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Taking a look at the typical large engine of a commercial plane, it looks rather unstreamligned. Nacelle drag is significant IIRC.

However, then I suddenly remembered these engines produce a large jet of thrust (most of the time). Does this air stream of thrust act as a good shield against nacelle drag? Does it effectively form a bell cone, or half of a seers-haak body, around the rear half of the nacelle?

If so, that would appear to cut the nacelle drag in half.

curious_cat
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DrZ214
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1 Answers1

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The nacelle is not some solid block. For the flow it looks like a hollow, slightly blocked tube.

The flow ahead of the nacelle is indeed slowed down in flight, but only a little, entering the intake face at a flow speed of around Mach 0.4 to 0.5. This compresses the air ahead of the nacelle when flying in cruise at around Mach 0.8, so what enters the intake is already slightly compressed. Note that this compression does not involve boundary layers or shocks, so it is very efficient. The effect of the compression is that some of the air flowing towards the nacelle is pressed aside to flow around it. To avoid flow separation, the intake lip is rounded.

Pre-compression helps, because it raises the pressure in the whole engine and increases its thrust. What might be looked at as drag is in fact increasing the thrust of the engine! Of course, by further compressing and heating the ingested air the engine accelerates it, so the outflow speed of the core flow is easily double its entry speed. The outflow fills up the cross section of the engine, so the outer flow never has to flow into a void and experience separation (which is responsible for most of the drag of blunt bodies). Instead, it meets a faster jet of air once it has passed the engine fairing and is accelerated during the mixing process.

Only if the engine is idling or off will the nacelle cause real drag.

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Peter Kämpf
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    Great answer! Also, greater air mass entering the nacelle leads to even more bypass air, improving both thrust and efficiency. – Timpanus Feb 10 '16 at 02:30
  • That third paragraph really helps. Just for the record, it was form drag I was concerned with, not skin-friction drag. I suppose another way of asking the question is, If the engine was throttled back to produce a jet exhaust equal to airspeed (so incoming airspeed = outgoing jet air speed), would nacelle drag be greatly reduced? – DrZ214 Feb 11 '16 at 07:02
  • @DrZ214: No, it would slightly increase. Most of nacelle drag is caused by friction, and throttling the engine will increase spill drag a bit and accelerate the flow at the end of the nacelle less, increasing the risk of flow separation. To be more precise, we need to clearly define what counts as nacelle drag and what counts as net thrust. – Peter Kämpf Apr 02 '16 at 07:07