Aerodynamics and streamlining computations

So, if I have a cylinder of length L and diameter D sticking out into an airstream at, say 210 Knots, what is the most effective way to streamline it?

A streamlined bubble (half teardrop) is convenient, but it also has a lot of skin drag. What is the break-even point? At what size does the blister have the same drag as the cylinder?

A streamlined mast (airfoil cross section) of length L (think pitot mast) also has a drag coefficient. Same question as above.

We all hate the little things that stick out.... Fuel drains, vents and so on. Is it better to leave them alone or to fabricate streamlined fairings for them? I know what the gut feeling is, but is it supported by science?

Aerdynamic Drag Video

vlittle said:

So, if I have a cylinder of length L and diameter D sticking out into an airstream at, say 210 Knots, what is the most effective way to streamline it?

A streamlined bubble (half teardrop) is convenient, but it also has a lot of skin drag. What is the break-even point? At what size does the blister have the same drag as the cylinder?

A streamlined mast (airfoil cross section) of length L (think pitot mast) also has a drag coefficient. Same question as above.

We all hate the little things that stick out.... Fuel drains, vents and so on. Is it better to leave them alone or to fabricate streamlined fairings for them? I know what the gut feeling is, but is it supported by science?

Click to expand...

You may find this video on the science of drag reduction (and other related videos) very informative:

https://www.youtube.com/watch?v=ftq8jTQ8ANE

Skylor

Great Reference book on Drag

I suggest going to this link and download ?Fluid Dynamic Drag? by Hoerner, it is a great book and a classic for aero engineers like me.

https://docs.google.com/file/d/0Bx0MqOfev7dnTnB0eFRIN2JQb0k/edit?pref=2&pli=1

For a cylinder perpendicular to the local flow the CD is 1.2 (assuming Reynolds number of 1x105 (210 mph at 15C, 3500ft and pipe diameter of .5?). If you streamline it in a teardrop without changing the diameter, the CD drops to .06. The base drag is the big contributor due to turbulent airflow and separation, skin friction is almost insignificant.

A properly shaped blister has a CD of .036 but the reference area is higher.

So here are some examples (all at 210 mph, 3500ft, std atmosphere):
1. A .5? diameter cylinder 2 inches long has an un-streamlined drag of .77 lbf, streamlined it is .038 lbf. A blister would be about .092 lbf.
2. A 2? dia cylinder 48? long (gear leg). Un-streamlined drag is 73.6lbf. Streamlined is 3.68 lbf, 20 times more w/o streamlining. Explains why you pick up 10 knots or so with the gear legs streamlined.

There obviously are a lot of other factors such as surface roughness, critical Reynolds number, intersection drag etc. that can be accounted for on a case by case basis.

The book is heavy in theory but has some clear examples. Since it was written in the 1951 (1st edition), there are some great examples that take you back in history, such as adding a blister fuel tank to an Me-109 drops the speed from 320 to 318 mph.

It covers, scoops, radiator drag, struts, vents, sea anchor drag, drag of cars, nacelles, aircraft floats, landing gear, tires, external fuel tanks, bombs, parachutes, airships, antennas, Mercury capsule - virtually everything you can think of!

Take a look and see if it helps you understand how to reduce drag on your plane.

TJ

Those are a couple of great answers. In summary, the easy answer is, it will always be better with a fairing or aero shape because the aero shape will have roughly 5% of the form drag of the cylinder and skin drag will never make up for this difference.

That said, on a practical side, if you had five 1/2 inch diameter 1 inch rods sticking out of your plane, at cruise you would lose about 1mph. If you just used more power and kept the speed the same, you would burn about 0.1gph more, or roughly 45 cents per hour cost. Difficult to justify making little fairings for fuel drains and such, but if you are after every last bit of speed or efficiency, definitely the way to go.

Tim

cylinders and fairings

the old conventional wisdom from airship design days was that a fineness ratio of 3:1 was the best tradeoff between skin friction and pressure drag.
But that was for body of revolution, not a cylinder. Also, it was for constant internal volume, which is different from constant diameter.

But it gives you some idea. For a cylinder, the optimum would be more fineness than that (more chord for a given thickness).

My dear old friend and mentor, R T Jones, used to have a plaque on the wall in his office. It had a 1" stub of 1/8" rod sticking out, and it had a equal-length stump of an NACA 0010 airfoil, with a 1" thickness and 10" chord. The claim was that the two had the same drag.

....as does Vern, I'm sure.

vlittle said:

We all hate the little things that stick out.... Fuel drains, vents and so on. Is it better to leave them alone or to fabricate streamlined fairings for them? I

Click to expand...

Or just buy them over here at JDair.com

The numbers shown are based on 2-D sections. These are a very good approximation for things like landing gear legs, but not so good for short protrusions like temperature probes or tank vents.

See Hoerner again for examples.

Also, streamlining of low aspect ratio protrusions is much less effective when not closely aligned with the local flow. This means very careful measurement of the local flow direction at cruise (or whatever condition you want to optimise for min drag) before fairing installation. Since the absolute drag effects are mostly small, it may not be worth the effort.

However, design to eliminate the protrusion will always give 100% improvement at any flight condition ...

The video was quite good. An airfoil shape is about 1/10 the drag of a cylindrical shape. Further reading says that a 1/2 airfoil (blister) is about 1/5 the drag (not exact, just rules of thumb).

This means that if the x-sectional area of the blister is 5x the cylinder, it's the same drag.

Therefore an airfoil shape (like a pitot mast) over a cylinder is better than a blister over a cylinder because the area is smaller and the Cd is lower.

So the JDAIR design is much better than a blister over a protuberance. Better yet would be a small afterbody fairing on the cylinder(aka fuel vent or temp probe).

QED.

vlittle said:

The video was quite good. An airfoil shape is about 1/10 the drag of a cylindrical shape. Further reading says that a 1/2 airfoil (blister) is about 1/5 the drag (not exact, just rules of thumb).
QED.

Click to expand...

It depends on the airfoil. For a good laminar flow section, its more like 1/30.

I thought the general teardrop ratio was like 2.5:1?

The other thing to consider is the boundary layer on the airplane. Is this protuberance near the front, where the boundary layer is thin and you have some semblence of laminar flow, or is way in the back buried under a thick boundary layer in turbulent flow? If the latter, streamlining may not even matter. Consider rhe example that flush rivets aft of the spar (roughly) really dont buy you a thing on a non laminar section like the RV uses. You could use universal heads and suffer no speed penalty.

Also consider that anything in the prop blast is likely to be turbulent.

OMG

"...I suggest going to this link and download ?Fluid Dynamic Drag? by Hoerner, it is a great book and a classic for aero engineers like me..."

Agreed. Classic light reading, for sure...

A fellow Aero E...

So my contention is that a blister fairing will actually increase drag because its greatly increased cross-sectional area, despite it's lower Cd.

The JdAir fairings have a much lower area, so they are 'better'? A custom after body fairing (essential a wedge on the aft part of the cylinder is 'best'?

Blister fairing:




Vs

http://www.jdair.com

Fairing of a cylinder and Formation Drag Reduction

Figure 50 on page 13-19 of Hoerner's book (https://docs.google.com/file/d/0Bx0M...t?pref=2&pli=1 ) shows the drag reduction due to different types of fairings added behind the cylinder. The best is 1/3 the drag a un-faired cylinder. However a streamlined fairing is 1/8 the drag of the un-faired cylinder.

Under the wing, due to high pressure the boundary layer is attached so a streamlined fairing does have a benefit. If a blister fairing is used then a small one is best to keep the area down. You can compare the drag by using a nominal Cd of .036 for a blister and calculate the area and then multiply the area and Cd and see how it compares to the area of the cylinder times a nominal Cd of .22.

Page 7-15, Figure 25 shows drag reduction due to flying formation. Formation c reduced drag by about 3% for the 2 airplanes in the lead and 37% for the one in trail.

Lots of interesting and good info in this book. -- Now time for me to get back to work!

TJ

Flying again! said:

Page 7-15, Figure 25 shows drag reduction due to flying formation. Formation c reduced drag by about 3% for the 2 airplanes in the lead and 37% for the one in trail.


TJ

Click to expand...

Sorry for the minor thread drift but the trailing AC in a line astern formation usually requires a higher power setting as he is flying in sinking air.

back to drag reduction

Lots of interesting discussion, but let's cut to the chase

Are, say, the JDAir fuel vent and drain fairings better or worse than an unfaired fuel vents and drains?

I have the drain fairings, thinking of adding the vent fairings (Rocket-style vents in wing roots), if only for the cool factor provided they are *no worse* than the unfaired fittings.

RV7A Flyer said:

Lots of interesting discussion, but let's cut to the chase

Are, say, the JDAir fuel vent and drain fairings better or worse than an unfaired fuel vents and drains?

I have the drain fairings, thinking of adding the vent fairings (Rocket-style vents in wing roots), if only for the cool factor provided they are *no worse* than the unfaired fittings.

Click to expand...

But the extra few grams of weight requires extra lift, which adds drag. Maybe your lighter wallet will counteract it.

Seriously, on inspection, the JDAir parts seem to be the best compromise. I have a complete set of blister fairings for the drains, vents and OAT probes on my Rocket. I even have fairings for the rudder cable attachment brackets and for the rudder-fuselage intersection. Even with everything installed there was no measurable improvement in airspeed, but it looked cool.

I can only get 210-212 KTAS (8500' WOT). This actually went down afer painting because my stripes had ridges. I subsequently sanded down these ridges forward of the wing spars and most of the speed returned.

I've followed Tom Martin's secret formulas for drag reduction so there may be nothing left to get by tweaking. The next step may be more horsepower, but that just burns more fuel and I'm looking for efficiency.

Perhaps if I just wash the bugs off....

Have you tackled cooling drag yet, by controlling the airflow through the cowling?

Snowflake said:

Have you tackled cooling drag yet, by controlling the airflow through the cowling?

Click to expand...

Bingo! Excellent question.

Snowflake said:

Have you tackled cooling drag yet, by controlling the airflow through the cowling?

Click to expand...

Quite a bit of mods for cooling drag. Plenum, prop shaft seal, Coanda strip, sealing of the gear leg fairings, oil cooler shutter and perimeter cowl seals. Summer temperatures are as high as I can tolerate so not excess airflow.

Airframe includes close-outs on empennage surfaces, flush skin joints, hinged wingtips and cool decals. Some other stuff that Tom Martin told me but is secret.

Drop by and I'll show you the secrets.

Drag Queen RV 10

I've been lurking on this forum for a couple of years now. I just could not resist this thread for coming out of the closet... so to speak.

All I can offer is it is all relative. You don't know what serious drag is until you fly a float plane or an amphibian with rigid sponsons, domed head rivets, a hull with monumental form drag an engine pylon and cowl on top by itself.

At the other end of the spectrum, compared to a Lancair, even a Rocket is a Drag Queen.

I've been waiting for somebody to put an RV10 on a pair of Aerocet Amphibious floats, but I might be the first. If ever a 10 needed more HP I think this will be the application. Back seats won't be needed as there will not be enough payload remaining to use them. You would really need to enjoy fishing in places where there is simply no other way to get there to justify one of these.