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Old 11-03-2009, 11:48 PM   #1 (permalink)
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Getting 45 MPG at 207 MPH .... can you pulse and glide an airplane?

Autopia has an interesting piece about an airborne fuel economy competition won by this funky looking plane getting 45 MPG at 207 MPH:



Source: Getting 45 MPG at 207 MPH | Autopia | Wired.com

Now the article says:

Quote:
If 45 mpg doesn’t sound like much, consider that an airplane’s engine is constantly working to keep the plane aloft and moving forward. There’s no coasting, which you can get away with in a car using the old pulse-and-glide.
I know there are a few pilots around here. What I wonder is whether that quote about P&G is true. I would think that you could pulse & glide an airplane, pulse = gain altitude; glide = well, gliding.

And theoretically you should be able to pin an airplane's engine's load pretty much indefinitely at exactly the right BSFC point for max pulse efficiency, no?

Full article: Getting 45 MPG at 207 MPH | Autopia | Wired.com

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Old 11-04-2009, 12:06 AM   #2 (permalink)
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Yes, if you had a plane that was essentially a powered glider. Bart Rutan's Voyager that made its nonstop around-the-world flight was exactly that, and traveled about 25,000 miles on about 8300 lbs of fuel. At 6 lbs/gal, that's only 20 mpg, but they were hauling over 9000 lbs. of weight on a plane that only weighed 939 lbs empty.
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Old 11-04-2009, 12:10 AM   #3 (permalink)
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I think it may be what happens when you put wings on a mustang perhaps?

Quote:
The Fuelventure 400 is more than a showcase for efficiency. Pilots are handicapped based on weight, number of passengers and speed. The idea is to encourage more of a real world effort rather than some ultra light weight, slow moving, hypermiler competition in the sky.
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Old 11-04-2009, 12:36 AM   #4 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
And theoretically you should be able to pin an airplane's engine's load pretty much indefinitely at exactly the right BSFC point for max pulse efficiency, no?
Pretty much. The large variation in automobile operation and lack of suitable gearing relative to displacement is the only reason a car has to pee and gee in order to optimize engine efficiency.
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Old 11-04-2009, 01:16 AM   #5 (permalink)
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It would not surprise me if you could max your cruise distance simply by adjust your rate of climb so you were at bsfc peak, then coast back down at VBG
(feathered prop would be nice).

Wish I knew the specs (CDA, etc etc).

FYI wikipedia entry: Rutan VariEze - Wikipedia, the free encyclopedia

puts it at 35mpg at 181 mph in "stock" form.
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Old 11-04-2009, 02:03 AM   #6 (permalink)
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There are several dozen variations of this plane including one done with really long wings that is close to a glider in performance. Most build them with a hotrod motor cause they will close in on 300 mph if done right..... and even plane people can never go fast enough...

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Old 11-04-2009, 02:07 AM   #7 (permalink)
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Quote:
Originally Posted by dcb View Post
It would not surprise me if you could max your cruise distance simply by adjust your rate of climb so you were at bsfc peak, then coast back down at VBG
(feathered prop would be nice).

Wish I knew the specs (CDA, etc etc).

FYI wikipedia entry: Rutan VariEze - Wikipedia, the free encyclopedia

puts it at 35mpg at 181 mph in "stock" form.
That's essentially how all commercial scheduled flights operate. They accelerate (pulse) to 30,000 ft cruising altitude where the air is thin and resistance is low. Then they semi-glide back down.

From my experience in smaller propeller planes, they do operate at best BSFC all the time after takeoff is completed.
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Old 11-04-2009, 02:35 AM   #8 (permalink)
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Quote:
Originally Posted by SentraSE-R View Post
From my experience in smaller propeller planes, they do operate at best BSFC all the time after takeoff is completed.
You made the key point right there.

There's no need for "pulse" as we think of it in automotive terms cuz cruise loads a/c engines accordingly for the duration.

That is another reason why you generally can't just pluck an automotive engine out of a car and stick it into an aircraft. The duty cycles are waaay different, with the auto engine loafing along most of it's life with brief periods of higher load, and the a/c engine working pretty hard most of it's life, with brief periods of even higher load. That's why we ought not laugh too hard at an a/c engine that uses, say, 330 cubic inches to put out 100 hp. It's not because the engineers were dense.

There are motorized gliders that I suppose could be considered "pulse-and-glider(ers). They aren't in it for the fe though. They are mostly for recreation- not seriously trying to get from a to b- and the motor gets the tow plane out of the equation and allows for more thermals-hunting per mission anyway.
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Old 11-04-2009, 08:18 AM   #9 (permalink)
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Airplanes can choose their speed and altitude, so they can adjust their situation to their best BFSC map and get great mileage, if they are designed for that purpose and can climb high enough.

Much of this same technology can be used in cars, as long as you disconnect the engine from the wheels. While the airplane can use altitude to store energy, cars can use hills for the same purpose, but with cars you have to be lucky enough to find the right hill.

The motor gliders are superb examples of P&G in airplanes, especially if they can find a thermal, which is similar to a draft in a car.

I have thought about a hydraulic power boost for takeoff in an airplane, so the engine size could be reduced,

We used to catch Mallards in a jon boat with a crab net. If you chased them, they would take off and land a few hundred yards away. After 3 or 4 chases they were too exhausted to get off the water and they would go underwater, and we would scoop they up with the crab net.

The point is they used so much more energy taking off than it took to maintain flight it was easy to take them alive.

I believe the same applies to vehicles, and the practice of having giant engines to allow acceleration. Short term energy high capacity energy storage for the seldom encountered surges of power necessary to drive, would allow much smaller engines to do the job. If steep grades were an issue the some form of pressurized induction would allow speed to be maintained on steep grades.

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Old 11-04-2009, 01:57 PM   #10 (permalink)
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Quote:
Originally Posted by SentraSE-R View Post
That's essentially how all commercial scheduled flights operate. They accelerate (pulse) to 30,000 ft cruising altitude where the air is thin and resistance is low. Then they semi-glide back down.
.
Except that their glides are interrupted by ATC commands to level off at intermediate altitudes. Some of the technologies associated with NextGen ATC will allow airliners to literally glide with engines at idle from upwards of 30,000 feet to landing.

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