Prop Question

[Larry Fruits]

 Does increasing propeller pitch, cause an increase in fuel consumption?

 Thanks;
   Larry

[Perry Rose]

Not in my experience.

[Motorman]

Lower rpm, lower fuel use.


MM

[Larry Fruits]

 Thanks for the replies guys. I was curious as to what others may have experienced when increasing prop pitch with reference to fuel consumption.

 I never paid much attention to it before, not noticing much, if any, difference in fuel consumption when increasing prop pitch, but a recent fight brought the question to mind.   

 I increased prop pitch by 1/2 inch and used the same launch RPM, since I wanted to increase flight speed a bit. The measured fuel load was the same for both props. With the lesser pitch prop I had 6 to 10 laps after the clover. With the 1/2 inch greater pitch prop, the engine quit going uphill after the last loop in the clover. I removed less than 1/4 ounce of fuel from the tank after that flight, so I am pretty confident that it just ran out of fuel. I saved the plane, but I don't care to experience that again.

 I will increase the fuel load to ensure an adequate amount and then work backward, reduce fuel load, for the proper run time.

 I was just curious if anyone else had had a similar experience when increasing prop pitch.

 Thanks;
   Larry

 
 

[Brett Buck]

Thanks for the replies guys. I was curious as to what others may have experienced when increasing prop pitch with reference to fuel consumption.

 I never paid much attention to it before, not noticing much, if any, difference in fuel consumption when increasing prop pitch, but a recent fight brought the question to mind.   

 I increased prop pitch by 1/2 inch and used the same launch RPM, since I wanted to increase flight speed a bit. The measured fuel load was the same for both props. With the lesser pitch prop I had 6 to 10 laps after the clover. With the 1/2 inch greater pitch prop, the engine quit going uphill after the last loop in the clover. I removed less than 1/4 ounce of fuel from the tank after that flight, so I am pretty confident that it just ran out of fuel. I saved the plane, but I don't care to experience that again.

 I will increase the fuel load to ensure an adequate amount and then work backward, reduce fuel load, for the proper run time.

 I was just curious if anyone else had had a similar experience when increasing prop pitch.
   Larry

 
 

    The prop with more pitch is, generally more efficient. However, if you leave everything alone, and raise the speed intentionally, you require more horsepower, and it doesn't take much of a speed difference to counter any improvement in the efficiency.

   One nice thing about electrics is that you can see and measure these sorts of effects directly, in a much simpler system. Also interesting, at least with governor-only systems, is that using more pitch uses a bit less power in flight (assuming you adjust to the same airspeed), but much more on the ground, so the run time or battery usage can sometimes wash out, depending on how long you take to launch it.

    Brett

[Larry Fruits]

    The prop with more pitch is, generally more efficient. However, if you leave everything alone, and raise the speed intentionally, you require more horsepower, and it doesn't take much of a speed difference to counter any improvement in the efficiency.

   One nice thing about electrics is that you can see and measure these sorts of effects directly, in a much simpler system. Also interesting, at least with governor-only systems, is that using more pitch uses a bit less power in flight (assuming you adjust to the same airspeed), but much more on the ground, so the run time or battery usage can sometimes wash out, depending on how long you take to launch it.

    Brett

 Thanks Brett.
 I think from what you are saying is that since I increased pitch to increase speed, I increased the power requirement and thus fuel consumption. And if the higher pitch prop uses more power on the ground prior to launch, the net result would be that the same amount of fuel would be consumed at a faster rate with the higher pitch prop. (The higher efficiency of the increased pitch could be canceled out due to the increased power requirement, plus hold time prior to launch). Do I have it right?

 Thanks;
   Larry   

[Randy Cuberly]

Well, the one caveat I see in all that is that to launch at the same RPM with the higher pitch prop you would probably have to run a somewhat leaner needle setting to achieve that RPM.  That means less fuel consumed in most cases.  What would come into play however that is an unknown is what is happening in the torque curve at the leaner setting how the engine is regulating as the airplane attitude is changed during maneuvers.  The engine might break a little harder thus consuming more fuel etc. 

Notice I said might...certainly would if it's trying to maintain the same RPM as it is loaded...that would use more fuel!

Interesting question.  As a rule I always add a bit more fuel for the first flight after changing anything!  I don't like surprises...

Randy Cuberly

[Trostle]

Lower rpm, lower fuel use.


MM

Based on what?  What empirical evidence do you have?  Seems that there are other opinions with some rational thought.

Just asking.

Keith

[Motorman]

Based on what?  What empirical evidence do you have?  Seems that there are other opinions with some rational thought.

Just asking.

Keith

Based on the fact that I'm the national record holder for control line endurance.

The thread started out as using a bigger prop for lower rpm then changed to bigger prop with the same rpm so, are you applying my statement to what the thread turned into? How rational is that, just saying.

MM

[Randy Cuberly]

Based on the fact that I'm the national record holder for control line endurance.

The thread started out as using a bigger prop for lower rpm then changed to bigger prop with the same rpm so, are you applying my statement to what the thread turned into? How rational is that, just saying.

MM

Actually Larry said " I increased prop pitch by 1/2 inch and used the same launch RPM, since I wanted to increase flight speed a bit. The measured fuel load was the same for both props. With the lesser pitch prop I had 6 to 10 laps after the clover. With the 1/2 inch greater pitch prop, the engine quit going uphill after the last loop in the clover. I removed less than 1/4 ounce of fuel from the tank after that flight, so I am pretty confident that it just ran out of fuel. I saved the plane, but I don't care to experience that again."

So the thread did start out with different prop same launch RPM!

Randy Cuberly

[Brett Buck]

Based on the fact that I'm the national record holder for control line endurance.

The thread started out as using a bigger prop for lower rpm then changed to bigger prop with the same rpm so, are you applying my statement to what the thread turned into? How rational is that, just saying.

MM

   Keith is pointing out that there are a lot more factors to it than just the RPM. It's conditionally true, but you can take a stunt engine, open the needle 1/2 a turn, get lower RPM, and run out of fuel much faster.  In fact it's very touchy in that respect, even a few clicks of the needle making it richer/slower on a Fox makes the difference between running out normally, and running out in the overheads. 

    Brett

[Motorman]

Well if you open the needle valve to get lower rpm then yeah it's gonna use more fuel. I just thought since he asked about a prop that the needle would be the same and the rpm would be lowered with more pitch/diameter. 

MM

[Larry Fruits]

 Guys, please don't get in an argument over my question. It's not that big of deal.

 I simply increased pitch only, and turned the needle in slightly to maintain the same launch RPM, looking only to increase flight speed. I wasn't thinking about it making the engine work a bit more and what it might do to fuel consumption, and it bit me.

 I think Brett's and Randy's explanation of the increased power requirement and regulation have confirmed my thoughts about needing to add a little bit more fuel when increasing pitch. The amount would depend on how much harder and longer the engine needed to perform at the higher power requirement.

 As Randy stated, I too, usually add a bit more fuel when changing props of unknown performance, but didn't this particular time and I got a surprise. That got me thinking and is what prompted the question.

 Oh, I did get the result I was looking for with the increased pitch, it's just that the fuel thing surprised me.

 Thank you all for your responses and confirming what I suspected when I sat down and actually thought about what I had done.

 Thanks,
   Larry

[John Leidle]

   I have the exact experience that Randy C. described.  Normally when you increase the load on the engine you open the needle in order to keep the same type of run.
           John

[Steve Helmick]

"I increased prop pitch by 1/2 inch and used the same launch RPM, since I wanted to increase flight speed a bit. The measured fuel load was the same for both props. With the lesser pitch prop I had 6 to 10 laps after the clover. With the 1/2 inch greater pitch prop, the engine quit going uphill after the last loop in the clover. I removed less than 1/4 ounce of fuel from the tank after that flight, so I am pretty confident that it just ran out of fuel. I saved the plane, but I don't care to experience that again."

I made a similar change recently, but in an attempt to launch at a slightly lower rpm (i.e., richer). By increasing pitch from 4.25" to 4.75" (both are strictly my opinions on same Prather gauge), I lowered the launch rpm by about 200 (to 10.3k) and increased fuel consumption back up to a full 6 oz, which was part of the goal. I also got much better line tension overhead and a more obvious signal of impending shut-down. The basis for this experiment came from reading Randy Smith's pinned articles and a short chat with Mike Haverly. It worked pretty much as expected, and I finally developed some skill with repitching CF props. Can't say that I like it, but now I feel like I can do it and get decent results.    Steve

[Douglas Ames]

Lower rpm, lower fuel use.


MM

Depends how its loaded.