145 IN HG. Should it be 13.5 to 14.5 PSI?
While my engine is a lycoming o-360 and it's running a fixed pitch prop (60in pitch) my fuel flow at takeoff is 16.2 gals (us) and levelling out out and at 2450rpm my fuel flow is 9.2 gals per hour (us) This is at sea level . At the start of take off my rpm is 2350rpm but quickly rises to 2550rpm . However on a full climb out from 2000 ft to 8000 ft ,the fuel flow sits around 16 gals a hour but due to gaining altitude the mixture has to be leaned off and fuel flow drops off to about 12 gals a hour . I have also observed at a fixed throttle setting and the nose is raised slowly the fuel flow increases by itself . I would love to have a set of graphs but so far haven't been able to source them .
sfarringer wrote:My recollection is that my fuel flow gage reads about 11 to 11.5 gph at full throttle climb at low altitude.
Of course, I don't really spend much time looking at the fuel flow gage during climb......... But I'll look again next time out and see how bad my memory is........
-10C temperature today.
Full throttle climb at 80 MPH indicated, fuel flow shows 10.7 GPH.
Full throttle straight and level, fuel flow shows 11.5 GPH
Full throttle climb at 80 MPH indicated, fuel flow shows 10.7 GPH. ~2400 in climb
Full throttle straight and level, fuel flow shows 11.5 GPH ~2550 straight and level
Full throttle climb at 80 MPH indicated, fuel flow shows 10.7 GPH. ~2400 in climb
Full throttle straight and level, fuel flow shows 11.5 GPH ~2550 straight and level
If you or anyone else with fuel flow need an excuse to go fly again, please record the following
Temp =
Full Throttle straight and level 2550 Fuel Flow =
Pull back throttle to 2550 (if it will without being full throttle) straight and level and lean Fuel Flow =
Pull back throttle to 2500 straight and level and lean Fuel Flow =
Pull back throttle to 2400 straight and level and lean Fuel Flow =
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