Horse Power help!

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BBerson

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Apparently flying is very inefficient compared to walking or riding a bicycle.
Certainly. It is much more work to pedal an airplane than a bicycle or walking.
But you could build an airplane that flies on a 1/4hp. It would not be very strong for typical conditions.
 

lr27

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Airplanes aren't NECESSARILY less "efficient" than cars. Some airplanes, in cruise, get better mileage than a small car. I wonder what mileage the DA-11 gets cruising at 125 mph with a 20 hp engine?

80 percent efficiency is pretty good, but I doubt very many airplanes see that at any time during their takeoff run. NONE get that at the moment of brake release on a calm day. A variable pitch prop can only help with part of the problem. At low speeds, you need a large diameter to be efficient. The famous Daedalus human powered airplane used an 11.3 foot prop to absorb a fraction of a horsepower at something like 15 mph, at low rpm.

If wheel bearing drag was significant, hubs would be smoking hot after landing. 1 hp is equal to 746 watts, or as much power as an ordinary microwave.
 

proppastie

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Actually one horsepower is doing 550 lb-ft of work in a second, such as lifting a 200 lb weight 2.75 feet every second. Last I checked, I could take my 200 pound self up a stairway faster than that.
which brings up another issue. ....I thought humans could only generate about 3/4 hp.
 

proppastie

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Probably just mental masterbation.....we can account for the loss and do accurate calculations.....that it is not intuitive should not be an issue.
 

pictsidhe

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Actually one horsepower is doing 550 lb-ft of work in a second, such as lifting a 200 lb weight 2.75 feet every second. Last I checked, I could take my 200 pound self up a stairway faster than that.

Billski
For how many seconds? 100W is the sustained output per day of a average healthy person. Athletes can double that. Peak hp from a horse is around 15. But if you want it to work for 10 hours, it is 1 horsepower. From a fit human, the peak is over 1. At the age of 18, I somehow got into a pissing contest with someone. It involved running up 3 flights of stairs, carrying sacks of flour. I won. I manged to do it with 220lb of flour on my shoulders. I was quite glad when he admitted defeat...

The English Electric Wren flew on 8hp, and managed 50mph 87.5mpg. It was not very spritely, however.
 

Dana

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550 ft-lb is a lever 550 ft long with one lb on the end,...or 1 ft long with 550 lb force on the end. It is hard to wrap my head around that it takes at least 15-20 of that to fly the most minimal of aircraft or PPG. Apparently flying is very inefficient compared to walking or riding a bicycle.
No, you're confusing torque with work. Torque and work have the same units, which makes it confusing, so we usually refer to torque as ft-lb (or newton-meters or whatever), that is the length of a lever times a force applied perpendicular to the end of the lever. You can have torque without motion if there is an equal opposing torque. Work is force applied over a movement in the same direction as the force; we usually say lb-ft to make the distinction. In metric units, one joule is one newton-meter of work.

Power is the rate of doing work, e.g. lb-ft per second or joules/sec. 550 lb-ft/sec is one horsepower; in metric, one Watt equals one joule/sec.
 

Jay Kempf

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Yeah, the old torque vs. HP debate that goes on in most motorsports forums and garages/hangars around the planet. Been lobbying for years that we should call that Torque that is graphed on an engine dynamometer power curve "Dynamic Torque." as it is unfolding at some RPM. The more classic Newtonian or Archemedian one we can call "Static Torque." Whaddyathink?
 

mcrae0104

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That is the price for being airborne. Once you have paid the cover charge, you can add some more power and climb or go faster or both and get some benefit for the cost of running the thing.
It is much more work to pedal an airplane than a bicycle or walking.
There is a paper entitled What Price Speed? that deals with this topic. It is interesting to see that some aircraft can be more efficient--relative to their speed--than some wheeled vehicles.Pages from EPPaper080229E.jpg
 

lr27

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As I recall, the right version of a fully loaded 747 can get the equivalent of over 110 seat miles per gallon on a medium long trip. I don't remember just how long, but I think it was more than 1,000 miles.
 

Aesquire

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Human powered planes figure on 1/3 hp. And they pick skinny little guys with big legs and thin arms. G loading designed for 1.x so the Gossamer Condor never banked more than about 30 degrees.
 

lr27

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The Daedalus airplane broke up in the air while, I think, setting up for landing. Fortunately from low altitude over water.
 

BBerson

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Swampyankee

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Great Zombie thread.....found it as I thought about how 550 ft/lb-sec, for one hp seems like a lot of force.
James Watt concluded that a horse could produce about 33,000 ft-lbf/second of power (he was wrong. Horses can routinely produce and sustain more than a horsepower, and there is significant feeling in the engineering and communities that he chose 33,000 ft-lbf/min knowing that this was significantly less than a horse could produce as he wanted to sell steam engines to replace those horses ;))
 

wsimpso1

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80 percent efficiency is pretty good, but I doubt very many airplanes see that at any time during their takeoff run. NONE get that at the moment of brake release on a calm day. A variable pitch prop can only help with part of the problem. At low speeds, you need a large diameter to be efficient. The famous Daedalus human powered airplane used an 11.3 foot prop to absorb a fraction of a horsepower at something like 15 mph, at low rpm.
The reason most props are not more than 80% efficient on takeoff is because most props are designed for best efficiency in cruise. If you wanted a prop to be 80% efficient at low speeds and low power, you could do that. You could even design it for that efficiency at zero speed, if you were measuring change in air energy in the thrust direction per unit power input. On the other hand, total propulsive efficiency is always zero when you are not moving- that is called "trivial case".

As to size for efficiency, you are now confusing Efficiency with Effectiveness. EffIciency is the ratio of work made good vs work input. In props that has to do with how much air moved by how much change in velocity is made on the air. Small props can have high efficiency, but will still be lower static thrust than a larger prop at same efficiency. Helicopters use large rotors for this reason - larger props mean more static thrust from any given amount of power. Daedalus needed a large lightly loaded slow turning prop to make enough thrust from one human to drag a large lightly loaded airframe into the air.

Billski
 
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lr27

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I think your version of efficiency may be good for fans, but for airplanes I think the correct unit of measure is thrust times airspeed divided by shaft power. If it doesn't show up as thrust, the airplane can't really use it, even if you knock over that guy standing 100 feet back, directly in line with the prop's axis.

You could put 100 hp into a teensy prop that transferred 80 percent of the shaft horsepower into the kinetic energy of a Mach 0.5 stream of air. However, it wouldn't fly a cub as well as a 40 hp engine turning 2,000 rpm with a large prop.
It's possible to define efficiency in useful ways and in less than useful ways. I once participated in a wave energy project with a device which we calculated as 140 percent efficient. If that had been true with an appropriate definition of efficiency, we would have had the trickier half of a perpetual motion machine. Alas, it was not to be.
 

Dan Thomas

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As I recall, the right version of a fully loaded 747 can get the equivalent of over 110 seat miles per gallon on a medium long trip. I don't remember just how long, but I think it was more than 1,000 miles.
I think a Greyhound bus could do better than that. 4 MPG (guessing here) times 45 pax is 180 seat-miles per gallon.
 

lr27

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Obviously, no airborne vehicle will beat a well faired bicycle with a teensy motor or a truly lightweight, streamlined rail vehicle, or a sailing ship At least on seat miles per gallon.
 

proppastie

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Obviously, no airborne vehicle will beat a well faired bicycle with a teensy motor or a truly lightweight, streamlined rail vehicle, or a sailing ship At least on seat miles per gallon.
Hill launch or winch launch glider.
 

wsimpso1

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For how many seconds? 100W is the sustained output per day of a average healthy person. Athletes can double that. Peak hp from a horse is around 15. But if you want it to work for 10 hours, it is 1 horsepower. From a fit human, the peak is over 1. At the age of 18, I somehow got into a pissing contest with someone. It involved running up 3 flights of stairs, carrying sacks of flour. I won. I manged to do it with 220lb of flour on my shoulders. I was quite glad when he admitted defeat...

The English Electric Wren flew on 8hp, and managed 50mph 87.5mpg. It was not very spritely, however.
Hmm, OK, let's do data.

Adult humans have about 100W of waste heat just sitting in a chair. That is 0% efficiency, but without it, that human is DEAD, so that poor efficiency is acceptable.

Couch potatoes may only be up to generating 100 W of motive power, either moving themselves around or walking on a drum windlass to run a crane on an ongoing basis, but professional cyclists can make 400 W continuously for several hours a day, every day. It is just a matter of how well trained the human in question is at processing oxygen and how many calories they consume.

And the point of my observation on my personal peak horsepower? When someone says something has horsepower, some folks see a galloping race horse. This 63 year old man can make a couple HP for a few seconds. When somebody says two to three horsepower, this 63 year old man does not see a galloping horse, he sees a critter with six to ten times his muscle mass at a modest walk turning a windlass all day long and hardly get worked up. One horsepower is a pretty anemic horse, but it does lift 550 pounds one foot per second. It is just a unit of work per unit time that Watt came up with to market his steam engines, and most students of the topic know that he underestimated how much a horse could do to his benefit.

Billski
 
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