Good thrust-to-weight ratio for electrics?
Jimbo156
ratio
for acceptable performance, for a slow-flying trainer? Mine is 52 oz
flying weight, 24 oz static thrust (initial with full charge).
Anthony R
"
Why is the Thrust to Weight ratio so important? This is the ratio that
determines whether you have a flying aircraft or a static display model.
Hans Bühr in "Mike's Jet Book" listed three flight performance ratios for
electric jets that work very well for the design model.
.3 or 1:3 thrust/weight is a good flyer.
.5 or 1:2 thrust/weight gives a good aerobatic performance.
.6 or 2:3 thrust/weight is excellent performance, jet-like!
"
On some very light aircraft I have heard of people having a 1:1 or greater
thrust to weight ratio.
"Jimbo156" <Rayt...@bigfoot.com> wrote in message
news:689d6298.02071...@posting.google.com...
The Natural Philosopher
Jimbo156 wrote:
That should be enough. According to motocalc what is more important is
the thrust to drag at normal flying speed. So if you have a treally
clean sailplane, you don't need a lot f thrust to keep it up there.
If its a scale biplane however..
A better rule seems to be watts per pound. 50 watts per pound. If your
combo is able to supply this, 50 per lb weight will fly the model
assuming a reasonable prop and motor efficiency. Anything up to 100W per
pound will get you very decent performance indeed, and 100W/lb is near
the magic 'unlimited vertical' performance level.
The problem is, you can get static thrust to be really very large by
gearing down to a big fine pitch prop, but this won't necessarily give
you enough *speed* to atually fly the model.
52oz - 3lb - needs about 150W or so to fly..thats around 20A on 8.4V -
which is at the top end of speed 400/480 territory wih 600/800/1100ma/H
cells. Or the bottom end of speed 600 territory ...assuming a simple
sort of setuop.
Why not get a copy of motocalc and see what it says?
Jimbo156
> The problem is, you can get static thrust to be really very large by
> gearing down to a big fine pitch prop, but this won't necessarily give
> you enough *speed* to atually fly the model.
>
Hmm...I don't really follow this. Seems to me that from Newtons' law,
more thrust will result in more speed/acceleration. How could
increasing static thrust (with all other factors being the same) not
increase accel/speed?
Mike Slaughter
put on a much larger engine, it will not increase speed significantly. Drag
increases with speed faster then speed increases with horsepower. It all
depends on the airframe.
Mike
"Jimbo156" <rAYT...@BIGFOOT.COM> wrote in message
news:b15c2278.02072...@posting.google.com...
Matthew Orme
It does work that way. Your acceleration does increase with higher pitch, up
to the pitch speed, which with high thrust props is slow.
Good example. A magazine writer did a review of one of our pylon motors. He
got a much bigger static thrust on a 6x2 prop than the recommended 5x5. Yet
the 6x2 would not fly the plane worth a crap when he tried.
If you want to fly electrics, the first thing you must throw out is any
notion of thrust (except if you want to do 3D). Props for electric are
generally never less than pitch = 1/2 diameter. Square props are often used
(pitch = diameter)
Think in terms of power, not thrust. You will have a lot more success if you
try to eliminate that word from your vocabulary. Even when proping for an
F5B plane (currently 24 cells at 120A), which vlimb vertically, we only
consider power, not thrust.
dasm...@pacbell.net
news:b15c2278.02072...@posting.google.com:
The lower the pitch of the prop, and the more slowly it's turning, the
faster its thrust will drop as the plane goes faster. Frex, a 12x4 prop at
5000 RPM might generate 25 oz of static thrust but would generate *no*
thrust at 20 MPH! (5000 RPM = 83 rev/sec, 4"=1/3 foot, 83 x 1/3 = 28 f/s =
20 MPH)
Of course, as the airplane goes faster, the prop will see less resistance
because it's not biting the air as effectively, so the motor will be able
to spin it faster and you won't see the thrust drop off quite so badly, but
the general principle still applies.
--
"Like all great things, Pinky, the idea is simple, but the execution - oh,
there lies the difficulty. There lies the genius!"
"Taking a little nap, is he?"
-Pinky and the Brain
The Natural Philosopher
Matthew Orme wrote:
>>>The problem is, you can get static thrust to be really very large by
>>>gearing down to a big fine pitch prop, but this won't necessarily give
>>>you enough *speed* to atually fly the model.
>>>
>>>
>>
>>Hmm...I don't really follow this. Seems to me that from Newtons' law,
>>more thrust will result in more speed/acceleration. How could
>>increasing static thrust (with all other factors being the same) not
>>increase accel/speed?
>>
>
> It does work that way. Your acceleration does increase with higher pitch, up
> to the pitch speed, which with high thrust props is slow.
>
Another way of saying the same thing, is that thrust decreases with
speed for any given prop approaching zero as teh pitch speed is approached.
If you want to think of it like the gear ratin on a car, where first
gear will give you massive acceleration...upt o walking speed :-)
> Good example. A magazine writer did a review of one of our pylon motors. He
> got a much bigger static thrust on a 6x2 prop than the recommended 5x5. Yet
> the 6x2 would not fly the plane worth a crap when he tried.
>
> If you want to fly electrics, the first thing you must throw out is any
> notion of thrust (except if you want to do 3D). Props for electric are
> generally never less than pitch = 1/2 diameter. Square props are often used
> (pitch = diameter)
>
I think that is a very crude and potentially inaccurate generalization.
Firstly, thrust does tell you if you can prop hang and go straight up,
or not..If (static) thrust does not exceed model weight, vertical
performance will not be unlimited. So the sttaic thrust does have
relevance there.
It also gives a crude guide as to whether your prop diameter is anywhere
near right. However, motoxcalc is a far better guide :-)
As to props having opitch in general between 1/2 and the same as teh
diameter, this is normally approximately true, but there are exceptions.
What tends to happen is that props of thse diameter to pitch ratios are
perhaps the most efficient 'movers of air' in a typical plane context,
and if the torque/RPM doesn't match teh clee/motor combo, we normally
stick a box in to get the efficeincy up and the match 'better'.
Slow-fly models and gliders tend to operate at slow airspeed and need
higher thrusts, as do pattern style models. These tend towards - or even
exceeed 0 the diamnetr/pitch ratios of 2:1. For instance the APC 10x4.7
slowfly prop is very popular in geared situations.
> Think in terms of power, not thrust. You will have a lot more success if you
> try to eliminate that word from your vocabulary. Even when proping for an
> F5B plane (currently 24 cells at 120A), which vlimb vertically, we only
> consider power, not thrust.
>
Thruts is to some degree assocuiated with efficiency of power
conversion. The trouble is, its not a direct relationship.
As usual, Mathew is dumbing it down to make a good point, but, I feel,
is in danger of dumbing it down a bit too far.
Certainly as a starting point to getting a model in the air, the old
'rule of thumb' of 50W/lb for a floaty slow model, and maybe 100W/lb+
for vertical performance or a ducted fan, is the way you establish what
CELL COUNT you need - given that anything much over a SubC type cell is
hard to obtaun and maybe not as efficient in power to weight or ebnegy
to weight density....after that you probably have to limit the current
to say 30-50A to not cook the cells, so that if you have a 6lb model,
you need 300-600W power, which might equate to say 16 cells at around
15-30A or so....then what you need to do is to find as motor that will
not burn out at 30A plus...and a suitable gearbox and prop that will get
that power into decent thrust/top speed in terms of the aircraft.
Because there are lots of combinations of props and boxes that will get
that sort of power input - but at inappropriate prop speeds and model
speeds.
In the end, get motocalc and experiment. It isn't absolutely accurate,
but every time I check a model with a different cell count and prop, the
sort of performance *change* that motocalc indicates is the sort of
change in performance the model seems to have.
As matthew is trying to say, what you *tend* to end up with is a prop of
between 0.5 (slowfly/glider) and 1 (warbird) type pitch to diamter
ratios. For example a very useable prop on a speed 600 can motor in 6-8
cells is a 9x4.5 folder. Goes well in gliders etc. Or you can, for very
similar current draw and power in, use a 7x6, if our model wants to do
the speed. For better effeceincy a box and maybe a 12x8 prop would give
intermediate performance on maybe a few more cells...Best thrust is
certaibnly there on teh lower pitched combos, but top speed - well the
7x6 will be the fastest combo there, and if the model is low drag, it
will scream around quite happily on that..:-) It won't go vertical
forever tho.
>
>
>
Mathew Kirsch
> Hmm...I don't really follow this. Seems to me that from Newtons' law,
> more thrust will result in more speed/acceleration. How could
> increasing static thrust (with all other factors being the same) not
> increase accel/speed?
The problem is, static thrust only works when the plane is static. As
soon as the plane starts moving, it gets a whole lot more complicated.
A smaller propeller spinning really fast will give the same amount of
static thrust as a larger propeller spinning slow. Given the same
perfectly aerodynamic airplane, the smaller prop will make the
airplane go faster due to pitch speed.
The pitch of a propeller is how far forward it will move through a
theoretical block of Jell-o in one revolution. Both a 9x6 prop and a
12x6 prop will move six inches through that block of Jell-o. Given the
same exact engine, the 12x6 prop may spin at 10000 RPM, and the 10x6
may spin at 15000 RPM. Both propellers move through the Jell-o at 6
inches per revolution, but the 10x6 is spinning much faster. That 10x6
propeller will move through the Jell-o faster, even though the 12x6
moves more Jell-o.