Forum: Technology

Can anyone comment on how much thrust is added from an engine at significant velocity?

For example.....Lets say an F-16 engine makes 29K lbs. of thrust stationary at sea level, it takes more than that to achieve mach2. So how much is achieved from ram-air?

-Scott

F-16.net Sponsor

boosted-lt1 wrote:

Can anyone comment on how much thrust is added from an engine at significant velocity?

For example.....Lets say an F-16 engine makes 29K lbs. of thrust stationary at sea level, it takes more than that to achieve mach2. So how much is achieved from ram-air?

-Scott

What makes you think it takes more than that to achieve Mach 2? In fact it's unlikely that an engine at Mach 2 and 50,000 feet will be putting out as much thrust as at sea level on a bench.

Hmmm, Is that true? I'm no expert by any means.

Here is how I think....Yes, there is less 'air' at 20K feet when compared to sealevel, so you think engine performance will suffer, right. But you have an airspeed above zero right? So at some point this airspeed might increase engine performence above a static figure. Does this happen?

What is the indicated airspeed on the airframe at Mach2?
What is the altitude commonly associated with Mach2?

-Scott.

The reason you need to be at a higher altitude to go mach 2 is because the speed of sound changes at different altitudes, if thats what your asking...?

An F-16 cant go the speed of sound at sea level with 29K of thrust because the air is much thicker at sea level and hence sound travels through it quicker. At 40,000 feet the air is much thinner so the sound cannot travel as direct.

I think thats correct anyways. allenperos or pat1 will jump in if im wrong.

-Aaron

TenguNoHi :

I do realize you need to get up a bit in thinner air to reduce the drag on the aircraft.

So is engine thrust proportional to airspeed?

I have much to learn. Very Happy

-Scott.

Here's an example just to give you an idea though it is just best recollection from something I read several years ago in Ben Rich's Skunk Works. At altitude they figured the J-57 that put out a bit over 11,000 pounds of thrust on the test stand at sea level only generated about 700 (that's seven HUNDRED) pounds of thrust at cruising speed and altitude.

Engines work by taking slow moving incoming air and accelerating it out the back. The force required to accelerate the air backwards translates to thrust forwards (equal and opposite and all that)

The air coming out the back at max throttle doesn't change much...so if the incoming air is coming in fast (cruise), there is less thrust than if it is coming in slow (Takeoff).

Most thrust occurs when plane is not moving.

TenguNoHi wrote:

The reason you need to be at a higher altitude to go mach 2 is because the speed of sound changes at different altitudes, if thats what your asking...?

An F-16 cant go the speed of sound at sea level with 29K of thrust because the air is much thicker at sea level and hence sound travels through it quicker. At 40,000 feet the air is much thinner so the sound cannot travel as direct.

I think thats correct anyways. allenperos or pat1 will jump in if im wrong.

-Aaron

Hi Aaron,

The speed of sound does not change as drastic as you might think. Any changes in the speed of sound due to altitude, are due mainly to changes in temperature (and composition to some extent). The thicker air at sea level will induce higher drag forces, which I think is the main difference between airspeeds achieved at altitude vs sea level.

Quote:

So is engine thrust proportional to airspeed?

Proportional to acceleration

Is the pressure increase by the turbines constant increase across all altitudes or what is it a function of... propulsion class in next semester unfortunately Sad

. Boosted, aircraft thrust is based on conservation of momentum. Yeah, the change in temperatur is really freaky for earth's atmosphere, the graph is kinda haywire. http://www.csgnetwork.com/atmoscalc.html This website has saved my butt so many times on my homework. I really wish I knew more about the propulsion systems to help you. Also, what's the deal with military thrust rating?

The Viper can break the barrier at sea level

TenguNoHi wrote:

The reason you need to be at a higher altitude to go mach 2 is because the speed of sound changes at different altitudes, if thats what your asking...?

An F-16 cant go the speed of sound at sea level with 29K of thrust because the air is much thicker at sea level and hence sound travels through it quicker. At 40,000 feet the air is much thinner so the sound cannot travel as direct.

I think thats correct anyways. allenperos or pat1 will jump in if im wrong.

-Aaron

I would like to add the there is less oxygen at 40,000 FT. You start loosing segments of AB at around 30,000 FT A F-100-229 engine is rated in full AB during perfect standard day conditions. Mil power is around 16,000LBS Full around 29,000.

I would also like to add that this is not a ram jet so forcing air into the engine is a bad idea.(so zero thrust added by ram air) That is why the F-16 intake is designed like it is, to prevent a ram air condition. Also mach is not a universal number like 700mph at 1000ft and 700mph at 40,000, Basicly Mach numbers gets lower and easer to obtain at higher altitude with less power settings (based on temp and pressures ect). Which is a good thing concidering you "loose thrust" at higher altitudes.

Great Thread... here lets keep this going!

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

229guy wrote:

I would also like to add that this is not a ram jet so forcing air into the engine is a bad idea.(so zero thrust added by ram air) That is why the F-16 intake is designed like it is, to prevent a ram air condition. Also mach is not a universal number like 700mph at 1000ft and 700mph at 40,000, Basicly Mach numbers gets lower and easer to obtain at higher altitude with less power settings (based on temp and pressures ect). Which is a good thing concidering you "loose thrust" at higher altitudes.

Although its been a while since I studied air breathing propulsion, I can say for sure that a turbojet or afterburning turbofan (which is pretty close to a turbojet) as found on all F-16 aircraft will benefit from increased airspeeds up to a certain point... at least when the afterburner is lit. I won't write the equations as that would be tedious, but instead I'll refer you to something I just found searching google a minute ago:

http://www.rmcs.cranfield.ac.uk/aeroxtr ... pparam.pdf

For afterburning turbofan or turbojet engines, the following simplified forumula for net thrust can be used and applied to the case of the F-16's propulsion system:

Fnet = Mdot_a * (Ue - Ua)

Mdot_a is the air mass flow entering the engine. Ue is the engine's exit velocity and Ua is the velocity of the aircraft (i.e. velocity of air entering the aircraft's inlet).

Mdot_a = rho_a * Aa * Ua

rho_a is the air density and Aa could be considered the inlet area (although this may not be technically correct). Thus, while Ue can be considered somewhat constant, it is generally going to be a lot faster than the aircraft's flight speed. An F100 or F110 engine in the F-16 will have exit velocities in excess of Mach 2 for AB operation. As flight speed increases, mass flow (Mdot_a) increases for constant area and density. So at constant altitude, an F-16's thrust will, in general, increase. The F-16's engine can only handle so much airflow however, so above a certain speed it may "spill" excess air around the sides of the inlet leading to spillage drag and a net decrease in thrust. Regardless, with Mdot_a multiplying the very high Ue in the (Ue - Ua) expression, you can see that more net thrust results so long as you can keep increasing Mdot.

Net thrust calculations are actually a lot more complicated than what I just presented, but I know for a fact that an F-16's AB thrust profile increases with airspeed up until a certain point where you see a leveling off. You wouldn't encounter the point where thrust begins to decrease significantly in level flight however. Once this starts to happen you're likely beyond the design limit of the airframe anyway (like in a dive from high altitude to low altitude).

An F100-PW-229 engine installed in the F-16C Block 52 puts out about 22,000 lbs of thrust at SL and zero airspeed but around 34,000 lbs (max) at Mach 1.1. Beyond that the thrust begins to decrease (probably do to the engine logic reducing fan RPM). At 40,000 ft. where Mach 2 can be reached, the Block 52/PW-229 will be putting out about 16,000 lbs thrust as it approaches max speed which is Mach 2.05. At 40,000 ft. and Mach 0.65 though (about 200 KIAS), AB thrust is only 6,000 lbs or so.

I hope that answers some questions.

-Raptor One

Raptor_One :

Great info Very Happy

-Scott.

Forum: Technology

Thrust increase with velocity?