Author Topic: Saving fuel  (Read 2008 times)

Offline bozon

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Re: Saving fuel
« Reply #15 on: April 22, 2014, 04:29:03 AM »
The P38s in the pacific were running at high throttle and reduced RPM at the advice of Lindberg. They initially feared that this will wear down the engines, but found it was not so bad.

Lots of energy is lost on moving parts. Less RPM means the parts are moving leas, though there some counter effects so low RPM and high throttle is not a pure gain.
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Offline earl1937

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Re: Saving fuel
« Reply #16 on: April 22, 2014, 04:13:54 PM »
What is the advantage/disadvantages of changing prop rpm vs adjusting the throttle?
:airplane: Rather than go into a lot of detailed engineering explanation, consider this: If you have your prop control or controls to the full increase position, the prop blades will be in their "flattest" position and will allow the engine to turn maximum RPM. This position is good for climbing or carrying heavy loads. It does nothing to promote hi speed in the full increase position.
If it is speed you are after, you must know at what RPM on Tachometer your prop blades are  the most efficient, with a full open throttle!
There are many factors to consider when operating air cooled aircraft engines. If you constantly run hi manifold pressure settings and low rpm's, you are going to burn a hole in the top of the cylinder piston.     
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Offline Puma44

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Re: Saving fuel
« Reply #17 on: April 22, 2014, 05:09:51 PM »
Here's an excerpt from an article about Lindbergh and the Pacific P-38s found via Google.  Interesting read.

"First one, then two pilots reported dwindling fuel and broke off for home. MacDonald ordered the squadron back but because Lindbergh had nursed his fuel, he asked for and received permission to continue the hunt with his wingman. After a few more strafing runs, Lindbergh noticed the other Lightning circling overhead. Nervously the pilot told Lindbergh that he had only 175 gallons of fuel left. The civilian told him to reduce engine rpms, lean out his fuel mixture, and throttle back. When they landed, the 431st driver had seventy gallons left, Lindbergh had 260. They had started the mission with equal amounts of gas.

Lindbergh talked with MacDonald. The colonel then asked the group's pilots to assemble at the recreation hall that evening. The hall was that in name only, packed dirt floors staring up at a palm thatched roof, one ping pong table and some decks of cards completing the decor. Under the glare of unshaded bulbs, MacDonald got down to business. "Mr. Lindbergh" wanted to explain how to gain more range from the P-38s. In a pleasant manner Lindbergh explained cruise control techniques he had worked out for the Lightnings: reduce the standard 2,200 rpm to 1,600, set fuel mixtures to "auto-lean," and slightly increase manifold pressures. This, Lindbergh predicted, would stretch the Lightning's radius by 400 hundred miles, a nine-hour flight. When he concluded his talk half an hour later, the room was silent.

The men mulled over several thoughts in the wake of their guest's presentation. The notion of a nine-hour flight literally did not sit well with them, "bum-busters" thought some. Seven hours in a cramped Lightning cockpit, sitting on a parachute, an emergency raft, and an oar was bad, nine hours was inconceivable. They were right. Later, on 14 October 1944, a 432nd pilot celebrated his twenty-fourth birthday with an eight-hour escort to Balikpapan, Borneo. On touching down, he was so cramped his crew chief had to climb up and help him get out of the cockpit.

The group’s chief concern surfaced quickly, that such procedures would foul sparkplugs and scorch cylinders. Lindbergh methodically gave the answer. The Lightning's technical manual provided all the figures necessary to prove his point; they had been there all along. Nonetheless the 475th remained skeptical. A single factor scotched their reticence.

During their brief encounter, MacDonald had come to respect Lindbergh. Both men pushed hard and had achieved. Both were perfectionists never leaving things half done. And both had inquisitive minds. John Loisel, commanding officer the 432nd, remembered the two men talking for long periods over a multitude of topics beyond aviation. If, as MacDonald had informed his pilots, better aircraft performance meant a shorter war, then increasing the Lightning's range was worth investigating. Lindbergh provided the idea, but it was MacDonald's endorsement, backed by the enormous respect accorded him by the group, that saw the experiment to fruition. The next day, the Fourth of July, Lindbergh accompanied the 433rd on a six-hour, forty-minute flight led by Captain "Parky" Parkansky. Upon landing, the lowest fuel level recorded was 160 gallons. In his journal entry Lindbergh felt ". . . that the talk last night was worthwhile. " The 475th had lengthened its stride."
 :salute



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Offline hitech

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Re: Saving fuel
« Reply #18 on: April 23, 2014, 01:03:32 PM »
Gross power output of an engine is linearly related to the volume of air fuel mixture traveling threw the engine.

There are 2 ways to "PUMP" more air threw the engine , one is make the pistons pump the air faster (i.E. higher rpm) the other is increasing the density/pressure of the air entering (throttle) the cylinder but keep the RPM constant.

Increasing RPM creates more friction in the engine, and hence less efficiency then increasing throttle which does not increase the friction.

HiTech

Offline earl1937

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Re: Saving fuel
« Reply #19 on: April 24, 2014, 04:24:54 PM »
Gross power output of an engine is linearly related to the volume of air fuel mixture traveling threw the engine.

There are 2 ways to "PUMP" more air threw the engine , one is make the pistons pump the air faster (i.E. higher rpm) the other is increasing the density/pressure of the air entering (throttle) the cylinder but keep the RPM constant.

Increasing RPM creates more friction in the engine, and hence less efficiency then increasing throttle which does not increase the friction.

HiTech
:airplane: Your point about the friction is correct, but what you forgot to mention is with high manifold settings the dynamic force of added fuel and air in the cylinder can and will do damage to the engine in the long run. Scored intake valves and holes burned into the piston heads. While"Lindy" did extend the range of the 38's as well as some other aircraft, the damaged to the engines in the long run resulted in a higher frequency of inspection and cylinder replacements. I can't exactly give you the bulletin which changed the inspection procedures, but the "Ponie" drivers in Europe experienced higher maint. time per flight hour than before using his method of fuel conservation.
The D and B model will go a long way on 2,000RPM and 32 inches of manifold pressure, but you will piss off the crew chief when he finds out about it.   
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Offline bozon

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Re: Saving fuel
« Reply #20 on: April 24, 2014, 05:39:45 PM »
:airplane: Your point about the friction is correct, but what you forgot to mention is with high manifold settings the dynamic force of added fuel and air in the cylinder can and will do damage to the engine in the long run. Scored intake valves and holes burned into the piston heads. While"Lindy" did extend the range of the 38's as well as some other aircraft, the damaged to the engines in the long run resulted in a higher frequency of inspection and cylinder replacements. I can't exactly give you the bulletin which changed the inspection procedures, but the "Ponie" drivers in Europe experienced higher maint. time per flight hour than before using his method of fuel conservation.
The D and B model will go a long way on 2,000RPM and 32 inches of manifold pressure, but you will piss off the crew chief when he finds out about it.   
If that is the only way to achieve the range to do the missions, then I suppose they were willing to pay the price. How much it damages the engine probably varies significantly from engine to engine. Maybe the Allisons were more resistant to such abuse than the Merlins.

Mosquito VI - twice the spitfire, four times the ENY.

Click!>> "So, you want to fly the wooden wonder" - <<click!
the almost incomplete and not entirely inaccurate guide to the AH Mosquito.
https://www.youtube.com/watch?v=RGOWswdzGQs

Offline Puma44

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Re: Saving fuel
« Reply #21 on: April 26, 2014, 12:41:38 PM »
:airplane: Your point about the friction is correct, but what you forgot to mention is with high manifold settings the dynamic force of added fuel and air in the cylinder can and will do damage to the engine in the long run. Scored intake valves and holes burned into the piston heads. While"Lindy" did extend the range of the 38's as well as some other aircraft, the damaged to the engines in the long run resulted in a higher frequency of inspection and cylinder replacements. I can't exactly give you the bulletin which changed the inspection procedures, but the "Ponie" drivers in Europe experienced higher maint. time per flight hour than before using his method of fuel conservation.
The D and B model will go a long way on 2,000RPM and 32 inches of manifold pressure, but you will piss off the crew chief when he finds out about it.    
Pissing off the crew chief is right at the top of the list of things a fighter jock can screw up.  



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