Weight or model in relation to motor thrust

Hello, Is there some sort of basic ratio of thrust an engine and prop must make in relation to a planes weight? I mean, say for instance a model aeroplane weighs 5lbs... what is a good thrust ratio an engine and prop should make? Is 50% (50% thrust over weight) a good ratio? More?

Thanks for your help Richard

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This is not going to answer your question directly because most do not measure thrust.

AFAIK the major impact of thrust is how long it takes to get off the ground or how well the model will do in the vertical plane. A Cessna 152 (full scale trainer) gross weight is around 1800 pounds (IIRC) and it has a 110 hp engine on the nose, surely not a real barn burner but a good plane to fly and reasonable in most of its capabilities. A model with the same ratio of numbers would be unacceptable. The typical .40 sized trainer weighs in around 6 pounds and has a 1 hp engine on the nose.

The full scale ratio is 1 hp to 16 or 18 pounds and the wimpy model ratio is

1 hp to 6 pounds. Many of the 3D capable aerobats have 1:1 or better hp to weight ratios.

The trainer can be flown acrobatically if the pilot plans ahead and uses the energy properly, which many do not.


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"CouldBeFlying" wrote in news:iNCdnS-bA_uP4hXenZ2dnUVZ snipped-for-privacy@rogers.com:

I'll concur with the other poster that T/W isn't everything, especially if the T/W is below 1.

My general rules of thumb are: T/W 0.25 probably can't take off but should fly from a hand launch. Unless it's very efficient (think powered glider) it'll require some skill to simply keep it in the air. Even if it's a powered glider, it will have a slow climb

0.35 - you can probably take off from a runway, and fly around just fine. Won't be much for aerobatics, though 0.5 Getting perky. No longer feels sluggish in normal flight, and enough power to get you out of trouble in a lot of situations 0.75 Serious aerobatics - you can go all over the sky. Not unlimited vertical, but you won't have to do any energy management for most maneuvers 1.0 In theory you can hover; in practice, you probably can't

1.2 Now you can do a decent hover and climb out of it once you decide to

1.5 The plane will jump nicely from the ground; this is where I'd like to be for a vertical take-off.

Now - having said all that - let's talk about the prop speed. Suppose you have a 12.25X3.75 prop (hey, I fly 'em all the time) and you're turning around 8000 RPM on a sleek little 4 lb airplane. The prop will be giving you about enough thrust to hover, so all's good, right? Well, the prop speed is only 28 MPH, and if your sleek little plane stalls at

30 mph, your plane may be able to hover but not fly level! I know that sounds like an extreme case, but I had an electric that was in that situation

With glow engines, you can also get into the reverse problem with an overpowered plane - even at idle, the engine may put out enough power to keep the plane in the air. This makes landings challenging; I had an old-timer that I usually had to deadstick to get it to land.

So - when you say "good" ratio, what kind of flying do you want to do?

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Mark Miller

Reply to
The Natural Philosopher

| >> is 1 hp to 6 pounds. Many of the 3D capable aerobats have 1:1 or better | >> hp to weight ratios.

Translation with units: 1 hp : 1 lb.

... which seems way overpowered to me though, even by R/C standards. And really, you don't need massive amounts of power to do 3D -- just lots of static thrust. Though of course the two are related.

Pylon racing, that's a good place for lots of power. A 3D plane needs a decent amount of power, but one horse power per pound still seems awfully high.

Picking a typical engine known for being relatively powerful (though finicky), the Tower Hobbies 0.46 engine is rated at 1.75 HP maximum and weighs 1.05 lbs. That gives a hp:lbs ratio of 1.66, but merely adding 11 oz of fuel brings the hp:lbs ratio to 1.00 -- and we don't even have an airplane involved yet. (But I am including the muffler.)

I've checked a few other engines, and haven't found any higher hp:lbs ratios yet ...

(And I won't even get into how tricky it can be to get an R/C IC engine to put out the rated power in the real world.)

As an extreme example of a 3D aerobat, my Tensor 4D weighs around 0.6 lbs, and I think the maximum power draw is around 100 watts, or 0.13 hp. That gives a hp:lbs ratio of 0.2. Though as I said, this is an extreme example. Also note that I'm measuring power into the motor, and the mechanical power emitted by the motor is probably around

20-30% less, which would make the hp:lbs ratio even lower.

... | > power to weight needs to be 1.2 to 1 to get a plane to accelerate from a | > hover IIRC

Obviously he's meant static thrust rather than power there. And in that case, the exact ratio varies based on who you ask, and it's not an exact ratio anyways, but he's basically right. | Thats two crap answers in succression. | | power is not thrust.

I was thinking the same thing, but decided not to say anything since the context tended to explain what they were referring to. | Power to weight cannot be expressesd as a simple ratio, since the units are | not identical.

Right, but people are loose with units around here anyways.

| Mind you, considiring the posters, its likley that they think it is.

I suspect not. But that was a nice dig anyways.

Reply to
Doug McLaren

| > Mind you, considiring the posters, its likley that they think it is. ... | ok cocky a jet engines power is rated in Lb's of thrust so power does = | thrust, so a power to weight of 1 to 1.2 is correct in this case

Ok, I was wrong, and TNP was right. You *don't* understand the difference.

The _thrust_ of a jet engine is given in lbs. Not the power. Power is different. The units of thrust will be force, like lbs or newtons. The units of power are either something like watts or horsepower, or (force * distance / time). To convert, one newton * meter / second is one watt, or one lb * foot / second is 1.36 watts.

But note that pinning down a definition of `power' in the terms of a plane engine or motor is somewhat difficult. Where do you measure it? Eleetrical motor manufacturers generally give it in terms of electrical energy in. Engine manufacturers give it in terms of mechanical energy out at the shaft. But if your plane is in a 3D hover and not moving, no work (in the physics sense) is being done on the plane at all -- all the power is `wasted' by the prop moving air around.

A jet turbine creates a large amount of power, probably a good deal more than a reciprocating engine of a similar size. But this power takes the form of exhaust propelled back at a very high speed -- this is good for going very fast, but not so good for 3D stuff. The static thrust isn't particularly high, but the power is. (Has anybody ever made a R/C 3D plane powered by turbine engines? :)

Of course, static thrust is just that -- static. Once you start moving, you're not looking at static thrust anymore. Since the exhaust velocity is so high on a turbine in most cases, thrust doesn't go down signifigantly until you considerable speed. It usually drops off far more quickly on a propeller, though it drops off slower if the pitch speed of the prop is higher.

In any event, if you wish to work on your understanding of concepts like thrust, power and work -- basic physics -- I can find some good references for you. Let me know.

Reply to
Doug McLaren

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Good post. mk

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Storm's Hamburgers

In article , Six_O'Clock_High

Reply to
Doug McLaren

In the full-scale world, power-to-weight is used all the time in aircraft specifications.


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| In the full-scale world, power-to-weight is used all the time in | aircraft specifications.

It is in the R/C world too. What's your point?

And in neither world can you accurately say that a given plane `has a

1:1 power/weight ratio' -- you'll need to define some units. And you'll find that in both worlds give power/weight ratios, they give you some units with your figure.
Reply to
Doug McLaren

snipped-for-privacy@frenzy.com (Doug McLaren) wrote in news:RY2jf.17135$Au1.6885 @tornado.texas.rr.com:

But since I like 'em over-powered, if T/W is only 0.25, it's not the "right plane" ;)

A little more seriously - it depends on how you're generating the thrust. If you have a relatively low-pitch prop, so that very little of it is stalled when you measure your static thrust, then the thrust is going to drop substantially as you pick up speed and you're likely to have a lot of trouble taking off. If you have a fairly high-pitch prop, then a lot of the prop may be stalled during the static test and the thrust will actually increase as you pick up speed, up to a point.

GA planes are generally a lot less haphazard about their aerodynamics and prop selection than the models are; we tend to use power to make up for other inefficiencies.

And I'll admit the 0.25 was just an estimate, and airplanes vary a lot. I confess that I don't have a reliable way to measure thrust yet, especially at low T/W.

Reply to
Mark Miller

TNP claimed that power-to-weight ratios made no sense. I should have been more specific: The ratio isn't some undefined number like

10:1; it's pounds per horsepower, so that the specification for a 150 horse Cessna 172, say, is 15.33 pounds per hp. The number gives a pilot an idea of the takeoff and climb performance. A lower ratio, such as 10 pounds per hp, won't increase cruise speed too much but will make the TO and climb spectacular. Static thrust is usually around 3 pounds per hp, so that the 172 might have 450 lbs thrust at the beginning of the takeoff roll, a little more as speed increases and the prop unstalls, and then it begins to drop off by the time climb speed is reached and the AOA of the prop blades decreases. You can see that the 2300 lb 172 will have around 5 pounds of weight per pound of thrust, so it isn't a good aerobatic machine :-)


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ROFLOL! My complements sir, I was too lazy to go to the manuals and get the exact numbers...

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In article , wrote: | >It is in the R/C world too. What's your point? | | >And in neither world can you accurately say that a given plane `has a | >1:1 power/weight ratio' -- you'll need to define some units. | | TNP claimed that power-to-weight ratios made no sense.

No, he didn't. He claimed that :

power is not thrust.

Power to weight cannot be expressesd as a simple ratio, since the units are not identical.

and he's right, even if he didn't say it in the ideal way. It can not be expressed as a dimensionless ratio, which is what people were doing.

| I should have been more specific: The ratio isn't some undefined | number like 10:1; it's pounds per horsepower

I think that was TNP's point.

Why am I defending TNP? :)

| Static thrust is usually around 3 pounds per hp

That depends greatly on the prop. Sometimes pilots will remove the stock propeller and replace it with a large one with a lower pitch to get more static thrust, which is useful in short field takeoffs. (It also tends to lower your top and cruising speeds and decrease fuel efficiency, but there's always tradeoffs.)

An _extreme_ example would be a helicopter, where the total power is probably a little higher than that of an airplane of similar weight, but the static thrust is higher than the weight of the helicopter. (And if not, it wouldn't even fly.)

To make this R/C related, you'll find that most 3D planes have larger props with lower pitch ratings. On the other end of the spectrum, pylon racers tend to have smaller props with higher pitch ratings. Adding a gearbox to an electric plane allows you to use a larger prop with a larger pitch rating, which is generally more efficient than a larger prop with a smaller pitch rating and no gearbox.

Reply to
Doug McLaren

I don't think I said power to weight ratio made no sense.

Power to weight makes eminenet sense. It translates directly to rate of climb.

What does not, is STATIC thrust to weight. It ignores the fact that pitch speed is also utterly relevant.

Reply to
The Natural Philosopher

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