Thrust vs Shaft Horse Power?

[Evan Williams]

A question for all of the pilots, engineers, and people who are a lot
smarter than me. Is there a simple rule of thumb for comparing the amount
of thrust or lift you can get from an engine which drives a set of propeller
or rotor blades to the amount of thrust you can get from the exhaust of a
non-turbo prop engine? For example, a T56-15 from a C-130 has 4,910 SHP and
a J85-21 from an F-5E has 5,000 lbs of thrust (These numbers are from a
twenty year old book of mine. For the sake of this discussion, lets just
assume they are correct). How much actual thrust can you get from the T56
on the C-130? I realize that there must be a lot of variables involved with
the propellers or rotors due to different airfoil shapes and the "lift" that
they provide and the pitch the blades are set at. But I was wondering if
there was a "general rule" that people went by. Keep in mind that math was
one of the main reasons that I dropped out of college and I just spend the
last twenty years as an Aviation Ordnanceman (Population Control) so any
large mathematical equations would go right over my head.
I apologize in advance for posting a question with little chance of
degrading into a political rant. ;)

Evan Williams

[Paul J. Adam]

In message t>, Evan
Williams > writes
>A question for all of the pilots, engineers, and people who are a lot
>smarter than me. Is there a simple rule of thumb for comparing the amount
>of thrust or lift you can get from an engine which drives a set of propeller
>or rotor blades to the amount of thrust you can get from the exhaust of a
>non-turbo prop engine?

Short answer... no.

Me clankie, me not know much, but actual thrust depends on atmospheric
conditions and airspeed and other factors too.

It seems to my non-specialist eye that props are better at turning
'power' into 'thrust' at low speeds, less so as speeds and altitudes
increase.

>For example, a T56-15 from a C-130 has 4,910 SHP and
>a J85-21 from an F-5E has 5,000 lbs of thrust (These numbers are from a
>twenty year old book of mine. For the sake of this discussion, lets just
>assume they are correct). How much actual thrust can you get from the T56
>on the C-130?

At what airspeed? In what atmospheric conditions?

Sorry to sound like a broken record, but the answer is 'it depends'.

>But I was wondering if
>there was a "general rule" that people went by.

If you want to haul cargo from short airstrips, turboprops.

If you want to haul lots of cargo from longer airstrips over longer
ranges, turbofans or *maybe* turboprops.

For general-purpose multirole fighters, turbofans.

For high-altitude interceptors, turbojets.


Others who know more will correct me, I'm sure.
--
He thinks too much: such men are dangerous.
Julius Caesar I:2

Paul J. Adam MainBox<at>jrwlynch[dot]demon{dot}co(.)uk

[Scott Ferrin]

On Mon, 19 Jul 2004 23:38:24 +0100, "Paul J. Adam"
> wrote:

>In message t>, Evan
>Williams > writes
>>A question for all of the pilots, engineers, and people who are a lot
>>smarter than me. Is there a simple rule of thumb for comparing the amount
>>of thrust or lift you can get from an engine which drives a set of propeller
>>or rotor blades to the amount of thrust you can get from the exhaust of a
>>non-turbo prop engine?
>
>Short answer... no.
>
>Me clankie, me not know much, but actual thrust depends on atmospheric
>conditions and airspeed and other factors too.
>
>It seems to my non-specialist eye that props are better at turning
>'power' into 'thrust' at low speeds, less so as speeds and altitudes
>increase.
>
>>For example, a T56-15 from a C-130 has 4,910 SHP and
>>a J85-21 from an F-5E has 5,000 lbs of thrust (These numbers are from a
>>twenty year old book of mine. For the sake of this discussion, lets just
>>assume they are correct). How much actual thrust can you get from the T56
>>on the C-130?
>
>At what airspeed? In what atmospheric conditions?
>
>Sorry to sound like a broken record, but the answer is 'it depends'.
>
>>But I was wondering if
>>there was a "general rule" that people went by.
>
>If you want to haul cargo from short airstrips, turboprops.
>
>If you want to haul lots of cargo from longer airstrips over longer
>ranges, turbofans or *maybe* turboprops.
>
>For general-purpose multirole fighters, turbofans.
>
>For high-altitude interceptors, turbojets.
>
>
>Others who know more will correct me, I'm sure.


It's been discussed here in the past and I don't recall there being
any definitive answer. The lift fan on the X-35 draws about 28000hp
off the main engine to produce 18000 pounds of thrust but then if you
hooked that same 28000 to a big helicopter roter you'd generate a LOT
more lift.

[Evan Williams]

Thank you for the responses. And the very simple explinations. :) I was
"out of town" for much of last year so I must have missed the previous
thread.

Evan Williams


"Scott Ferrin" > wrote in message
...
> On Mon, 19 Jul 2004 23:38:24 +0100, "Paul J. Adam"
> > wrote:
>
> >In message t>, Evan
> >Williams > writes
> >>A question for all of the pilots, engineers, and people who are a lot
> >>smarter than me. Is there a simple rule of thumb for comparing the
amount
> >>of thrust or lift you can get from an engine which drives a set of
propeller
> >>or rotor blades to the amount of thrust you can get from the exhaust of
a
> >>non-turbo prop engine?
> >
> >Short answer... no.
> >
> >Me clankie, me not know much, but actual thrust depends on atmospheric
> >conditions and airspeed and other factors too.
> >
> >It seems to my non-specialist eye that props are better at turning
> >'power' into 'thrust' at low speeds, less so as speeds and altitudes
> >increase.
> >
> >>For example, a T56-15 from a C-130 has 4,910 SHP and
> >>a J85-21 from an F-5E has 5,000 lbs of thrust (These numbers are from a
> >>twenty year old book of mine. For the sake of this discussion, lets
just
> >>assume they are correct). How much actual thrust can you get from the
T56
> >>on the C-130?
> >
> >At what airspeed? In what atmospheric conditions?
> >
> >Sorry to sound like a broken record, but the answer is 'it depends'.
> >
> >>But I was wondering if
> >>there was a "general rule" that people went by.
> >
> >If you want to haul cargo from short airstrips, turboprops.
> >
> >If you want to haul lots of cargo from longer airstrips over longer
> >ranges, turbofans or *maybe* turboprops.
> >
> >For general-purpose multirole fighters, turbofans.
> >
> >For high-altitude interceptors, turbojets.
> >
> >
> >Others who know more will correct me, I'm sure.
>
>
> It's been discussed here in the past and I don't recall there being
> any definitive answer. The lift fan on the X-35 draws about 28000hp
> off the main engine to produce 18000 pounds of thrust but then if you
> hooked that same 28000 to a big helicopter roter you'd generate a LOT
> more lift.

[Guy Alcala]

Evan Williams wrote:

> Thank you for the responses. And the very simple explinations. :) I was
> "out of town" for much of last year so I must have missed the previous
> thread.
>
> Evan Williams
>
> "Scott Ferrin" > wrote in message
> ...
> > On Mon, 19 Jul 2004 23:38:24 +0100, "Paul J. Adam"
> > > wrote:
> >
> > >In message t>, Evan
> > >Williams > writes
> > >>A question for all of the pilots, engineers, and people who are a lot
> > >>smarter than me. Is there a simple rule of thumb for comparing the
> amount
> > >>of thrust or lift you can get from an engine which drives a set of
> propeller
> > >>or rotor blades to the amount of thrust you can get from the exhaust of
> a
> > >>non-turbo prop engine?
> > >
> > >Short answer... no.
> > >
> > >Me clankie, me not know much, but actual thrust depends on atmospheric
> > >conditions and airspeed and other factors too.
> > >
> > >It seems to my non-specialist eye that props are better at turning
> > >'power' into 'thrust' at low speeds, less so as speeds and altitudes
> > >increase.
> > >
> > >>For example, a T56-15 from a C-130 has 4,910 SHP and
> > >>a J85-21 from an F-5E has 5,000 lbs of thrust (These numbers are from a
> > >>twenty year old book of mine. For the sake of this discussion, lets
> just
> > >>assume they are correct). How much actual thrust can you get from the
> T56
> > >>on the C-130?
> > >
> > >At what airspeed? In what atmospheric conditions?
> > >
> > >Sorry to sound like a broken record, but the answer is 'it depends'.
> > >
> > >>But I was wondering if
> > >>there was a "general rule" that people went by.
> > >
> > >If you want to haul cargo from short airstrips, turboprops.
> > >
> > >If you want to haul lots of cargo from longer airstrips over longer
> > >ranges, turbofans or *maybe* turboprops.
> > >
> > >For general-purpose multirole fighters, turbofans.
> > >
> > >For high-altitude interceptors, turbojets.
> > >
> > >
> > >Others who know more will correct me, I'm sure.
> >
> >
> > It's been discussed here in the past and I don't recall there being
> > any definitive answer. The lift fan on the X-35 draws about 28000hp
> > off the main engine to produce 18000 pounds of thrust but then if you
> > hooked that same 28000 to a big helicopter roter you'd generate a LOT
> > more lift.

I expect Pete Stickney will be giving you his patented thrust vs. horsepower
lecture shortly, if he's not on vacation or otherwise occupied. If youre in a
hurry, you could do a google search restricted to this group with
as the author and "Maximum Speed of Airliner at Low
Altitude" as the subject, and you'll get one of his recent posts which
discusses this subject in some detail.

The short version is that propeller thrust = horsepower @ 375 mph. HP is
greater than thrust below that speed, less above.

Guy

[Evan Williams]

<snip>
Thank you

>
> I expect Pete Stickney will be giving you his patented thrust vs.
horsepower
> lecture shortly, if he's not on vacation or otherwise occupied. If youre
in a
> hurry, you could do a google search restricted to this group with
> as the author and "Maximum Speed of Airliner at Low
> Altitude" as the subject, and you'll get one of his recent posts which
> discusses this subject in some detail.
>
> The short version is that propeller thrust = horsepower @ 375 mph. HP is
> greater than thrust below that speed, less above.
>
> Guy
>
>

[Peter Stickney]

In article >,
Guy Alcala > writes:
> Evan Williams wrote:
>
>> Thank you for the responses. And the very simple explinations. :) I was
>> "out of town" for much of last year so I must have missed the previous
>> thread.

> I expect Pete Stickney will be giving you his patented thrust vs. horsepower
> lecture shortly, if he's not on vacation or otherwise occupied. If youre in a
> hurry, you could do a google search restricted to this group with
> as the author and "Maximum Speed of Airliner at Low
> Altitude" as the subject, and you'll get one of his recent posts which
> discusses this subject in some detail.

Just back - I'll get to it tomorrow - right now I'm wading through the
"British Night Bombers vs. the P-38" thread.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster

[Cub Driver]

>The short version is that propeller thrust = horsepower @ 375 mph. HP is
>greater than thrust below that speed, less above.

Guy, is this the same as saying that a turbojet's thrust in pounds at
375mph is the same as a horsepower rating for that engine?

Put another way: is a 3,000-lb thrust turbojet equivalent to a 3,000hp
recip at 375mph?

(I haven't been reading this thread either. Yesterday I discovered
that if I posted first, my newsreader downloaded all the new posts,
and if I then downloaded all the new posts (as I did) I wiped out the
day's work of all these brilliant people.)

all the best -- Dan Ford
email: (put Cubdriver in subject line)

The Warbird's Forum www.warbirdforum.com
Expedition sailboat charters www.expeditionsail.com

[Eunometic]

"Evan Williams" > wrote in message t>...
> A question for all of the pilots, engineers, and people who are a lot
> smarter than me. Is there a simple rule of thumb for comparing the amount
> of thrust or lift you can get from an engine which drives a set of propeller
> or rotor blades to the amount of thrust you can get from the exhaust of a
> non-turbo prop engine? For example, a T56-15 from a C-130 has 4,910 SHP and
> a J85-21 from an F-5E has 5,000 lbs of thrust (These numbers are from a
> twenty year old book of mine. For the sake of this discussion, lets just
> assume they are correct). How much actual thrust can you get from the T56
> on the C-130? I realize that there must be a lot of variables involved with
> the propellers or rotors due to different airfoil shapes and the "lift" that
> they provide and the pitch the blades are set at. But I was wondering if
> there was a "general rule" that people went by. Keep in mind that math was
> one of the main reasons that I dropped out of college and I just spend the
> last twenty years as an Aviation Ordnanceman (Population Control) so any
> large mathematical equations would go right over my head.
> I apologize in advance for posting a question with little chance of
> degrading into a political rant. ;)
>
> Evan Williams


Power = Speed x Thrust.

Power in Watts,
Speed in meters per second,
Thrust in Newtons (about 101 grams).

Thus a jet moving at 440mph (200m/sec) with a 2000kg (20,200N) jet
engine is equivalent to a 200 x 20,200 = 4040000 watts (4040kW,
5200hp).

Propellers are between 75% to 85% efficinet so we would have to add
about 25% to those figures to get the shaft horsepower equivalent.

Ofcrouse the equation gets a little silly (non linear) about Mach 0.66
or so as compressability reduces propeller efficiency.

Turboprops posses substantial jet thrust and inefficient engines in
terms of shaft power often perform better due to jet thrust in faster
aircraft.

The WW2 spitfire merlin possesed about 300lb (1330N) jet thrust. At
440mph (200m/s) and 75% propellor efficiency this equated to 353kw or
460hp. This is one reason NOT to use a turbo charger.

[Peter Stickney]

In article >,
(Peter Stickney) writes:

> Just back - I'll get to it tomorrow - right now I'm wading through the
> "British Night Bombers vs. the P-38" thread.

Here I am - and here it goes:
Basically, aircraft engines have two different types of power delivery
- Constant Power, like a reciprocating engine, or a turboprop's output
shaft, and
- Constant Force, like a jet engine, or the exhaust thrust portion of
a turboprop.
Turning Power into Thrust is something that starts out simple,
but, as Real Life intrudes, gets a bit complicated.
I'll stick to SAE/British Engineering units for now, too.

To start with, there's a very simple formula that relates Force to
Power: 1 HP = 550 lb(force) ft/sec, or basically the amount of oomph
needed to lift 550 lbs 1 ft in 1 second. When you look at it, you
see that we've got a Force (lbs) and a speed (ft/sec). So, the way to
determine how much horsepower you're getting from a particular force
(thrust) is HP = (Thrust(lbs) * Speed(fps))/ 550 ft/sec .
Since 550 ft/sec is 375 mph, let's rewrite it as:
HP = (Thrust (lbs)* Speed (mph))/375 mph.

Or, rearranging things a bit, Thrust = HP * 375/Speed (mph).

So, for a constant Horsepower, the Thrust is very high at low speeds,
and it tapers off as speed increases.
For a Constant Force type of engine, teh Thrust Horsepower generated
increases as the speed increases.
For example, let's round your C-130 example (Assuming it's all Shaft
Horsepower - more on that later) to 5,000 HP, and let's see what
we get for Thrust.
We'll also take your J85 from the F-5, at 5,000# thrust, and see what
we get for Horsepower.

Constant Power Constant Thrust
HP Speed Thrust HP Speed Thrust
MPH Lbs MPH Lbs
5000 100 18750 1333 100 5000
5000 200 9375 2666 200 5000
5000 300 6250 4000 300 5000
5000 400 4697 5333 400 5000
5000 500 3750 6667 500 5000
5000 600 3125 8000 600 5000

So, as you can see, with constant power, teh thrust starts out very
high, and drops off rather markedly as speed increases. This is great
for hauling a Heavy Herc off of a hot runway, but if you want to go
much faster than 500 mph, you're going to need an awful lot of
horsepower. (That's why late war & post- WW2 fighters topped out in
the high 400 mph range, and why the propeller driven speed records
haven't changed mich in about 70 years.
The jet starts out with a _lot_ lower power, but since the thrust
remains constant, it really picks up as speed increases.
This is why it's possible for a high-powered piston-engined fighter to
outperform a jet (especially an early jet, like the Me 262, which was
really rather underpowered) in the lower speed ranges. A jet at low
speeds doesn't have a lot of excess power for accelerating, climbing,
or turning, and the prop has tons of it.

That was the simple part. :)
WHen it gets complicated, we start throwing in stuff about Propeller
Efficiencies and adjustable pitch propellers vs fixed pitch, & so on.
Propellers are complicated beasts, and they tend to fudge things up at
either very high or very low speeds. We can get into it later, if you
want.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster