How many Watts equal 1 HP?

[Mike C]

I have read the answer to this question before but can't seem to find it using the search functions. My nephew and I were talking about this tonight in regards to how many watts of power it takes to produce the same thrust using the same prop with electric vs. glow. We were wondering about 1 HP because that would give us a basis to figure any glow or gas engine that the maugacturer give the HP rating for.

[Rod]

Mike, 745.7 watts = 1 horse-power

[gkamysz]

Don't go by the max rating for HP that engine manufacturers publish. That is a MAX figure and very seldom is an engine for sport use operated at that figure. HP drops off quite a bit when the RPM is not at the quote figure usually 16+kRPM. It would be better to find actual prop test data and back calulate HP based on prop size and RPM.

400W will fly a .20 size aerobatic model very well. While you will see many .20 size engines with 1.0+ HP ratings. That will give similar performance.

Greg

[ChrisP]

The air doesn't care at all whether the prop is being turned by a rubber band, an electric motor, an internal combustion engine or whatever. The air only 'sees' a propeller with a certain diameter and pitch turning at a certain number of rpm's. So an electric motor with 745 watts will turn the same prop at the same rpm's as a glow motor with 1 horsepower.

When designing my models I use German test data on model propellers which give the rpm's for each prop based on an input of 100 watts. This is what Greg is referring to.

Remember that the power requirements for a prop are in a cubic relationship to the rpm's. If you want to turn the prop twice as fast you need eight times the power ! Many glow pilots just don't realize how much extra power is really needed for 'only 300 rpm's more'.

[jrb]

The actual power produced by an engine is different (less than) its rated power because of elevation & temperature effects. In general, 3.5% loss for each 1000' above sea level & 1% loss for each 10°F above its rated temperature. If an engine rated @ 10hp standard conditions (sea level/50°F) is operated @ 2000’ & 80°F at best it only produces 9hp.

[escapee]

& of course, watts from the battery does not = watts @ the propeller though some setups can get sorta close.
This is really more like an art form than a science? at least tinker form. :)

Does somebody know what the typical difference is between current measured statically vs flying?

[Dick Huang]

Mike C,
A typical Cross-scavenged glow engine develops about 2 hp/in^3.
This would typicall be the Fox 0.35 stunt engine; using 2 * 0.35 = 0.7 hp or 522 watts. An OS Max 0.20 would develop 2*0.2=0.4 hp or 298 watts.
Dick Huang:)

[Sparky Paul]

Originally posted by escapee
& of course, watts from the battery does not = watts @ the propeller though some setups can get sorta close.
This is really more like an art form than a science? at least tinker form. :)

Does somebody know what the typical difference is between current measured statically vs flying?
.
This on-line calculator includes computations for
static and inflight parameters. The accuracy of the results isn't known.
I did a regression analysis on the static and inflight thrusts for several props and battery combos looking at watts and thrust, and got reasonably consistent answers, but if the flight information is correct ????

http://custompaintjob.com/ezcalc/dma.asp

[vintage1]

I always thought it was 763W per bhp but http://www.onlineconversion.com/power.htm gives it as 746....

That's a great web site for these conversions by the way.

Bear in mind also that we can gear the electrics and get much better prop efficiencies than a glow at slow to moderate speeds, so its possible to fly an electric on a lot less power than the equivalent glo engine...

[ChrisP]

Originally posted by vintage1
I always thought it was 763W per bhp but http://www.onlineconversion.com/power.htm gives it as 746....A few years back it became a legal requirement in Germany to specify the power of car engines in Kilowatts instead of Horsepower. The reason was, as you infer, there was never a common definition of 'horsepower'.
In fact I think at the time German horses were about the laziest at a measly 735 W.

[vintage1]

Worse still, there was no agreement whether you measured it at the flywheel, or out of the cars wheels.

AND remember that whilst BRAKE horsepower is 745watts or whatever, HORSEPOWER was the bore in pikes divided by the stroke in mm times the number you first thought of (or something like that), and was indeed only so defined in order to rate cars for taxation.

It led to a lot of UK cars being built with long stroke engines ISTR,. as this lowered the effective horsepwower rating for a given swept volume..

...additional info for completeness....I found this

"Up until 1920 taxation of motor vehicles was based roughly on a guinea per 4 horsepower, this was in addition to a petrol tax of 3d per gallon. However on the 21st January 1921 the Motor Taxation Act which taxed private cars at £1 per horsepower was enacted. This was created by using a complicated R.A.C formula; it was worked out that one horsepower was equal to 2 square inches (12.9sq.cm) of piston area, and came to be known as the 'Treasury Rating'."

There you go. You really wanted to know that didn't you?