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How much does Horse Power matter with gas engines?

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How much does Horse Power matter with gas engines?

Old 02-15-2020, 09:06 PM
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HelloWorldC++
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Default How much does Horse Power matter with gas engines?

Hello. Im wondering how much does Horse Power matter for Gas engines?

From this resource rcplanes.online/calc_thrust.htm
If i use a prop 25x12 at 5000 RPM it will produce 15kg of static trust, it will use power of 5.48 Horse power.
Does that mean that i need a gas engine that is in the 5.48 hp range to produce that thrust?
I also see some manufactures dont list the Horse power of the engines

Last edited by HelloWorldC++; 02-15-2020 at 09:16 PM.
Old 02-16-2020, 08:20 AM
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Personally I haven't considered a manufacture's HP rating for 30 years,, and didn't really give it much credence then

Most guys "just know" if a motor is enough or not based on experience,,

Tell us what plane and motor are you considering, and you'll likely get good advice
Old 02-16-2020, 01:45 PM
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Horse power actually means very little when it comes to how much power an engine kicks out. By definition, one HP is how much force it takes to lift 33,000 pounds one foot in one minute.
Torque, on the other hand, is a measurement of force in a twisting or rotating motion. Since you want to spin a prop, the amount of torque is more important than how many HP you have. That is one reason aircraft engines don't turn the same amount of RPM that a boat or buggy engine does(not counting DF types in this), it takes more torque to spin an airplane prop than it does to spin a boat prop while a buggy engine turns the input on an reduction gearbox that multiplies torque while reducing RPM to usable levels. A second reason is that you can spin a prop too fast and it will either fail or it will go supersonic and stop giving thrust, but I digress here. Thus, if you compare three .21 sized engines, you will find the following:
Aircraft 21s will give you around 17KRPM
Boat 21s will give you around 35KRPM
Buggy 21s will give you around 50KRPM
Just for the record, these are approximates, with no load on them and based on manufacturer's literature

Last edited by Hydro Junkie; 02-16-2020 at 01:57 PM.
Old 02-16-2020, 01:54 PM
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I am working on a custom plane that will be built from fiberglass. The MTOW will be 30kg but if i can get a 4 stroke engine i can lower that number to 20-25 KG MTOW since most of it will be fuel.
Anyway, the plane will have mostly just fly like a trainer. The WCL is about 13-14 at MTOW of 30KG and Aspect ratio is 13:1

basic specs:
WCL: 13-14 (measued with MTOW of 30kg)
MTOW: 30 KG (If i can get a efficent 4 stroke gasoline engine then i can lover the MTOW down to 20-25kg)
Wingspan: 4.3m
Aspect Ratio: 13:1
Fling time: 10 hours or more

Right now im considering these 3 options.

ROTO 85 FS AL - 80 cc Max RPM is about 6000 and will use a 26x12 prop
ROTO 130 FSi AL - 130 cc
or SAITO FG-100TS - 100cc

What would be a good choice between these 3? is it possible that the Saito FG-100TS can turn a 16x12 prop?

Last edited by HelloWorldC++; 02-16-2020 at 01:56 PM.
Old 02-16-2020, 01:59 PM
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Why do you need a plane to fly for 10 hours?
Old 02-16-2020, 02:01 PM
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Will have a onboard camera soo will be used for mapping and/or surveilance
And ariel photography/recording over longer distances

Last edited by HelloWorldC++; 02-16-2020 at 02:05 PM.
Old 02-16-2020, 02:32 PM
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Originally Posted by HelloWorldC++ View Post
Will have a onboard camera soo will be used for mapping and/or surveilance
And ariel photography/recording over longer distances
For what purpose and where?
Old 02-16-2020, 06:08 PM
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With the latest rules planned by the FAA, your plane will be illegal before it's completed
Old 02-16-2020, 06:59 PM
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A lowly OS .61 a 7 lb model with 1000 sq inch lifting surface lifts over 22 lbs of lead taking off from a grass runway. No ailerons spoilerons only on outboard section of wing.

Old 02-16-2020, 09:18 PM
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Originally Posted by HelloWorldC++ View Post
...

What would be a good choice between these 3? is it possible that the Saito FG-100TS can turn a 16x12 prop?
If you actually meant to ask about a 26x12 prop - yes. That's right in the recommended range for that engine. A 16x12 would kill it in short order.

10 hours??? Even though they are good on fuel consumption, that is an awful lot of fuel it will have to carry.........
Old 02-17-2020, 03:54 AM
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Originally Posted by Hydro Junkie View Post
With the latest rules planned by the FAA, your plane will be illegal before it's completed
Not everyone on this board is from the US or under the FAA's rules
Old 02-17-2020, 03:01 PM
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Originally Posted by BarracudaHockey View Post
Not everyone on this board is from the US or under the FAA's rules

Which is exactly why we should know more about the OP and his intentions prior to answering any questions. We already have the FAA on our backs, how easy would it be for them to use threads like these to illustrate that we are all to quick to offer help to those who could use that help to inflict harm.
Old 02-17-2020, 04:26 PM
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Originally Posted by BarracudaHockey View Post
Not everyone on this board is from the US or under the FAA's rules
I know that. Since he hasn't listed his location, I felt the warning was appropriate since, unless I'm badly mistaken, well over half of the people on this forum are from North America where the FAA does have a say
Old 02-17-2020, 05:49 PM
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If somone is giving specifications in metric units odds are they are not US based.
Old 02-17-2020, 05:59 PM
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Originally Posted by speedracerntrixie View Post
If somone is giving specifications in metric units odds are they are not US based.
And that was where I was having the issue. He was giving prop sizes in inches, HP rather than watts and thrust in kilograms. Kind of makes it hard to know how to respond
Old 02-17-2020, 06:15 PM
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I've got props from Europe, Asia and the US and all of them are dimensioned in inches. As far as I know APC is the only brand that gives metric dimensions. One bade at the hub references inches the other metric.


For IC engines, HP is pretty universal. Watts usually applies to electric power. However HP can be converted to watts and vice versa, just less common.


Either way, the OP is almost certainly not in the US so FAA rules ( when they take affect 5 years from now ) don't apply but he should investigate what the laws are in his own country. We as good US citizens should not be giving away the farm until we know what his intentions are. It seems to me that this is not a hobby project, if a commercial venture then consultants should be paid.

Last edited by speedracerntrixie; 02-17-2020 at 06:48 PM.
Old 02-17-2020, 08:34 PM
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I'd say the bottom line is that if the OP knows enough to design and build an RC plane that can successfully fly for 10 hours, he also knows enough about basic gas engines to make a good choice. Let's not debate laws or accuse anyone of malicious intent without any evidence. Time to move on.

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