Air Inlet Drag

[Callum]

I was at a lecture where one of the head guys from the Bloodhound SSC landspeed record attempt was speaking. He was talking about the general design of the car.

He came to talk about the jet engine air inlet and said something like "Sadly i'd like to think of the inlet as drag-free but we really can't assume that"

It got me thinking WHY we can't assume that, surely the inlet is at a lower than ambient pressure - it's almost anti-drag?

Any ideas?

Pictures of the car can be found on their website: http://www.bloodhoundssc.com/

[raymondu999]

Maybe because it's a big honking cannon in the way of the air?

[mep]

Just a quick guess about it:
With increasing speed you will soon arrive on a point where there is more air streaming on the inlet than the engine will suck in. Not to forget the car is doing supersonic speed. I can imagine the fan has to be designed in a way that the inlet speed is subsonic to make the fan blades work. The airspeed has to decrease first to build up pressure in the turbine. Then after the combustion chamber it will be accelerated.

For sure a interesting topic to investigate.

[marekk]

This car is capable of +1000Mph, way beyond speed of sound, and as no compressors are able to work with supersonic flows, they have to slow inlet air to much lower speeds (depends on engine's design point, usually about Mach 0,3). This means drag (called ram drag or inlet drag).

Another source of drag would be stagnation on intake lips.

[Richied76]

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

[Just_a_fan]

The SR71 engine was designed to use bypass tubes at high speed and become, in effect, a turbojet / ramjet hybrid.

The engine doesn't alter the aero rules it just uses them differently.

For those with suitable interest:
http://www.sr-71.org/blackbird/manual/1/1-33.php

You can view the entire flight manual if you have the time. Some fascinating stuff...

[N12ck]

you have got to remember there is over-spill in an air inlet, so some air that is forced through that some of it will over-spill back out, so drag is still there, also with a inlet you have more surface area so you get surface drag

[Greenish]

Forward-facing inlets like the one on the SSC or an F1 car are designed to use some of the "ram pressure" to force more air into the engine. This means slowing down the air to recover static pressure from velocity. Therefore there is in fact backpressure - not suction - in these inlets, at anything over a moderate forward speed This leads to the spillover mentioned above, and the inlet becomes a drag source. Because the air entering the inlet "disappears" it's not as bad as a bluff body, but not drag-neutral. Of course if the air is sped up by combustion and/or heated by the radiator, and re-injected carefully in the flow downstream, you may end up with some net thrust for the system.

In supersonic inlets like the SSC, the inlet is shaped to position shock waves to slow the air down to subsonic and recover lots of pressure. This is draggy and finicky (thus the moveable nose cones) but necessary for the compressor to work right. In subsonic ones the goal is usually just to ingest the required amount of air through as small of a hole as possible, without making too much of a mess around it, and without causing additional pressure losses inside the duct.

[Ray]

You have to slow the incoming air into the jet engine down to subsonic speeds. Otherwise you'd have a compressor stall. At 10,000 feet it may not be so bad to have that happen, but in a land speed car and compressor stall could easily be deadly.

[riff_raff]

Callum,

Drag is a somewhat subjective term. And drag losses should always be considered in relation to their relative gains, whether it's lift/downforce, or increased engine power due to higher intake duct pressures.

As for a turbine engine intake at supersonic speeds, the other posters made some excellent points. With the Bloodhound SSC example, its turbine engine likely must have subsonic airflow at its compressor face, otherwise its compressor airfoils may experience stall. So the Bloodhound SSC inlet duct is designed to diffuse and slow the intake air velocity to subsonic at the compressor face.

The example given for the mach3+ SR-71 engine inlets is even more interesting. As Just_a_fan notes, the SR-71 engines were combined cycle engines, operating as both turbojets and ramjets. If you look at a picture of those engines, you'll see a variable geometry cone in the inlet. Above about mach 2, the turbine engine's compressor and turbine stages were completely ineffective, and contributed nothing to thrust. They basically just got in the way. At mach 2+, all of the engine thrust came from the intake duct ram air compression effects. Surprisingly, the SR-71 achieved some amazing efficiency numbers (ie. mpg) when flying at high altitudes & high speeds.

As for air ducts in general, almost all ducts are designed to produce a net gain in performance. Engine air intakes increase horsepower, and well designed radiator ducts result in a net increase in thrust.

Great discussion.
riff_raff

[Callum]

Brilliant replies everyone, thanks a lot - This forum is great

But it has not got me with a further question: Why must air inlet speed be subsonic? - something to do with the turbine blades not physically being able to turn fast enough?

[Greenish]

Getting pretty far from F1 here, but sustaining supersonic combustion is very very hard (look up SCRAMJETs and note that none have ever been made operational or even tested very successfully). The flame fronts from most fuels other than hydrogen can't even keep up so it just gets blown out.

But even before you get to the combustor, the compressor in a jet engine is essentially a spinning cascade of airfoils. The lift/drag ratio of those airfoils goes way down as they go sonic. Also, the position of the shock waves in on the spinning compressor blades would move around, and impinge on the other blades, generally making the whole thing less stable, noisier, hotter, and requiring more difficult structural design. That's not to say that high-speed jets don't have locally supersonic regions in their compressors and have to deal with trans-sonic flow - they do - but it's much harder to manage if the inflow itself is also supersonic.

Finally, bringing the flow from supersonic to subsonic in a controlled manner within the intake allows maximum pressure recovery from the flow.

[Callum]

Good stuff, thanks Greenish

[Robert.Gardner]

As I see it, with a Turbojet, there is the problem that at very high speeds, the compression of the intake air, causes a rise in temperature throughout the engine to the point, that the turbine blades melt, the tip clearance increases losing efficiency, and nozzle choke would occur.

The engine in question, a Turbofan, features active tip clearance control (by directing bypass air to cool the turbine casing), uses other bypass air to mix with the hot combustion gasses, at a ratio of 0.4:1, an afterburner, also it uses a convergent-divergent (de Laval) nozzle to accelerate the exhaust gases past supersonic speed. A more efficient style of engine for supersonic flight compared to a turbojet.

Other factors may also be at play, but I'm no rocket scientist.

[peter kent]

Hi folks,
Hope you're still interested in this ageing topic and haven't moved on to other things.
I hope you don't mind me clarifying a couple of things mentioned re SR-71.

Hi Just_a_fan,
The SR-71 engine was not a turbojet/ramjet.
We can convince ourselves that it had nothing 'ramjet' about it in one of 2 ways.

The easy way is by taking note that both the people that made it (P&W) and those that used it (Lockheed) -and the flight manual- called it a turbojet with bypass bleed. No mention of the word 'ramjet'.

The 2nd way is more interesting and you could call it the D-I-Y way. We will prove for ourselves that there is no ramjet mode for the J58 engine.

First, take a typical afterburning turbofan of the 70's operating at Mach 2. Follow the air path through the engine from compressor inlet to exhaust nozzle.
ie 50% of the air goes thro the fan, then down the bypass duct to the afterburner.
the other 50% also goes thro the fan, thro the core to the afterburner.

Now take the J58 at Mach 2, just to keep apples to apples. Again, follow the air path from compressor inlet to exhaust.
ie 20% of the air goes thro the 1st 4 compressor stages, then down the bypass tubes to the afterburner
the other 80% also goes thro the 1st 4 compressor stages, then the remaining 5 stages and combustor and turbine to the afterburner.
BTW, the above holds all the way up to M3.2

We see there is a distinct similarity in where the air goes, so we have just shown to ourselves that there is nothing worthy of the term 'ramjet' for the J58 anymore than there is for the other engine (F14/ F111 or whatever)

So how did the term turbo/ramjet originate?
My suspicion is that back in the 50's when all sights were set on ever higher speeds, Mach 3 was in the exotic ramjet regime so enthusiasts presumed the engine must have had some ramjet connection.
This link is a good complement to the flight manual
http://www.enginehistory.org/Convention ... lsion2.pdf

Hi riff_raff,
The only reason the SR-71 could run at M3 was because the engine was processing air thro a 9 stage compressor, combustor, turbines and afterburner. To do this it was burning 8000 gall/hr. Now this engine flow induced a large airflow around the engine at the same time. The induced flow plus the engine flow together produced the intakes thrust contribution. The engine was running with compressor exit at 1300 F, turbine 2000 F and afterburner at 3200 F (see above link) and provided 17% of thrust. It's worth realising that it was only because the engine was moving so much air that it could boost its own thrust dramatically with the extra airflow (intake thrust 54%, rest final nozzle).

Hope it all makes sense.
PK