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ake109
07-05-2007, 08:30 PM
Question.

On fighter plane specs, its common enough to see 2000hp at take-off, 1700hp at 20,000ft 1200hp at 40,000 ft etc etc.

But I never see the same thing for jet engines. Its always something like - 10000 lbs static, 15000 lbs re-heat/afterburner. Never any mention of altitude.

How does the thinner air of higher altitudes affect thrust on jet engines? I am sure it does, but how much exactly?

ake109
07-05-2007, 08:30 PM
Question.

On fighter plane specs, its common enough to see 2000hp at take-off, 1700hp at 20,000ft 1200hp at 40,000 ft etc etc.

But I never see the same thing for jet engines. Its always something like - 10000 lbs static, 15000 lbs re-heat/afterburner. Never any mention of altitude.

How does the thinner air of higher altitudes affect thrust on jet engines? I am sure it does, but how much exactly?

LStarosta
07-05-2007, 08:35 PM
Jets are usually very inefficient at low altitudes, and very much more efficient at high altitudes in terms of ground speed, range, and fuel economy.

ake109
07-05-2007, 08:57 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by LStarosta:
Jets are usually very inefficient at low altitudes, and very much more efficient at high altitudes in terms of ground speed, range, and fuel economy. </div></BLOCKQUOTE>

Well, I have read that one about the Me262 having a much higher range at high alts but then so do pistons.

But there has to be a critical altitude where the thrust drops right? Otherwise the jet would just go faster and climb harder as it goes higher into the thinner air. Which is not true since some jets have even lower ceilings than pistons.

Not just that, are there any hard numbers anywhere? There are hard numbers showing hp vs altitute for many piston engines but I have yet to see even _one_ for jets (thrust vs alt).

LStarosta
07-05-2007, 09:04 PM
I don't know about thrust charts or anything like that. There is a cutoff point, otherwise there would be no ramjets and scramjets, and the spacecraft wouldn't have rockets. Like all combustion engines, jets require oxygen, so there is an altitude at which the compressor turbines cannot feed enough oxygen for this combustion to take place. However, the better compression ability of a turbojet compared to a turbocharged or supercharged piston, or even a turboprop engine is what gives them improved high altitude performance.

What I meant though is that jets are VERY inefficient at low altitudes compared to piston engined aircraft. The amount of money you save flying at FL350 in a DA-20 is insane compared to 5,000 or even 10,000 feet.

erco415
07-05-2007, 09:55 PM
While I don't have any charts to show you, for the most part jets do lose power with altitude, but at a much reduced rate compared to piston engines. A jet is little more than a glorified turbosupercharger anyway. We get some pressure recovery with higher airspeeds and through a more efficient intake arrangement. Plus we don't have to worry about diminishing prop efficiency (better power to thrust conversion) as altitude increases. For better numbers, surely someone at Pratt&Whitney/GE/RollsRoyce could help you.
As far as jets being greatly less efficient at low altitudes, this has been abated somewhat. To illustrate, our Lear 25 with it's CJ610 turbojets (non-afterburning version of the engine used on the T-38) which is 40 year old technology, burns the same amount of fuel on the ground -at idle- as it does at 40,000 feet at 98%RPM (max cruise power). Takeoff fuel flows are in the 5000lb/hour range whereas at cruise we usually see 1100lb/hour. Contrast this with our Lear 35 (10 years newer) with it's Garrett TFE731 turbofans (a development of the apu on the DC-10) where the effects of altitude on fuel flow is greatly reduced. Burning approx 1000 lb/hour at altitude vs. 1400 or so cruising in the high teens/low twenties. The point being, the modern turbofan won't completely hose you down low - though it's happier up high.

ake109
07-05-2007, 11:00 PM
Thanks for the replies.

I asked because I have a good rough idea how piston engine power vs alt curves are like but had absolutely not idea what a jets thrust vs alt curve looks like.

I am sure the curve is greatly different amongst the various designs (e.g. The J58 won't have a curve like the TF30) but since I have yet to see one, I am very curious.

leitmotiv
07-06-2007, 12:01 AM
I am not an expert on this but check out the info on the B-36 in the B-36 DETAIL AND SCALE. The author notes the early '50's jets lost a great deal of thrust at very high altitude. The B-36 was designed to boom along at 40,000 feet. At that altitude, supposedly, the early '50's jets were staggering. The USAF confidently believed the behemoth bombers would be able to tear MiG-15s and MiG-17s to pieces with their 20mm batteries while the jets were just about at a stall (the turboprop bomber was tearing along just fine at 40K).

The-Pizza-Man
07-06-2007, 04:13 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ake109:
Thanks for the replies.

I asked because I have a good rough idea how piston engine power vs alt curves are like but had absolutely not idea what a jets thrust vs alt curve looks like.

I am sure the curve is greatly different amongst the various designs (e.g. The J58 won't have a curve like the TF30) but since I have yet to see one, I am very curious. </div></BLOCKQUOTE>

The reason you won't find jet thrust curves is because thrust depends on speed and altitude, where as the horsepower a piston engine produces is only dependent on altitude(except for a bit of ram air effect), as it's only the torque by the rpm of the engine. However, the thrust a piston engine produces will also vary with altitude.

The thrust ratings you see for jet engines generally taken at sea level on a test stand, that's why they are "static" thrust. It only gives you an indication of the performance of the engine. It doesn't tell you specifically how they will perform, 2 engines with the same static thrust could perform very differently at the same flight condition.

Kettenhunde
07-06-2007, 05:56 AM
You can SWAG the effect of altitude on thrust production by using the following formula:

Thrust = Thrust at sea level * (density ratio^X)

Thrust at 30,000ft = (50,000lbs) * (.3741^.8)
=22,800lbs of thrust at 30,000ft.

X is most accurately assumed to be .8 below the tropopause and 1 above it.

Sometimes you will find X=1 for the entire analysis.

All the best,

Crumpp

Zoom2136
07-06-2007, 08:34 AM
Jet engine (and not a Turbo Fan engine like most airliners use today) is more efficient at altitude... at low altitude they are not as efficient a Prop driven plane...

At low altitude

- Turbo props (best)
- Turbo fan (without afterburners)
- Piston
- Jet (without afterburners)

At altitude

- Jet (best)
- Turbo Fan
- Turbo Prop
- Piston

Best overhal... Turbo Fan