F-8 Crusader Inlet

KJ_Lesnick

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Was the F-8's inlet a single-shock inlet, or a multi-shock inlet?

KJ Lesnick
 

shockonlip

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KJ_Lesnick said:
Was the F-8's inlet a single-shock inlet, or a multi-shock inlet?

KJ Lesnick

I vote for single shock if you don't count the normal shock.
Or two shocks counting the normal shock.

The answer depends on what the freestream Mach number is.

It also depends on what the half-angle measurement of the F-8's nose
spike is in degrees, the bottom half of the spike actually. If you go
look at some nice hi-res F-8 nose shots, you will see a definite
shaping of the bottom half of the inlet spike for this purpose.

So given that I don't really know what the answers to the above
questions are, and given that I have a Taylor-Maccol solver on my
computer (TM is a algorithm that solves for supersonic flow over
cones).

I did a quick lookup on the Wicki page for the F-8 which
indicated Mach 1.2 (intial versions I guess).

I then played with different F-8 inlet spike half angle measurements
with my TM solver.

What I found is a solution with a freestream Mach of 1.2, with a
cone half-angle of 15 deg, which gives you a downstream mach
number of .98, which means after one cone shock due to a 15 deg.
half angle cone at Mach 1.2, the flow is subsonic downstream of
the single shock at Mach .98.

The cone shock angle or theta-s, is ~61 deg for freestream Mach at 1.2
with a cone half-angle of 15 deg.

That means that the cone shock off the F-8's spike tip at ~61 deg.,
should encounter the F-8's chin inlet lip (this would be full capture)
at Mach 1.2 top speed.

If that is true then the normal shock is sitting right at the F-8's cowl
lip.

I don't know if my estimates are correct for F-8 inlet spike half-angle,
and it doesn't really matter, but it illustrates how much fun it is to
play with this stuff.

I think it also indicates that the F-8's inlet is one cone shock and one
normal shock, probably at the inlet lip.

You can also do some exploration looking at later F-8 models as well.

Say the F-8E, which wiki says is Mach 1.86 capable. At Mach 1,86
a cone half-angle of 35 deg does roughly the same thing, namely, the final
mach no. downstream of the cone shock is .96. Theta-s is 58 deg, so
still close to 61 deg so it would roughly also be at shock on lip at M1.86
freestream.

So again, what we're theorizing, is that even though we may be wrong
about the cone angles, what Vought did, is for the faster F-8's, they
put a fatter inlet spike to handle shocking down a higher freestream
mach no. to subsonic at the F-8's inlet lip.

So this means you should be able to tell a difference between XF8U-1 say
and an F-8E. Go look. I think you will see skinnier cones on the earlier
models and fatter cones on the later faster models.

Now you know one reason why. The fatter cone could also hold a larger
radar I guess.

It would be fun some day and go into a museum and measure the actual
half-angles for different models of F-8. Or get some nice side shots of F-8
inlet spikes and try measuring off those.

Larry
 

KJ_Lesnick

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Shockonlip,

I vote for single shock if you don't count the normal shock.
Or two shocks counting the normal shock.

The normal shock counts, so it's a twin-shock inlet...


I got some questions

- Regarding the nose-cone diameter: Since you are unsure of the exact diameter of the nose cone, can you guesstimate a range of degrees the nose would be (in other words can you figure out how far off you could be?)? If it's not too much to ask that is...

- Did you factor in the fact that the inlet is wider than it is tall? Does this have any adverse or beneficial effects on your predictions?


KJ Lesnick
(I'm trying not to sponge or anything -- this isn't exactly information you can just find with a google search)
 

shockonlip

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sferrin said:
Now do the XF8U-3. ;)

OK, I will work some numbers on it.

But up front it is a 3-shock inlet.

An external cone shock formed by the spike, an internal oblique shock
formed by the sugar scoop, and then a normal shock which is inside
the inlet in the throat. So the XF8U-3 is a mixed compression inlet.
The XF8U-3 also has the side bypass doors to match the flow and it also
has the sugar scoop cut into the inlet sides which helps the normal
shock from being dislodged outside the inlet on a pressure build up
downstream of the inlet throat due to like a J75 transient.

I'll get my protractor out tomorrow and see if I can measure some angles.
 

shockonlip

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KJ_Lesnick said:
Shockonlip,

I vote for single shock if you don't count the normal shock.
Or two shocks counting the normal shock.

The normal shock counts, so it's a twin-shock inlet...


I got some questions

- Regarding the nose-cone diameter: Since you are unsure of the exact diameter of the nose cone, can you guesstimate a range of degrees the nose would be (in other words can you figure out how far off you could be?)? If it's not too much to ask that is...

- Did you factor in the fact that the inlet is wider than it is tall? Does this have any adverse or beneficial effects on your predictions?


KJ Lesnick
(I'm trying not to sponge or anything -- this isn't exactly information you can just find with a google search)

Indeed a twin or two shock inlet.

Since the compression is all outside here, it would then be considered an
external compression inlet.

I suspect there is a duct divergence or subsonic diffuser, somewhere inside
as well before the J75 engine face, which would create a subsonic pressure
rise and reduce the mach no before the engine face. But I don't know for sure.

It's not the diameter, but the half angle of the cone. Or more appropriately,
the lower half angle which is actually the compression surface for the inlet.

You are correct, I don't know any actual half-angle numbers. The idea was
to play around with half-angles and see if an external compression inlet is
feasable. It is.

No I didn't do any vertical width versus horizontal width compensation. I don't
think that is necessary for our purposes. Actually you only asked how many
shocks and I think we answered that.

The idea was just to figure a right angle cone and get a rough estimate. Some
of the actual cones for some of the models appear to be somewhat ogival as
well, again no compensation for that.

Using a cone of a close half-angle should be close enough for our purposes.

I will dig up a good F-8 side photo (maybe several models) and see if I can measure
any lower half-angles. If you want to try to do that as well, I can try your numbers.

The numbers don't need to be exact.
 

Tailspin Turtle

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F8U-3 Inlet Study

See:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19710064978_1971064978.pdf

Note the boundary layer removal slots in the pictures. (Ignore the missing access panel in the one with a flight test mechanic.)
 

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KJ_Lesnick

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shockonlip,
Indeed a twin or two shock inlet.

Since the compression is all outside here, it would then be considered an
external compression inlet.

Really? I would have never known that. What advantages does an external compression inlet have over an internal compression inlet or a combination of internal and external compression?

No I didn't do any vertical width versus horizontal width compensation. I don't
think that is necessary for our purposes. Actually you only asked how many
shocks and I think we answered that.

Still it doesn't seem to be all that crazy a question when you think about it.


While I'm at it: I was looking at a picture of an F-8's inlet duct and I noticed some things. The nose-cone seems to continue deep inside the inlet-duct. I'm not sure if the shape of the duct is straight at the entry to the duct or slightly divergent. Regardless it narrows down a bit down the length of the fuselage. I'm wondering if anybody has any diagrams of the inlet duct of the F-8 assuming it's not classified?

An inlet with a divergent entry generally suggests the airflow is subsonic past the inlet leading edge but there are exceptions to this rule. The XB-70's outer inlet wall does look divergent. However when you consider that the inlet shortens considerably and the shape of that wedge-splitter in the middle -- the area of the duct actually does progressively narrow as you go deep inside the duct. The point I'm getting at is that the change in duct area (the whole package) is also highly important. With the cone going deep inside the inlet I'm wondering if the straight or even divergent shape of the duct is more than compensated by the cone?

I'm just curious because past the "chin", the duct does seem to narrow down a bit as it goes down the length of the fuselage it would appear. Now granted the duct could be getting taller in height and such which would negate the narrowing in width (and this odd duct shaping could be done for a number of things including area ruling) but I'm not sure as I don't know what the inside of the inlet looks like...


KJ Lesnick
 

Tailspin Turtle

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F8U-3 Inlet Duct

Shockonlip: "I suspect there is a duct divergence or subsonic diffuser, somewhere inside
as well before the J75 engine face, which would create a subsonic pressure
rise and reduce the mach no before the engine face. But I don't know for sure."
 

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shockonlip

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I've been too busy to post in the past few weeks.

Thanks TT for posting the excellent support pix and documentation on the XF8U-3
inlet and the NASA paper on similar inlets. I want to fully digest those when I get a
moment. I realy enjoy this stuff !

Hadn't even noticed (hard to see) the BL bleed on the XF8U-3 spike before. Of coarse
it makes sense to be there. I'm also wondering if it has something to do with the normal
shock sensing/control.

Thanks again!!

Can't wait to see your book!

Larry
 

shockonlip

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KJ_Lesnick said:
Shockonlip,

I vote for single shock if you don't count the normal shock.
Or two shocks counting the normal shock.

The normal shock counts, so it's a twin-shock inlet...


I got some questions

- Regarding the nose-cone diameter: Since you are unsure of the exact diameter of the nose cone, can you guesstimate a range of degrees the nose would be (in other words can you figure out how far off you could be?)? If it's not too much to ask that is...

- Did you factor in the fact that the inlet is wider than it is tall? Does this have any adverse or beneficial effects on your predictions?


KJ Lesnick
(I'm trying not to sponge or anything -- this isn't exactly information you can just find with a google search)

Yes, the way to approach it, when one doesn't know the exact number, is to do your best measuring the spike
angle from the best side view you can find, and then assume a range of some number of degrees (maybe 2-3deg)
above or below and then work the numbers. That's all you can really do.

One thing that I really think is neat is the shape of the F-8's spike. You can see a lot if you look at it
closely. I have included several pix I got off the net. Credit and copyright to the people named on the pix.
If you look at photo1 and photo2 attached here, you can see several interesting things:

1. In photo1, if you look at the angle from the tip, of the lower half of the spike, it is much larger than the
angle of the top part of the spike (on photo1, imagine a horizontal line from the pitot tube to the spike
tip that you use to delineate the top part of the spike from the bottom - then look at the different angles
between the top and bottom).

photo2 lets you see the angle from the tip, of the side of the spike. That also is a smaller angle than the
bottom of the spike. I believe the reason for this is that the cone shock off the side and top of the spike
is desired to be smaller and weaker to reduce the wave drag (ie: as air flows through a shock it slows
down - the weaker the shock the smaller the amount of air slow down) over the top and sides of the
fuselage.

2. The shock over the bottom of the spike is just for the inlet. So it will be picked so as the cause a normal
shock at the entrance of the cowl lip.

I believe the above is also what causes the 'smile' you talk about or the cause of the wider than taller inlet cowl.
 

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shockonlip

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KJ_Lesnick said:
shockonlip,
Indeed a twin or two shock inlet.

Since the compression is all outside here, it would then be considered an
external compression inlet.

Really? I would have never known that. What advantages does an external compression inlet have over an internal compression inlet or a combination of internal and external compression?

No I didn't do any vertical width versus horizontal width compensation. I don't
think that is necessary for our purposes. Actually you only asked how many
shocks and I think we answered that.

Still it doesn't seem to be all that crazy a question when you think about it.


While I'm at it: I was looking at a picture of an F-8's inlet duct and I noticed some things. The nose-cone seems to continue deep inside the inlet-duct. I'm not sure if the shape of the duct is straight at the entry to the duct or slightly divergent. Regardless it narrows down a bit down the length of the fuselage. I'm wondering if anybody has any diagrams of the inlet duct of the F-8 assuming it's not classified?

An inlet with a divergent entry generally suggests the airflow is subsonic past the inlet leading edge but there are exceptions to this rule. The XB-70's outer inlet wall does look divergent. However when you consider that the inlet shortens considerably and the shape of that wedge-splitter in the middle -- the area of the duct actually does progressively narrow as you go deep inside the duct. The point I'm getting at is that the change in duct area (the whole package) is also highly important. With the cone going deep inside the inlet I'm wondering if the straight or even divergent shape of the duct is more than compensated by the cone?

I'm just curious because past the "chin", the duct does seem to narrow down a bit as it goes down the length of the fuselage it would appear. Now granted the duct could be getting taller in height and such which would negate the narrowing in width (and this odd duct shaping could be done for a number of things including area ruling) but I'm not sure as I don't know what the inside of the inlet looks like...


KJ Lesnick

I was able to find a shot that I think we both wanted to see - see photo3 attached.
Credit and copyright to the indicated person on the photo.

It still looks pretty straight to me (although we have the tunnel effect going on where
distances narrow). We do have the step up though at the end of the duct to match
the compressor face (which can be actually seen in the distance). I suspect that there
is a widening there as well, similar to the XF8U-3 diagram that Mr. TT shared with us.

I want to say that I don't see any obvious changes in area until the inlet duct goes up.
What do you think?

So I suspect that the flow is high subsonic in this part of the duct. The drag due to the
skin friction on the walls will take some of that speed away.

Then I suspect we will encounter a subsonic diffuser when the duct goes up and the
speed will reduce and static pressure go up to the proper speed and pressure for the J57's
compressor face.
 

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KJ_Lesnick

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I don't know if this could physically happen, but what would happen if the shock off the nose went into the duct?

Kendra
 

sferrin

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KJ_Lesnick said:
I don't know if this could physically happen, but what would happen if the shock off the nose went into the duct?

Kendra

A 40mm cannon would go off beneath the pilot. ;)
 

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KJ_Lesnick said:
SFerrin,

So it would be quite a loud bang eh?

According to the test pilots it was a very loud bang. as one said it "unleashed with the sound and fury of a 40 mm cannon going off under the seat".
 

sferrin

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KJ_Lesnick said:
Did they specify what mach number this happened at?

I doubt it was as simple as a specific Mach number. There were probably several variables.
 

KJ_Lesnick

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I'm wondering what affect angle of attack has on the shock-wave angles...

I don't know if it would be all that much (a small fraction of a mach number) but the sharpness of a cone to an extent affects the angle of the shockwave a bit and at a positive angle of attack the cone would "seem" to the airflow as being a little sharper/flatter on the top and blunter/deeper on the bottom. Granted the shape of the lip is unswept...


KJ Lesnick
 

KJ_Lesnick

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I just wanted to bring something up...

I was reading about the F-11F-1 and according to what was said about it, it could reach at least Mach 2 and altitudes of over 80,000 feet in testing. What I'm wondering is, how is it that the F8U Crusader outperformed it, yet was slower...

Honestly, I'm kind of confused here especially if a program which actually shows shockwave angles specifically limits the F8U to Mach 1.85


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Tailspin Turtle

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KJ_Lesnick said:
I just wanted to bring something up...

I was reading about the F-11F-1 and according to what was said about it, it could reach at least Mach 2 and altitudes of over 80,000 feet in testing. What I'm wondering is, how is it that the F8U Crusader outperformed it, yet was slower...

Honestly, I'm kind of confused here especially if a program which actually shows shockwave angles specifically limits the F8U to Mach 1.85


KJ Lesnick

That Tiger was the F11F-1F with the J79 engine. Only two were built, with the Navy wanting some inflight experience with the J79 and Grumman wanting better performance for the F11F for international sales and hoping for the F8U to disappoint. As it turned out, the F11F-1F was too little and too late.
 

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