RC Extreme power

My Feedback: (24)

For all those out there that can understand RPM to HP would like your comments on something

I see different HP ratings for different motors at different RPM

I see one motor that say's it is rated at 6.5 HP but when I see some of the RPM readings of 68-6900 rpm on a 22-10 prop and other engines rated at 5.2 HP doing 7000 on a 22-10, It confusses me to no end.

I know different props will give different reading but not that much.

How are these people coming up with these rating ?????

skiman762

guesstamation most likely

Ken6PPC

Great imagination?

Props can make some differences, certainly. However, not THAT much difference.

I like using Pe Reivers' Prop Thrust Calculator. http://www.mvvs.nl/

Rcpilot

My Feedback: (78)

They are just guessing.

It's called:

"Pulling something outta your arse."

OR

S.W.A.G --- Silly Wild Arse Guess

You don't REALLY believe they put these engines on little bitty dynos--do ya?

The HP ratings of motors means about as much as the price of tea in China. They are exaggerated from charts and graphs. They don't mean diddely squat.

mmattockx

The formula for HP is:

HP = Torque * RPM / 5252

Torque is in ft-lb for this formula
5252 is a constant that comes from 1 HP = 550 ft-lb/sec and rpm being revs/minute instead of seconds.

It depends entirely on where the engine is tuned to run. You could indeed have a high rpm engine that turns a 22x10 at 6800rpm, yet makes 6.5hp at a much higher rpm on smaller props. Conversely, a 5.2hp engine may turn that same 22x10 at 7000rpm if it is tuned to peak at that rpm with that prop load.

I would like to think that they are indeed running the engines on dynos to get these numbers, but I doubt it. Prop and RPM data combined with an accurate prop/load/power calculator like Pe Reiver's is a decent way to work back how much power your engine is putting out. Note that is only because Pe has put in an uncountable number of hours developing accurate prop load factors and refining his spreadsheet to reflect his test results. Anything else is just a guess, probably a bad one at that.

Mark

Jake Ruddy

My Feedback: (40)

I have to say I have never looked at HP numbers on any rc engines.

I used to install high end car audio many years ago. It was always funny to me to see the customer who would say why is that 50watt x4 (at 4 ohms - but could do 400 watt x 4 at .5 ohms and be happy) $600 when I can go to radio shack and buy a 400watt x 2 amp for $199? Little did they know that the $199 amp would never do half that power and wouldn't sound half as clean as the high end amp would.

These numbers are all marketing hype.

Does a car really get 35mpg?

Rcpilot

My Feedback: (78)

Off Topic:

Actually, my wifes 1998, 4-door, Honda Civic LX, 4-speed automatic. 1.6L 4-banger will get 35mpg at 73MPH. That seams to be the sweet spot. 65MPH yields less mileage per gallon. I used to travel about 80 miles -- 1-way to work. I drove it like an old lady. No hard acceleration on the ramp to the highway. No hard acceleration to pass. Just put the cruise on and try to anticipate traffic ahead so that I could just ease up on somebody and pass them without holding up traffic in the left hand lane.

It still gets 31mgp at 88--90MPH.

You just about HAVE to do 85MPH on I-25, north of Denver, or you'll get run over. It's a regular super-speedway out there. [X(] Kinda scary. Glad I don't have to run that highway every day anymore.

I keep the tires aired up, oil changed, air filter changed, fuel filters etc.. It's gotta be perfectly tuned up to get that mileage. I can see the difference about every 20,000 miles when the air cleaner and fuel filter need changing. It drops down into the low 30's or high 20's on the fuel mileage. Even the tranny fluid must be changed every 30,000 miles or it drops about 1/2mpg.

On Topic:
HP numbers for our airplane engines don't mean a whole lot. In reality, most of them are pretty close in power.

Your average 50cc will turn a 22-8 around 7000RPM. 21-8 about 7400RPM. 23-8 around 6600RPM.

Average 60cc engine will turn a 22-10 around 7200RPM. 23-10 around 6800RPM. 21-10 around 7800RPM.

Most 80cc engines will swing a 26-10 around 6700R--6800RPM.

None of them are exponentially more or less powerful than the competition. You can buy an old, used up engine and it may be down on power by a few hundred. But, those old, used engines are RARE as hens teeth. Most of them get wrecked before they ever see 200 gallons of fuel. 200 gallons isn't anywhere NEAR run out for most gas engines. If you just put some oil in it and don't heat it up--they should run 500 gallons or more.

If your curious about which one to buy for a particular plane, I'd suggest you build the plane and then figure out how much firewall weight it takes to balance it. Buy an engine that weighs close to that amount and bolt it on.

the Wasp

My Feedback: (1)

food for thought,,

in the past I researched the Brison 40cc against the MVVS 35cc (gassers w/mufflers)

these 2 engines will pretty much turn the same RPM with a Zinger 20x8 pro, about 7700 RPMs (peaked),,

but put a 20x10 on them and the Brison comes threw,,, about 7100 for the Brison and about 6500 for the MVVS,,

so larger diameter props with less pitch are more efficient,, so if you go down in diameter and up in pitch you will loose pulling power at lower RPMs, so with that, this tells us that a 20x10 over all is the best prop for the Brison while a 20x8 is best over all for the MVVS,,


my MVVS 35 against my G38,, my MVVS peaked out with a 3W 20x8 at 7370, while my G38 with the same prop peaked was 6600,,

so with all this, when you read someone is getting 8300 on their G38 with an 17x10 or 18x8, they really would be better-off turning 7100 with an 19x8 or 20x6,,

Jim

RC Extreme power

My Feedback: (24)

I know that higher RPM does not = more power, Found that out when I built my thrust stand, Had a 21-8 on engine and got 7800 rpm 25 lbs of thrust put on a 22-10 and got 7000 rpm and thrust went up to 29 lbs.

Jezmo

I absolutely agree with your post 100%, my findings have been essentially the same. My only comment is toward the final car question. Without naming the manufacturer other than to say it is one of the US big three, I just purchased a 2008 vehicle brand new at the beginning of the year. After it got some miles on it (in the thousands) the mileage is almost what the sticker said it would get on the highway for around town mileage. On the highway it gets about 7 to 8 miles to the gallon better than the sticker rated highway economy. I just thought you would be interested in knowing there are anomalies out there to the golden rule of marketing hype. Rare but they do exist.

blw

My Feedback: (3)

I've been driving a S 2000 for 2 1/2 years now, and it gets somewhere from 29 to 30 miles per gallon just about all the time. It doesn't seem to matter if I shift at the economical points, or run it into VTEC most of the time through the gears. Maybe half of my driving is on the Interstate, and it doesn't seem to matter what speed I cruise. Sometimes, I've used a tank of gas that was mostly red lining it through the gears and some fast driving on the Interstate when guys wanted to get it on, the mileage seems to be the same then too. Must be the ECU.

rmh

Honda at one time--had some of the most flat torque curves i have ever seen. My first was a new 1977 Accord -I was astonished at how that thing performed- having a GTO at the same time (a real POS) a A VW van ,I was a slowly reforming hot rodder ( took more years tho to get past the urges )
The idea of torque vs HP- goes on and on and as some know they are simply "numbers" meaningless -unless you look at the application
The Peak Torque rpm on our model engines is elusive for most to find.
A loadprop (a calibrated prop which requires a known amount of power to turn) is all in all the best approach
these are impossible to find now --
Over the years -I have fouund several props (as others noted ) which have known rpms on certain engines.
These allow comparative data -which is reqlly all we relly need - just run n record.

soarrich

My Feedback: (98)

I ran Pe's spreadsheet, it seems pretty accurate. I had one of the 50 single Gr Pro type engines in a 17 pound Sukhoi. I was getting 6200 rpm with a 22x8, Pe sheet said that was 19 pounds of thrust, the plane could just hover with a VERY slow pull up, so I would say it verifies the spreadsheet.

the Wasp

My Feedback: (1)

after writing my reply above last night I read another post on another page,,

these guys were tell their RPM numbers, then a couple of them said their engines went from idle to top RPM almost instantly, this tells me they are spinning the wrong prop, they are not loading their engines, all they are doing is spinning the prop, kind of like driving a car down the hi-way at 60 in first gear, they are not really pulling much air, these guys need to go up in diameter or pitch..

Jim

soarrich

My Feedback: (98)

+
With that in mind and seeing Milton's thrust video I bought a digital fishing scale, so now I'm going to be measuing thrust to pick a prop, the tack will be just for engine set-up.

the Wasp

My Feedback: (1)

I had a 3W 19x8 on my G38, this prop measured 18&5/8 inches long,, I got 7000 RPMs from that engine tuned for flight, peak RPM was 7200,,

my MVVS 35 with a 3W 20x8 pulls much better at 7100 RPMs

Jim

Jezmo

My car also has variable valve timing (like VTEC), gets 27 to 29 mpg around town, and gets anywhere from 34 to 38 mpg highway. I am very impressed with the way it runs and the gas mileage, but since it was made by one of the big three US makers I'll just have to wait and see how it holds up. It's big enough for me to put all of my planes in to take them to the field as well as all of my field equipment. I can even get my Giant Spacewalker in it (82" wingspan). VTEC and the ECU must be how they're getting all the mileage. Have a great one Barry.

Edited for punctuation.

mmattockx

Rich,

Be careful with that. Measuring static thrust is only a useful tool if you are concerned with low speed 3D flying and hovering. If your goal is more IMAC or pattern style flying at higher speeds, you will be very disappointed by the results.

Milton, in your example you either ran different brands of props or your engine had a drastic fall off in torque above 7000rpm. According to Pe's calculator, your engine was putting out 7% more horsepower at 7000rpm than at 7800rpm. That equates to 19% more torque at 7000rpm than 7800rpm, which is a very drastic drop off over a short rev span. If you used different props, then all bets are off because they will have different load factors and efficiencies.

The real problem is that most people don't really understand torque and horsepower and what they mean. For the most part, it also isn't important, it is what prop an engine will swing at what rpm that matters for our purposes. But a thorough understanding does make it easier to optimize airframe/engine/prop combinations.

Just to point this out - the mileage figures are NOT marketing hype, they are EPA test figures that are produced independently of the OEM.

There, now I feel better.


Mark

soarrich

My Feedback: (98)

Just to point this out - the mileage figures are NOT marketing hype, they are EPA test figures that are produced independently of the OEM.

There, now I feel better.


Mark
[/quote]
I only have an interest in 3D, IMAC is of no interest to me.

The mileage figures are going to be down about 20% starting in 2008, they are using a different test to get the mileages.

mmattockx

If 3D is all you do, then static thrust is a big part of the equation and is a very useful indicator for you.

That ought to get people worked up - "But last year it got 25% better mileage..."[X(]


Mark

RAMMJETT

i will take a shot .. hp is work over time .meaning how mutch work can be done in a given amount of time. if you rev higher you do more work in less time.a engines horsepower can be rated a any rpm and will increase with rpm.do not confuse this with torque witch is greatest at a given rpm.that is why you must prop at max torque witch is at a point mutch less than therotical possable max hp.wow hope that helps.Roger

RAMMJETT

in a nut shell a 10 hp engine is only 10hp at it's max rated rpm you could prop for this rpm.but you would be so far out of the max torque range the engine would be ineffective.max torque is usally at 75% of max rpm .so in fact a 10 hp motor will yeald in most cases no more than 7.5 hp.it would be so nice if they would rate engines torque this is a mutch more valuble mesurment

RAMMJETT

i guess i forgot torque is applied force(how hard the engine can push)

RAMMJETT

here i go again.the way i was tought.imagine 2 piles of sand one with a big guy and a shovel the other a small fast guy with a spoon. they both move the same amount of sand in the same time .same horsepower.(work over time) and that is the way we buy our engines .tell you nothing about what it can do at any one given point of time .torque.(applied force)

RTK

My Feedback: (1)

Good example rammjet