Forum: Technology

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

229guy wrote:

Thank you, I do understand AB operation, but I was origianly focusing on engine w/o ab. As you know when you add ab you have a fresh supply of unburned air mixed with a lot of fuel. Very Happy

And yes will add thrust whenever activated

I edited that last post to un mislead people but you posted it before I got around to it!

I responded to your edited post as well. Regardless of whether the AB is in operation or not, airspeed will affect the net thrust for the same reasons. All the AB changes in the basic thrust equation is the exit velocity of the engine... it greatly increases it. The airflow still multiplies the inlet and exit velocities by the same amount. Just take a variety of different cases and do the math.

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So...

Thrust (at a constant altitude) will increase with airspeed up until the engine is getting all the air that it is designed to handle (about .7M for a Viper?). Then will it stay constant or drop off?

Higher altitude means less thrust b/c of the lower density? That lower density offsets the lower temperature?

Thanks for the great info guys, I've got the basic pilot understanding: left hand forward = fast. Always been told (and demonstrated) that the engine does better at higher airspeeds (to a point I guess). We'd do a max AB acceleration from L/D max up to 100kts above that and time it, then do it from a higher airspeed and the faster we were, the better our acceleration. I had the laymans understanding of more air = more power, like in my car. Am I right to say that is true up until that point where the intake/engine has it's 256lbs/sec?

Thanks again!

Thrust available will remain nearly costant as velocity (airspeed) increases. Fuel flow is directly proportional to thrust output

Perhaps someone can shed some light on this question...
Lets put a F-16 in the trim pad and start the engine and run it up to full burner and keep it there.

Zone A is 10ft in front of the engine Zone B is the beginning of the inlet. Zone C is the engine front face and Zone D is the engine exhaust.

1. At full power does any one know what the airspeed is at zone B, C and d?

Now lets introduce hurricane Viper starting with a 200MPH steady wind at zone A going straight into the intake.

From Raptor One and my previous discussions there should be a thrust increase at zone D which leads to my questions

2. deleted

3. At what (airspeed) point does hurricane Viper start to provide extra thrust at zone D

4. What point does hurricane Viper overcome the engine.

Any one?

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

200 MPH isn't all that fast in terms of increased airflow to the engine. It probably wouldn't make a substantial difference in terms of thrust output. The questions you are asking are not the kind you could answer without having extensive data on the F-16's inlet/engine... a lot of which you'd need a security clearance for. What I can say is that when an F-16 or any other aircraft is at static conditions, the engine is sucking air into the inlet. How much air it can suck in depend on a variety of things. How fast the air is accelerated into the engine at static conditions is something you could calculate/simulate or perform tests on the aircraft itself to determine. But it's not one of those numbers that gets pulled out of someone's hat. Smile

,

I somehow knew thats what someone would Say!

Thanks

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

Destro wrote:

Thrust available will remain nearly costant as velocity (airspeed) increases. Fuel flow is directly proportional to thrust output

Thrust availabe being nearly constant as velocity increases is what they tell you, in general, for a turbojet engine without thrust augmentation (afterburner). This is definitely not true when you add thrust augmentation into the mix and is not always the case for turbofan engines.

Hello

I've got one question connected to main topic. As we know, for example, F-119 engine can generate about 22k - 26k lb of static 'dry' thrust.

Is there any possibility, that as F-22 increases its speed, the thrust generated by F-119 become greater then static thrust?

Regards

I doubt 'more' thrust at speed than static, the Raptor can only hit those speeds at altitude where the air density is lower, which will reduce an engine's thrust; but lower air density at altitude also reduces drag Wink

Being 'almost' a turbojet though the F119 will have a high specific thrust and low thrust lapse at MACH 1.5 or 2. Remember the engine was 'designed' to hold MACH 1.5+ without burners at altitude. Considering this the thrust of the F119 must be higher than that of the aircraft.

I highly-doubt the F119's thrust higher than static/sea-level, but it's still going to be higher than any other engine out there today! Two Cents

It's late for a description/definition, so for reference: http://www.answers.com/topic/turbofan

Good article; Look for "specific trust" and "thrust lapse" in the article.

Keep 'em flyin' Thumb

TEG

Hello

TEG ---> thanks for answer. I were asking because of this chart (found on book about aircraft aerodynamic and performance):

http://img114.imageshack.us/i/thrustchange.jpg/ (where P is overall 'dry' thrust and 'P + dopal.' is thrust with afterburner)

As we can see, the engine achieves its maximum thrust at speed, not at static condition.

However, I also found this chart which present slightly different point of view:
http://img402.imageshack.us/i/thrust.jpg/

We could see that thrust at speed is suppossed to be smaller than in static condition.

Regards

For those interested at how sea-level static thrust differs to installed and net thrust , you may read NASA Technical Memorandum 86042. It's very interesting to see how the F-100 EMD engines in an F-15 and rated 27500lbs at sea level static thrust were performing troughout the flight envelope and how you can use DEEC to improve performances of an engine.
That's also give some reason to be prudent about "real" thrust/weight ratio
regulary published on this site.

Thanks for that document.

I tried doing some extrapolating from the chart that was provided in it, and here's what I got (numbers are approximate). Let me know if I've made any mistake in interpretting these data.

SEA LEVEL THRUSTS:
@M0.0 = 20,800 lb.
@M0.2 = 22,700 lb.
@M0.4 = 24,100 lb.
@M0.6 = 25,900 lb.
@M0.8 = 27,100 lb.
@M1.0 = 30,500 lb.
@M1.2 = 31,000 lb.

THRUSTS AT 50,000 FEET
@M0.0 = 2,440 lb
@M0.2 = 2,560 lb
@M0.4 = 2,950 lb
@M0.6 = 3,330 lb
@M0.8 = 3,850 lb
@M1.0 = 5,540 lb
@M1.2 = 6,620 lb
@M1.4 = 7,700 lb
@M1.6 = 8,920 lb
@M1.8 = 10,200 lb
@M2.0 = 12,100 lb
@M2.2 = 12,800 lb
@M2.4 = 12,800 lb

Looks like installed, static, sea level thrust is ~21,000 lb, so I'm guessing that the 27,500 lb figure mentioned was the uninstalled, static, sea level thrust?

Yes your numbers are right, and yes the 27500lbs is for uninstalled thrust at static sea level. All published engine thrust are always at those conditions. That's in part due to the fact that the same engine will perform in a different manner in another aircraft. Thrust has a close relation to engine inlet overall design.

Forum: Technology

Thrust increase with velocity?