I've got a question for you. I've got to design an inlet spike, in a fully ducted installation. The outside size and shape I've got figured out. Once it heads into the nacelle I've got 2 thoughts, as you can see below. My instinct says that it should be shaped like image A below... the pointy one. However, a couple years ago, I spoke with someone that builds our model turbines, and he was adamante that inside ducts should end rounded prior to turbine intakes, as shown in image B.
He was referencing how my F-15 ducts came together and ended in a sharp seam, when in fact if it were fatter, and could be rounded where they joint, it would be much better. Seemed backwards to me.


Thoughts? A or B?

Thanks,
Lance

B would give less turbulent air into the turbine intake.

I have to disagree. I think this shape gives the least turbulence.

Of the two B would be better but it could be improved with a slightly more teardrop shaped aft contour.

Yes pardon my attempts at using Paint, on both of them above, the transitions around the fattest part would be smoothed as much as possible.

The root of the question, is how the center is shaped, in that it ends up blunt (rounded), or ends in a sharp point, pointing at the turbine face.

Lance

Sorry for the poor pic but this is an SR71 intake
it has a movable spike and a stator to smooth the air stream

brian

I am curious why the person you spoke to said they would prefer a more blunt face in front of the turbine, as in your F-15. The type of compressors we use on models are not as intolerant to turbulent flow as a full scale axial flow turbine. But I see no reason to not use reasonable design rules, and minimize turbulent flow creation. Typically, any kind of quick or sudden expansion in the duct will create a turbulent flow. I would prefer to design with smooth contours inside, and slowly reduce the diameter of the centerbody to more of a teardrop shape, as the above posters have shown or mentioned. ( Mach1's pic "C" ) [8D]

I assume your turbine is inside a bypass, and you are using a fully ducted system. If not, it probably doesn't matter. A short blunt aft facing area would be lighter, and you could probably slide the turbine forward a bit more. But it wouldn't be a cleaner flowpath.

Looking forward to seeing your project fly - next time with turbines! [8D]

I would try something like this. It's what I will put in the B-58

At the speeds we operate with model turbines I dont think it really matters. If it were for a ducted fan, then you need all the inlet help you can get. Scotty

I like the one in Mach1's diagram and think plan A and B both have big problems, creating a LOT of drag and turbulent flow. Mach1's drawing has the least drag and smoothest flow entering the turbine.

What if you were to mill the basic shape(s) out of foam and put them to the test? Mount them in the intake of your SR, put some streamers on the end of the cone, set a high speed fan at the intake and let it rip. Then observe the streamers from the exhaust end and see which setup is less prone to turbulance i.e. which streamers are more smooth in the airstream. Cant be any worse than the speculation so far

mb

Guys

The "spike" as you call it ONLY functions in supersonic flight and as such has no relevance in the realms we operate our models. I wont go into the aerodynamics, just leave your intake unobstructed.

PF

Well he is making a scale model that has intake spikes. Everyone know that it's not for supersonic flight ! He just want clean air. Hell he could leave the wings off but it wouldn't look right.

Lance,

Do a quick google search and type in {inlet spike flow data simulation} and you will quickly find your answer. From what I see, I would have to go with a design similar to the one Hustler58 shows.

Ok, the external shape is fixed (has to be scale). The issue once you get inside the duct is how to taper away the cross section area of the spike without messing up the flow. The problem is that you now have increasing cross section area as the flow goes downstream, and so its going from low pressure, high speed flow, to high pressure low speed flow, and thus it wants to separate. Then the flow gets to the engine. A bunch of the air goes into the compressor, while the rest goes around the engine. BUT, a lot of the duct area is now filled with engine and the net cross section is small again.

So the issue is that you have this area of large cross section area just ahead of the engine, and thats likely to cause flow separation on the centerbody unless the centerbody is really long with a gradual taper.

The way around this is to "area rule" the duct. Namely, put a constriction on the outside of the duct, such that the cross section area stays pretty constant in the area between the inlet throat and the engine intake. As long as the cross section area is constant, the flow can handle some pretty tight bends without separating.

Note, in general, you want ANY afterbody to come to a point, IF the taper is gradual enough for the flow to stay attached. Its the same as a wing trailing edge. However, if you cannot have it that long, then you taper it as much as you reasonably can and then round it off. If the engine has an electric starter in front, you can blend the back of the spike into the front of the starter.

However, our turbines with the centrifugal compressors are pretty tolerant about turbulent inlet air. Chances are that regardless of what you do , the engine will still run. A good duct design will have a bit less drag and a bit more thrust, but it wont be dramatic. (A bad duct like this would totally kill the performance of a ducted fan, since its a low pressure ratio, high mass flow device.)

The attached drawing is just a sketch. I didnt calculate any cross section areas.

One thing to remember.. ANY airplane that uses a movable spike inlet had it designed for supersonic flow. The rules for our low subsonic flow are very different. The designers also had to make the inlet work over a wide range of throttle, altitude, and mach number. So dont just blindly copy what they did!

Bob parks

If anybody knows how to streamline something, It's a toss-up between the Lockheed Skunkworks and Mother Nature. As far as I can tell, the Skunkworks ain't talking, and nature is right in front of you so just look at the body of a fish- or a bird for that matter. Both have cross-sections which are amazingly similar to post #3. Maybe I've oversimplified things a bit, but once again, you're not going supersonic.

LOL they just did, one message above yours!

Thanks everyone, especially Bob. I've got my by-passes made and based on the good advice above, got my rough shape of the spike brought down on the lathe last night. The one thing not done, is the outer lead in ducting from the inlet lip to the start of the by-pass. Based on the info above, I'll bring it in sharper than I would have otherwise, letting it follow the back of the spike in, based on keeping a constant, approximately 20 square inches that it's seeing at the inlet face, and also let the back of the spike flow into the starter bullet on the engine. I've got the engines set about as far forward as I can, to help with balance issues, so I've got allot going on in a short space. The below pic, is a crude pic of what I'll look like. I can get close with this without a massive amount of effort... I don't have the bulge in the ducting around the starter bullet, but could add that later if it seems to have performance issues.

Thanks again all, much appreciated,
Lance

Lance,

If you can make it like that, and its really pretty constant cross section area between the inlet throat and the engine, then it should be fine. These engines are pretty tolerant, as long as you give them halfway decent airflow.

The only issue, which might just be the simplification in the drawing, is that you dont want any convex sharp corners in the duct, they should be smoothed out.

For example, the concave corner between the back of the spike and the starter is fine, but the sharp convex corner on the outer wall where it goes from the taper to the cylinder should be blended.

Bob

You got that right!!!!!!! If that was the one and only Mr. Bob Parks my good friend from San Jose, then I will put QED or rather QEF to anything he said.

That's Bob all right. I love chatting to him, as he always has the most amazing projects on the go.

I contacted him recently for some advice on a suspect CG specified for my Jag, and ended up on the phone for ages listening to stories that from anyone else would have had me doing the "Bull*t" cough - instead I just marvelled once again at how fortunate some of us are to be allowed to see even a small window of what Bob does. I could sit and listen to that stuff all day long.

Gordon

Bob,
Yea, It will be blended much better than my stick figure drawing. I wasn't joking when I said that was done with MS Paint, in about 5 minutes. Mostly just to get across concepts. I've now got my spike/cone part shaped out, and it's ready to take primer. Last night I was just rough cutting out the disks that will be shaped down for the 6 inches that funnel from about 7.5 inches in diamater down to about 5.75. The by-passes turned out really sweet, and flow into Tam's tail pipes just perfect.

I'm kind of progressing on many fronts at once, so I've not gotten one section done yet for an update... engine airflow stuff, nose gear is done, about half done on making the molds for the tip of the nose and tip of the tail.

I measured the unducted version right at a 20% loss, at a total of 42 lbs of thrust. It will be very interesting to measure it after all this work on the ducting.

Thanks again all,
Lance