My friend sent this to me recently. A pretty interesting, although old, look at drag on an aircraft.

That's a great video. The only other topic that I wish he would have shown is that drag is proportional to the square of velocity.

It always fascinates me that round has about the worst drag coefficient of any commonly used shape. It really makes me hate my round nose strut!

That was great! Really makes you appreciate the value of fairings on gear legs.

Zane wrote:That was great! Really makes you appreciate the value of fairings on gear legs.

In an inverse way, I have used this round tube drag thing to help heat my shop.

I have a large homemade wood boiler (burns clean, with NO smoke) for my radiant floor heat system in my shop, hangar and house, backup to the solar hot water system. During initial startup, I use a small fan that's rigged to the combustion air intake, this really kicks it in the ass and makes starting a fire super easy, like blowing on twigs when starting a campfire.

But I found that if I run the fan for more then a few minutes, the stove flue temps get way too high, (using an internal flue thermometer) so I have another fan that kicks on automatically that cools off the stack and blows it around the shop. Experimentation showed that keeping it simple was the best way: no ducting around the 8" stove pipe was needed, just the airflow hitting the round pipe, somewhat counter intuitively, was the most effcient way to remove the excess heat. Something called the Coanada effect http://en.wikipedia.org/wiki/Coand%C4%83_effect is responsible for the air hitting the pipe and then, at least at the speeds involved here, clinging to the pipe all the way around it, thus effectively acting a pretty good heat exchanger. In other words, what makes a round tube so draggy, works to my advantage in this situation. I was going to duct the airflow around the pipe, in an effort to get it to stay attached, it wasn't needed.

My new cub style landing gear on The S-7S, since I'm not trying to cool it off has every round tube, including the cabane V, faired. The results after fairing were dramatic, well worth the small effort involved. Reducing drag, like reducing weight, works everytime it's tried!

Tom - Working on Joel to make some removable FG fairings as I can't seem to get to it. Think he's on board. I've got some of this fairing on the way for the cabane. Got a great deal.

http://streamline.8k.com/FAIRINGS.htm

I replaced the lift struts on my Maule with sealed struts. A friend had an ultralite with round tubing for landing gear and some other supports. We cut the struts and fitted them over the round struts and supports, and even on the slow speed of this craft, he was able to see about 5 MPH increase in airspeed.

Gene

That is great. While watching it, I had flashbacks of my Intro to Aerodynamics course from college. The instructor for that course has us watch a whole series of these movies for his class. The library still had them on film when we watched them. There were some others that were pretty amazing as well.

I wondered what the drag would be from the round tubing on the Alaskan Bushwheels (ABW) heavy duty gear legs and Goodyear 8.50x6 tires on my Maule MX-7-180 so I did some research and analysis. The results indicate the amount of drag in terms of pounds of force, but not in terms of loss of airspeed or increased fuel consumption associated with overcoming the drag.

The formula for calculating drag is:
Drag Force (Newtons) = ½ * air density (kg/cubic meter) * frontal area of the surface (square meters) * coefficient of drag * square of the velocity (meters/second)

Assume the following:
Air density = 1.23 at + 15C temperature
Frontal area of the ABW gear legs = 2” * 36” or 0.05 square meters
Frontal area of Goodyear 8.50x6 tires = 8.5” * 20” or 0.11 square meters
Coefficient of drag for a sphere = 0.47
Velocity at 100 kts (115 mph) = 51.44 meters/second
Velocity at 83 kts (95 mph) = 42.70 meters/second

Drag factors at 100 kts (115 mph):
2 ABW gear legs = 70.77 Newtons or 15.91 pounds of force
2 Goodyear 8.50x6 tires = 167.09 Newtons or 37.56 pounds of force

Drag factors at 83 kts (95 mph):
2 ABW gear legs = 48.75 Newtons or 10.96 pounds of force
2 Goodyear 8.50x6 tires = 115.11 Newtons or 25.88 pounds of force

These are very approximate numbers. The surface area measurements are not precise. The shape of the ABW gear legs is complicated by the airflow around the tubes behind the front tube, which increases the coefficient of drag. The shape of the tires is not a sphere, which decreases the coefficient of drag slightly. Altitude, temperature and water vapor affect air density but this analysis assumes sea level altitude and ignores water vapor. Higher altitudes and more water vapor are associated with less air density and decreased drag.

Drag increases as the square of airspeed, so the largest effect is at cruise speed where the engine must produce much more power to overcome the drag force and maintain airspeed. During lower airspeed maneuvers, such as takeoff, climb and landing drag is much lower so there is much more engine power available.

The extra drag of round and rectangular sections versus an airfoil is surprisingly large. Wow.

The draggiest thing on my plane of course are the floats. The float strut attachment points have always been of concern to me. Does anyone sell fairings for use on float attachment points to smooth out the airflow? If not, it sounds like a business opportunity for a small shop.

Wow, that's amazing. It's no wonder the 182 RG feels so much draggier with the gear down than the fixed 182--round gear legs! That's a pretty simple yet profound way of demonstrating it, I bet the Dept. of Energy could turn that into a 4 million dollar research project that nobody could understand when they were finished.