Air Inlet Drag

[PlatinumZealot]

when your talking super sonic with a jet engine, this may sound crazy but the SMALLER the inlet the better. Look at the SR71 blackbird spy plane. The inlet cones extend out to create an aerodynamic anomily where the friction hitting the surface excelerated the air inside the engine at a much higher temp and speed. the faster the black bird goes the less fuel it uses and the more aerodynamic it becomes with reduced inlet size. Its hard to explain. Wikipedia the SR71 and go to the engine part. its an interesting read in my opinion. turns the rules of Aero on its head in a small but not insignificant way

I have learnedof that phenomena, it is called "Reheat." It also happens at the blade tips of a gas turbine. Where the gas gets heated even without the aid of combustion, due to friction at the tips of the blades.

[peter kent]

Hi n_smikle,

Interesting stuff. But there's no magic going on here. Hope the following helps.

Friction at blade tips is all bad news because that means they are rubbing which they should never do. If they do, they lose material and the clearance gets bigger. Bigger tip clearance is like having crummy piston rings.

Good sealing piston rings keep the gas pressure on top of the piston where you need it.
Small turbine tip clearance (but no rubbing!) keep the gas on the blades where you need it ( not leaking round the tips!)

The sequence of events is: friction at the blade tips, material rubbed/melted away, bigger tip clearance, more leakage, less power, more fuel needs combusting to make up for it, more fuel costs more money. So it's bad news at every step.

Regards
PK

[Tommy Cookers]

let's not forget when looking at any jet or gas turbine etc etc .......

all such engines always did, and still do ingest more air than can combust with the fuel, otherwise they would melt
(research, even now, sees a 'stoichiometric' engine as a distant goal)

late in WW2 piston engined aircraft were getting jet thrust from the exhaust and from coolant heat in jet ducts
jet aircraft got much of their thrust from the spare air (as above) they pulled in and compressed and accelerated
deconstructed, the two types of propulsion could be seen as quite similar

today's big turbofans get 84% thrust from the fan and 16% from the 'jet'
(and some of this 16% is from the spare air that is in the jet flow)
the same as the piston engined aircraft can do/did

Bloodhound SSC is for low supersonics, so different to SR71 intakewise etc ?

[olefud]

late in WW2 piston engined aircraft were getting jet thrust from the exhaust and from coolant heat in jet ducts
jet aircraft got much of their thrust from the spare air (as above) they pulled in and compressed and accelerated
deconstructed, the two types of propulsion could be seen as quite similar

today's big turbofans get 84% thrust from the fan and 16% from the 'jet'
(and some of this 16% is from the spare air that is in the jet flow)
the same as the piston engined aircraft can do/did

Bloodhound SSC is for low supersonics, so different to SR71 intakewise etc ?

The WW2 exhaust thrust was due to the Meredith effect. It didn’t completely null the radiator drag, but significantly reduce it.

I assume fan jet don’t go supersonic essentially for the same reason prop planes don’t.

[Tommy Cookers]

The WW2 exhaust thrust was due to the Meredith effect. It didn’t completely null the radiator drag, but significantly reduce it.

I assume fan jet don’t go supersonic essentially for the same reason prop planes don’t.

the exhaust thrust came from proportioning the stacks to increase velocity IMO
(slight loss of crankshaft power, negligible loss of efficiency, worthwhile gain in jet thrust at high speed and altitude AFAIK)
increased engine power couldn't always be used by the prop, anyway
the M effect was using coolant heat only in jet action radiator duct ?, later developments incl. exhaust heat, entrainment effects etc

all F-15,16,18 etc supersonics have turbofans for range, (usually ?) supersonic only with reheat)
now eg F-22 still turbofans but somewhat supersonic without reheat for fast cruise (reheat for combat)
not big fans though, but they have plenty of spare air for reheat
presumably increased temps with progress in turbojets reduced spare air, and encouraged turbofans ?

[peter kent]

when your talking super sonic with a jet engine, this may sound crazy but the SMALLER the inlet the better. Look at the SR71 blackbird spy plane. The inlet cones extend out to create an aerodynamic anomily where the friction hitting the surface excelerated the air inside the engine at a much higher temp and speed. the faster the black bird goes the less fuel it uses and the more aerodynamic it becomes with reduced inlet size. Its hard to explain. Wikipedia the SR71 and go to the engine part. its an interesting read in my opinion. turns the rules of Aero on its head in a small but not insignificant way

Hi n smikle,
Hope I can help out here with a couple of pointers to explain there were no mysterious anomalies with friction happening in the intake, nor more speed with less fuel.

If we look at what is happenning to the air in the SR71 we see that it is basically no different from what's happening to the air in a supercharged car engine.
I know you are familiar with the car stuff but I have put it in just to highlight the similarities with the SR71 engine.
The car has an engine with piston compressors, ie air is compressed on the compression stroke.
The plane has an engine with a turbomachine compressor, ie lots of spinning blades.
If you put a S/C on the car it compresses the air B4 it goes into the engine compressors.
If you fly the plane at some speed the intake compresses the air B4 it goes into the engine comressor.
The car S/C compressor usually compresses the air with a spinning parts.
The aircraft intake compresses the air by slowing it down, when it is supersonic thro shockwaves, and then some more while below the speed of sound until it gets to the engine compressor.

The car S/C puts up the pressure and temperature (bad)
The aircraft intake puts up the air pressure and temp (bad). Since temp is bad you wouldn't want friction adding to it (which it doesn't measurably).
We fit a cooler after the S/C which helps with the car problem.
In the plane the high temps are something that have to be lived with. They spoil the engines performance so it needs more fuel for the same output.

In the airplane, the faster you want to go the more fuel you have to put in, until at SR71 cruise the two engines are burning 8000 gal/hr each ( ref AEHS website)

So, there is no mysterious friction effect giving thrust for nothing.

Hope you can pick something which makes sense out of the above.

Intakes for Mach 2+-type speeds have typically been complicated and difficult to design because of the way air behaves at those speeds but they don't do anything magic, just slow the air down.

PK

[Tommy Cookers]

having not properly followed this thread,
is the compression process efficiency inherently (or potentially) improved by being supersonic ?
Concorde's engines were about 55% efficient in cruise, this is remarkable, and much higher than subsonic
(or is this just due to better matching of jet speed to flight speed ie better 'Froude efficiency' ?

[olefud]

having not properly followed this thread,
is the compression process efficiency inherently (or potentially) improved by being supersonic ?
Concorde's engines were about 55% efficient in cruise, this is remarkable, and much higher than subsonic
(or is this just due to better matching of jet speed to flight speed ie better 'Froude efficiency' ?

I’m just being deductive here, but given the efficiency of ram jets at say mach 3, it would seem that any free “ram” compression is compression you done have to pay for by the parasitic powered compression stage.

[peter kent]

having not properly followed this thread,
is the compression process efficiency inherently (or potentially) improved by being supersonic ?
Concorde's engines were about 55% efficient in cruise, this is remarkable, and much higher than subsonic
(or is this just due to better matching of jet speed to flight speed ie better 'Froude efficiency' ?

I’m just being deductive here, but given the efficiency of ram jets at say mach 3, it would seem that any free “ram” compression is compression you done have to pay for by the parasitic powered compression stage.

Concorde and XB70 and SR71 were impressive in their day because compared to subsonic aircraft of the 60's they had high Froude eff as well as the ram eff.
However, 50 years of engine development means todays big fan engines now match those levels of high overall eff.
They have even higher Fr eff now with the high BPR and the engine overall pressure ratios are over 40:1. In the 60's you had to go to mach 3 to get 40:1 in the intake.