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
This is not going to answer your question directly because most do not
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
The trainer can be flown acrobatically if the pilot plans ahead and uses the
energy properly, which many do not.
"CouldBeFlying" wrote in
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
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
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?
| >> 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.
| > 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
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
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.
| 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.
firstname.lastname@example.org (Doug McLaren) wrote in news:RY2jf.17135$Au1.6885
But since I like 'em over-powered, if T/W is only 0.25, it's not the "right
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
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.
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 :-)
| >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.
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
What does not, is STATIC thrust to weight. It ignores the fact that pitch
speed is also utterly relevant.