Need help to understand airflow

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

Eugene

Well-Known Member
Joined
May 26, 2017
Messages
1,559
Location
Merrill, Wisconsin, USA
I need you guys to explain something to me. On one hand for any object moving through the liquid ,shape is very important to reduce drag. we know that shape of the front part of this object is equally important as shape of the back side.

On other hand I am hearing this explanation that airflow on the back side of abruptly shaped object will form turbulent Triangle that will work like a bearing and this void will be naturally filled. So, we should stop worrying about building anything for this area?

I believe Peter Garrison at one point was telling me that all this fancy looking Spinners that you guys using on canards complete nonsense.

I also see how they shape fuselage on icon A5 right before propeller. They must have the same logic as far as airflow concern.

when we standing on the bridge and looking down we can see how water is moving around concrete columns. You can clearly see still water forming that kind of area that’s really not moving at all and acting like filler.

so, do I see it correctly? In certain area like prop spinner for pusher or backside of fuselage after body doesn’t need to be perfect?

B812170E-D85B-4D5C-ADBB-2DF9BCFD1619.jpeg
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,502
Location
US
I don't have have the answer, but you'll recall that the "Kammback" tail on some cars was the result of aerodynamic research that indicated that a car with an abruptly cut off tail could have drag fairly close to the ideal streamlined taper, and was a lot more practical for a car (bumpers, widely spaced rear wheels, etc). Maybe there is published research on this, and on the best way to do it (VGs back there? Boat tail?).
For an airplane, most designers try to bring the flow back together along the surfaces of the empennage, nacelles, etc, if possible. But, maybe in your case it isn't possible...
P.S. I agree that thee idea of not worrying about the trailing edge is hard to accept. In wind tunnel tests of the drag of various simple shapes, a tapered aft body is generally more important than the front end of the shape.
 
Last edited:

patrickrio

Well-Known Member
Joined
Aug 15, 2020
Messages
328
I think that this is mostly from research on automobile aerodynamics. They found that it was BETTER to have the full "aero teardrop" shape for aerodynamics, but not practical because of length required. Also, crosswind variation at ground level has more effect on a full teardrop and makes it harder to engineer good lane tracking.

So they have developed techniques over time to minimize the aerodynamic cost of truncating the aero teardrop that also make the car track better.
 
Last edited:

Aesquire

Well-Known Member
Joined
Jul 28, 2014
Messages
2,856
Location
Rochester, NY, USA
The answer to the question "chop it off, or fill in the taper" is complex, because as noted above, there is a lot less drag with the tear drop trailing edge than a cut off one. But you can't always do that, for other practical reasons. A full size van needs to be 2/3 again longer to twice as long to get optimum drag reduction. Not only parking, but injuries from inattentive pedestrians in parking lots make that impractical.

The sharp edge cut off shape is better than a rounded one. the "bubble" behind is fairly symmetrical, while a half circle that fills in the space will generate oscillating vortexes that will pull the shape back & forth and actually make more drag than a flat.

As to prop spinners... If you have a very streamlined shape, like Miller-Bohannon JM-2 Pushy Galore - Wikipedia then it does make a big difference. On a Skyboy with an unfaired engine, probably a waste of effort and not needed weight, ( even a carbon fiber spinner has some mass & all those screws & a backing plate ) and IMHO you can get a lot of drag reduction with a properly faired engine and only then would a spinner be worth while.

Pity, since it's a relatively easy and inexpensive thing, and on a different airplane like a Berkut it's "low hanging fruit". But unfortunately, that branch isn't near the bottom of your drag tree. Looks cool, though, and that may be reason enough. :)
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
3,932
Location
KBJC
My understanding is that it's best to complete the shape, which minimizes drag by minimizing separation.

If you don't have the length available to complete the shape, one option is to taper it more sharply, but this leads to separation and lots of drag. Truncating the shape, as shown in the OP's sketch, leads to less drag than a teardrop that converges too abruptly.

In the case of those "pressure recovery" style pusher spinners, you will notice that many of them have the very tip (maybe the last half inch?) truncated. This leads to cleaner separation than the rounded tip you would otherwise have. It's the same principle used when the last 1% of an airfoil is clipped off. You want the geometry to help support the Kutta condition, but the material used doesn't lend itself to a razor trailing edge (not to mention the everyday practicalities of such an edge).
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
15,069
Location
Port Townsend WA
If a multi-bladed, high pitch fan is placed at the chop off it might help to round off the edge for fan inflow.
The fan changes the design. Might need to spin the fan with more rpm than the prop?
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
14,976
Location
Memphis, TN
If you look at a Piaggio it has long spinners. Probably way to long to be practical for most homebuiders. The spinner is an extension of the nacelle so the shape should be following that. As for what you can do, I would get a long pointy one that is off the shelf, and move to another task.
 

ypsilon

Well-Known Member
Joined
Mar 1, 2016
Messages
296
Location
Austria
“Big whirls have little whirls,
That feed on their velocity;
And little whirls have lesser whirls,
And so on to viscosity.”


Each little whirl you drew feeds from energy, that has to be put into the system from outside.

A streamlined back will always be better than filling the void with eddying. If you cannot have a streamlined trailing edge for practical reasons, there are still ways to reduce the drag. Just search for "bluff body aerodynamics". There is a lot of research going on in the field.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,041
Location
Saline Michigan
My understanding is that it's best to complete the shape, which minimizes drag by minimizing separation.

If you don't have the length available to complete the shape, one option is to taper it more sharply, but this leads to separation and lots of drag. Truncating the shape, as shown in the OP's sketch, leads to less drag than a teardrop that converges too abruptly.

In the case of those "pressure recovery" style pusher spinners, you will notice that many of them have the very tip (maybe the last half inch?) truncated. This leads to cleaner separation than the rounded tip you would otherwise have. It's the same principle used when the last 1% of an airfoil is clipped off. You want the geometry to help support the Kutta condition, but the material used doesn't lend itself to a razor trailing edge (not to mention the everyday practicalities of such an edge).
Thatone
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,041
Location
Saline Michigan
In the case of the uncowled pusher, the engine has a churned and slowed down wake as large as the uncowled shape fed into the prop. That case is pretty awful for both drag and for conversion of power to thrust. The spinner might help straighten some of the flow and help the prop be more efficient, but I suspect its effect will be lost in all of the other mess. Imagine two regimes of flow through the prop: The inner one of slow and churned up air and the outer one that is close to free stream air speed. The prop ends up operating at too little AOA outboard and too much AOA inboard.

In the case of a cowled pusher, where most of the air will be smoothly flowed around the cowling while a modest fraction of the flow will be go into an expanding diffuser (reducing velocity), through the HX's, over the engine, through a contracting outlet and back up to something resembling flight air speed, before being dumped into the prop. The cowling will make for a smaller and much faster inner regime with less difference between it and the free stream flow, making for a more efficient prop and more power expressed to drive the airplane. I expect that the spinner with a smoothly rounded "back" plate (it fairs the front end of the air encountering the prop hub) will make for air smoother and faster into the inner end of prop. Once you have that smoothly rounded "back" plate (it is forward of the prop in a pusher), you might as well have a nicely shaped spinner to keep the flow smooth.

As the air exits the prop, the spinner is then a contraction in the prop outflow. Know this: the outflow from the prop is sped up significantly from free stream, so the spinner shape does matter. Running with a spinner does make for more efficiency, but the shape is always a compromise. A smooth convex curve works great on a tractor prop, keeps surface area down, but it does little for pressure recovery. Sexy double curved pressure recovery spinners add a little more area and I presume weight too, but buy you some thrust. I do not know which way benefits the soon-to-be-cowled and aerodynamically cleaned up EugeneMachine the most. My money is on a spinner with a rounded front plate being significantly beneficial, I just do not know which spinner will be most beneficial.

Billski
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,502
Location
US
Eugene,
Another option (before you mount your new spinner and cowl):
1) Behind the prop, use some brackets to mount a large mixing bowl or even a small snow saucer (open end into the breeze). A bit like a rigid parachute.
2) Make a cowl consisting of 20 plastic wiffle bats protruding from around the engine and into the breeze (like a porcupine).
3) Fly the plane and take a video showing your tachometer and max airspeed.
4) Remove the snow saucer and wiffle bats.
5) Fit a nice looking cowl and your new handsome spinner. Fly the plane and get a video of the new higher max airspeed.

Now, you are done and ready for the story swapping. "Yeah, I did a lot of reading, testing, and cleanup on the engine pod drag. It made a big difference. Over the course of the program I gained 20 knots and decreased my fuel flow in cruise by 20 percent. Here's the video. The modifications paid for themselves in one year of flying.' etc, etc.😉
 

drgondog

Active Member
Joined
Sep 24, 2015
Messages
32
Location
Scurry, TX/USA
I don't have have the answer, but you'll recall that the "Kammback" tail on some cars was the result of aerodynamic research that indicated that a car with an abruptly cut off tail could have drag fairly close to the ideal streamlined taper, and was a lot more practical for a car (bumpers, widely spaced rear wheels, etc). Maybe there is published research on this, and on the best way to do it (VGs back there? Boat tail?).
For an airplane, most designers try to bring the flow back together along the surfaces of the empennage, nacelles, etc, if possible. But, maybe in your case it isn't possible...
P.S. I agree that thee idea of not worrying about the trailing edge is hard to accept. In wind tunnel tests of the drag of various simple shapes, a tapered aft body is generally more important than the front end of the shape.
I would disagree the last statement. One of the most important design considerations forward of max cross sectional area of an aircraft is to minimize the velocity gradient from spinner to Max Cross section. The Mustang had three monstrous drag advantages built in to the design: 1.) Conical section rule for fuselage cross sections moving aft, 2.) Low Drag NAA-NACA 45-100 airfoil with low velocity gradient and small area cross-section of wing/BL leading up to 37.5% (T/C Max) and aft - which also increased Mcr, and 3.) Meredith Effect which was finally optimized on the XP-51G/P-51H.

FWIIW a salmon/trout cross section is superior low drag profile to teardrop. Look to NAA-NACA 45-100 Low Drag wing and compare to other Clark Y, NACA 23016, etc conventional airfoils during WWII.
 
Last edited:

patrickrio

Well-Known Member
Joined
Aug 15, 2020
Messages
328
FWIIW a salmon/trout cross section is superior low drag profile to teardrop. Look to NAA-NACA 45-100 Low Drag wing and compare to other Clark Y, NACA 23016, etc conventional airfoils during WWII.
Isn't this because of variation in necessary height vs width of the fuselage? I think that if required height and width were exactly the same at the same datum measure location, the resulting best aero taper from that location would be to a point not a line -- I think.

I think that the salmon shape is best if required height is greater than required width? Or if you need to create thrust with a tail surface and also have optimal forward and lateral vision????
 

Dan Thomas

Well-Known Member
Joined
Sep 17, 2008
Messages
6,048
Each little whirl you drew feeds from energy, that has to be put into the system from outside.
Yup. All I know is that turbulence represents lost energy. We want to minimize turbulence, so airplanes are pointier at the back than at the front. We never see an airplane with a chopped-off aft end.

Highway trucks sometimes get modified to reduce that drag. What Are Those Odd Panels on the Backs of Trucks?

1623285616057.png

1623285636718.png
 

Eugene

Well-Known Member
Joined
May 26, 2017
Messages
1,559
Location
Merrill, Wisconsin, USA
Yup. All I know is that turbulence represents lost energy. We want to minimize turbulence, so airplanes are pointier at the back than at the front. We never see an airplane with a chopped-off aft end.

Highway trucks sometimes get modified to reduce that drag. What Are Those Odd Panels on the Backs of Trucks?

View attachment 111560

View attachment 111561
One of my customers is trucking company owner. We talk about this couple of times. He claims that benefit of this "improvement" is so minimal that is very hard to measure
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
8,041
Location
Saline Michigan
One of my customers is trucking company owner. We talk about this couple of times. He claims that benefit of this "improvement" is so minimal that is very hard to measure
But look at it. The transition from a straight top and sides to the tail is a sharp slope change, inducing separation. Then you only get a modest reduction in area before it ends. Usually, if you are going to clip off the tapering afterbody, you do 80-90% of the taper before clipping it off, not after doing around 50% of the reduction, like on these trailers. Even then, I would bet that they get way more aero drag from driving the tractor and all those tires through the air. You can not do anything about all that frontal area, but you can smooth out the front end to get lower Cd, fair in the wheels on tractor and trailer to keep drag from them down, and then do something off the back end.

I know this about tractor-trailer rigs. Driving our pickup truck (2006 F150 with a cap on the box), we can feel the difference in the amount of pull we get behind trailers with a plain back end and no skirt and those with the tail and side skirts. We can feel the draft much further back with a plain trailer. We don't notice it so much with the C-Max and hardly notice it at all with Focus, but in that pickup, you can tell.

Billski
 
Last edited:
Top