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

In message et, 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.