Pusher cowling ideas

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WINGITIS

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I was hoping to find exact dimensions like this for pressure recovery spinner that I am building

View attachment 115337
The Spreadsheet is simple, just select the sheet with the type of spinner you want, such as that OGIVE, then change the base sizes, diameter, length etc in the YELLOW cells and it generates the build dimensions in the columns to the left....

You know the diameter you will need, you can choose the length/diameter ratio off some existing plane that you think is good and adjust the rest.

Here is an example I just did...10 x 14 Inches.

OGIVE SAMPLE.png
 
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WINGITIS

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In terms of proportions here are some guides, you can also treat your RECOVERY type as just multiple cones, if that is what you want.

There are MANY STUDIES to read up on or just go for one that is proven.

Because as you can see from this data, you are chasing VERY SMALL margins of improvement......

SPINNER EXAMPLE 1.pngSPINNER EXAMPLE 2.png
 
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WINGITIS

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I understand. Very small difference between different shapes as far as performance go. I will stick with what I got and whatever looks good to me. Thank you!
Indeed what looks good and whatever can match the shape of your engine cover flow wise...

That Ogive does a fairly good job though...

tempImageNO2KltUD.png
 

Eugene

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  • First is that as long as your section is bigger than zero and you are more than a couple percent aft of the stagnation point, your velocity adjacent to the surface is higher than free stream;
  • Second is that as long as the section thickness is decreasing, the velocity is decreasing, and that means the pressure is increasing.

My god, this pressure subject does increased my blood pressure and my head is spinning big time! I used to feel so smart about myself??!! Where did that confidence go???

main-qimg-2519c707730b5240f9e3ac98d773f7f1.jpeg

We have said before that first half of the shape would make a perfect tractor spinner and second part would make into pusher spinner. So, first half will be in low pressure, except the very front part? And back part part will be in positive pressure?
 

Eugene

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Indeed what looks good and whatever can match the shape of your engine cover flow wise...

That Ogive does a fairly good job though...

View attachment 115350
I agree it looks pretty good. At 1:1.4 ratio and 14 inch diameter spinner will get 19.6 inches long. I was planning to end my project at about 22" or so. Maybe to go this road will be a little bit more practical. I will think about it. Thank you!

IMG_6052.jpeg
 

wsimpso1

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My god, this pressure subject does increased my blood pressure and my head is spinning big time! I used to feel so smart about myself??!! Where did that confidence go???

View attachment 115349

We have said before that first half of the shape would make a perfect tractor spinner and second part would make into pusher spinner. So, first half will be in low pressure, except the very front part? And back part part will be in positive pressure?
Eugene,

I keep trying to get this message through, but from the way you explain it back to us, I do not think I am getting through. So I will try another way.

This stuff I keep writing to you is all for modest AOA and comes right out of TOWS. It is real and scientifically supported for about a century.

At the stagnation point, which is the very point where the flow divides to go up or down over the foil, velocity is theoretically zero and pressure is equal to free stream q. this region is so small, it can be difficult to measure.

The velocity increases rapidly as we slide aft from the stagnation point, and local pressure against the surface goes exactly opposite of local velocity. As you get Into the aft end of the foil, velocities are decreasing towards free stream velocity, and pressure, while remaining below ambient does head that way. Velocity stays above free stream until you get behind the wing, and pressure stays below ambient pressure until there too. You can see this right off the plots in Appendix I of TOWS. We have known this for a century.

Yes, the forward part of a fat airfoil will make for a pretty good fuselage with a tractor prop spinner taken from the very front. Yes, the aft part would make a pretty good pusher spinner. A fair shape that looks good should work fine. When I say a fair shape, I mean from engine cowling to off the back end. Look at it from the side and from above. Smooth flow from cowling off the spinner Is the objective. Abrupt shape changes are bad.

The folks that race and pursue speed records with pushers have favored “tangent” or pressure recovery spinner shapes with a square cut end rather than have it go all the way to a sharp point. Dr Kamm taught us a long time ago that the losses from the cut off can be less than the losses from having the long tail.

Billski
 
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Eugene

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This stuff I keep writing to you is all for modest AOA



The velocity increases rapidly as we slide aft from the stagnation point, and local pressure against the surface goes exactly opposite of local velocity. As you get Into the aft end of the foil, velocities are decreasing towards free stream velocity, and pressure, while remaining below ambient does head that way. Velocity stays above free stream until you get behind the wing, and pressure stays below ambient pressure until there too.

Yes, the forward part of a fat airfoil will make for a pretty good fuselage with a tractor prop spinner taken from the very front. Yes, the aft part would make a pretty good pusher spinner. A fair shape that looks good should work fine. When I say a fair shape, I mean from engine cowling to off the back end. Look at it from the side and from above. Smooth flow from cowling off the spinner Is the objective. Abrupt shape changes are bad.

The folks that race and pursue speed records with pushers have favored “tangent” or pressure recovery spinner shapes with a square cut end rather than have it go all the way to a sharp point. Dr Kamm taught us a long time ago that the losses from the cut off can be less than the losses from having the long tail.

Billski

I am trying very hard to be smart about this. But something doesn't click. I am not doing this on purpose. I promise

main-qimg-2519c707730b5240f9e3ac98d773f7f1.jpeg

OK, this airfoil look almost symmetrical to me. And looks like we will have low pressure everywhere on top and everywhere on the bottom. Because velocity will be higher and as a result pressure will be low.

main-qimg-2519c707730b5240f9e3ac98d773f7f1 2.jpeg

So, we said that we can use this shape as backside of fuselage or spinner for pusher propeller. And pressure will be low?

If this guys decided to make fat tail boom, it will become low pressure tail? But because they decided to make it skinny, it is high-pressure tail?

tempImageKjzpGM.pngScreen Shot 2019-09-27 at 21.24.40.png

If I made my tail boom like a picture of the law, it should become high-pressure tail boom. And airflow will stay attached. But airflow can stay attached on high-pressure or low-pressure areas. So, what difference does it make?

IMG_4250.jpeg
 

mcrae0104

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View attachment 115448

OK, this airfoil look almost symmetrical to me. And looks like we will have low pressure everywhere on top and everywhere on the bottom. Because velocity will be higher and as a result pressure will be low.
Yes, it is very nearly symmetrical, and it will have relatively low pressure over almost the entire top and bottom surface. Consider, though, that the camber line (in red, below) is positive, which indicates that the curvature over the top is greater than the curvature over the bottom. So while it is true that the bottom has a convex curve, and produces a "lifting" force (in the downward direction), the top creates a greater "lifting" force in the upward direction.

camber.jpg
 

WINGITIS

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If your spinner is in separated flow, you can pretty safely make it any shape you want! 😁
Agreed, I am just not sure what HOT AIR flow is coming out the back of the vents around the spinner, can we assume they calculated all that at Cessna or not?

There used to be one flying from my local club airport in the Seventies, it did make a lot of noise from that rear engine on climb out....
 

Eugene

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Here is a real life airflow question. I don’t know if you can see this packet that I show in red on the picture, but seems to me that there will be flow direction change. My guess that I will need to fill this packet so air is flowing smoothly.

0901B5C4-C470-49D0-99ED-122648A80FC2.jpegBD06BB84-9DA3-445B-982A-A08FFA9B50D9.jpegFB854362-5881-41B9-9422-CBF2EB9E266C.jpegE0BAF5EC-2D8F-4AC4-B08C-8ABE68FAE635.jpeg83A6DCB8-99A3-4778-90BD-C2F36028DA49.jpeg691765E1-E7E1-40B1-9CBD-F9D3FAC432BF.jpegA809F330-F7E9-4F96-B48D-17E8E680FAE8.jpegF62F9ED4-C6B8-4FB4-BC0F-CB230A87F8DE.jpegFD9772AF-0655-442A-83D7-685616E6D7D0.jpeg167D0E18-84FF-49E7-AD5A-32E054DF2517.jpeg
 

wsimpso1

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Indeed what looks good and whatever can match the shape of your engine cover flow wise...

That Ogive does a fairly good job though...

View attachment 115350
If you were able to look at how the air all the way around the cowl flowed to and then over the spinner, you could pick an ideal shape of the cowl and spinner together. That is really what you want to happen here, is have a smooth flow without separation over the cowl and spinner. If it were mine, I would avoid pulling the flow in and then running it nearly straight, then tapering it again. Since we can not avoid some sort of taper to a point or nearly to a point, I fill in the reversed curve there at the aft end of the cowling, and then do the pressure recovery spinner.
 
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