Forum: Technology

The "round two" topic had gotten stale.

Care for some mo'?

1) I hear the term "kinematics" and "kinematic advantage" stated in relation to fighter jets. I'm not exactly sure what these terms mean. I figured that it might refer to an aircraft's ability to launch a missile at high speed so that the missile spends less of its fuel getting up to speed and therefore has more fuel to burn in order to increase its range. Is that correct? However, I heard on a video about the F-35's EO DAS that the sensors give it a kinematic advantage in WVR combat. So now I'm confused again. What does the term "kinematics" encompass?

2) Can the tips of the fan, compressor and turbine blades exceed the speed of sound in a jet engine? What effect does this have on the engine's performance? Is that one reason why jet engines are so loud?

I can't think of any more at the moment.

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Kryptid wrote:

2) Can the tips of the fan, compressor and turbine blades exceed the speed of sound in a jet engine? What effect does this have on the engine's performance? Is that one reason why jet engines are so loud?

Jet engines as they exist today "can" a little here or there, but if they do too much you'll get a compressor stall. (Bad thing) You'll get local supersonic air sometimes at tips due to rotational speeds combined with the flow speed of the air. (But as far as I know most of this is accidental or incidental due to rotational speed, and supersonic flow limits performance more than helps.)

Now NASA has played with the idea, and yes it is possible if designed into the Fan/Engine from the start. See "Supersonic Through-Flow Fan engine (STFF) or Supersonic Through-Flow Rotor and Supersonic Counter-Rotating Diffuser (SSTR/SSCRD)" Cool

As for the compressor or "inlet" noise... Mad

"Inlet" noise is generated by interaction of air-flow pressures and turbulence between the rotating blades and stationary vanes within the compressor.

I suppose supersonic fan flows would only make it worse, but at those speeds and altitudes, I doubt anyone on the ground would notice. According to NASA's study, during take-off and landing, the speeds within the compressor would be subsonic and stay that way until about MACH 0.8 Wink

Keep 'em flyin' Thumb

TEG

Thanks TEG. I could always count on a good word from you.

3) I watched a video from Northrop Grumman about the EO DAS on the F-35. It's been posted here before. From what I saw, they seem to imply that the EO DAS will allow the F-35 to target and launch a missile at an enemy aircraft regardless of where it is positioned relative to the Lightning (whether above it, below it, behind it, etc). Is that true? Has it been demonstrated yet or is it just a projected ability of the production F-35s? If it is true, then it would seem that it would give the F-35 a significant advantage over practically any other airplane in WVR engagements. Even the F-22. Flankers beware!

Per the vid, yes.

The DAS system has been working in the test AC for a long time. HOBS missiles are a reality and the Block 2 upgrades for the AIM-9X will give it LOAL. The AIM-120D is also getting better HOBS ability.

The new JDRADM is also going to come with HOBS as it is being developed specifically for the F-22 and F-35 to be internal.

The F-22's MLD (Missile Launch Detector) is the precursor to DAS and consists of 6 sensors like the F-35's DAS. Word is being done on the MLD to upgrade it to airborne detect and track abilities like DAS.

deleted double post

4) In one of my other topics, "shockwave ingestion", it was mentioned that shockwaves colliding with the compressors in a jet engine is bad news. I figure that this is one of the reasons that variable-geometry inlets are used. However, some high speed aircraft seem to get by with fixed inlets (i.e, Crusader III, Raptor). Do longer intake ducts delay the collision of shockwaves with the engine face when compared with shorter ducts (thus allowing aircraft with fixed inlets to attain high speed)?

No Kryptid, fixed inlets do something a little different. There are two kinds of shocks you need to worry about, oblique and normal (angled and normal to the flow, respectively). The air is still moving supersonically behind an oblique shock, there's some math to it, but kinda hard to get into it here. Behind a normal shock the flow is subsonic. Oblique shocks help pressure recovery, normal shocks hurt it, real simple rule.

What a fixed inlet does is bounces the shock around in the inlet a couple times then take it to a normal shock. That way, you're getting no shocks meeting the engine face. To part of your question, longer inlets mean you can bounce the shocks more, producing more oblique shocks and increasing pressure recovery. Somewhere though longer inlets become too heavy and have to be limited.

To your first question, whenever you hear the words "kinematic" think energy. When dealing with fighters its almost solely kinetic (velocity based) though potential (altitude) is not to be totally discounted. Regaining energy can be done through chemical energy (fuel and engines). So for something like "kinematic advantage" means an energy advantage either being faster than the other guy, attacking from above, etc. "Kinematic lob distance" is another term thrown around occasionally and it refers to how your energy affects how far your missiles can fly.

Salute

Absolutely beautiful explanation of the shockwaves, Bunny-breath.

Just last Monday I had to show a yute the deal with "diamond" airfoils and the oblique shockwaves and pressure differentials. Sheesh, we had those doofers back in the late 50's.

The noise from our motors is a bear. The T-birds used BUC versus normal fuel control as it had less of the "scream", and the jets were all pointing right at the crowd when they started up. Maybe TEG can expound on that phenom.

later,

Quick and dirty on BUC and noise (Or SEC and noise) in the F100 engine.

Wait a minute; BUC!?! Who the talks about BUCs anymore? Brother Gumms you're dating yourself! Cheers

When running an F100 in a mode other than Primary, the CIVV (Compressor Inlet Variable Vanes) are positioned to "fail-safe" to keep the fan compressor out of most stall situations. By changing the position of the vanes, airflow changes in the fan, as does the angle at which it meets the 1st fan blades. These simple changes in flow and AOA changes the pitch/noise levels of the engine at the inlet.

The PW-229s of the current T-Birds are much "quieter" in comparison to the PW-200 or PW-220s used previously. I doubt anyone except the T-Birds would get away with starting engines in anything other than primary mode Wink

I don't think the practice is used today. (from what I can hear)

Keep 'em flyin' Thumb

TEG

5) What feature of certain jet engines that allow them to reach very high altitudes (70,000+ feet) whereas others cannot? I guess I'm referring to engines onboard planes like the U-2, MiG-25, SR-71, XB-70 and MiG-31. At first I thought it might be the high speed of the aircraft (like the SR-71 and MiG-25) that allowed for the thin air to be compressed as it entered the engine. Then I remembered that the U-2 was subsonic, but it can still attain altitudes up to 85,000 feet. Are these engines simply able to compress air better than other engines?

Everything you listed but the U-2, the design feature is speed. It allows huge quantities of air to be crammed down the engines throat. There's a lot of math on what it takes to get up to that height, but the simple parts of it are that speed dramatically increases the mass flow through the engine (rho*V*A).

The U-2 on the other hand, we'll its extremely low drag aircraft, and probably also quite light, so doesn't need much thrust (look at those wings, I believe they're 14:1 aspect ratio). If you want to add another one to your list, add the Global Hawk, those things orbit at like 60-70k ft IIRC. Those are another where you have a combination or things to make them work. I'm pretty sure the inlet is also wide open on both those so they can use the area term to get more mass flow.

Old topic, but I've thought of something else that I'd really like to know.

6) Does anyone know the relationship between the speed of an aircraft and the resulting temperature on the skin generated from air friction at said speed? I'm not asking for predictions regarding a made-up aircraft or what not. Let's say that we use the SR-71 as an example. If I know the temperature (Kelvins is probably best) on the nose of an SR-71 that is traveling at Mach 3 at 80,000 feet (or the equivalent air pressure thereof), is it possible to extrapolate from that what the temperature would be at different speeds and altitudes/air pressures? Could you calculate the temperature, for example, of an SR-71 doing Mach 3 at sea level (I know it can't, but I'm asking hypothetically)?

Forum: Technology

Question Block (Round Three)