Archive for May 27th, 2016


Some notes on Rantoul’s Hangar 3, home of this year’s NATS.

In Uncategorized on May 27, 2016 by propblocks

Hangar doors on SE side of the building

Hangar doors on SE side of the building

The hangar doors face to the south east. Around half the doors are single layer safety glass from approximately 5’ to 35’, with the rest being 15’-35’. In the morning, there is a significant amount of solar gain through these windows, heating the floor and any tables set in front of the glass. As might be expected, the heat produces a convection current going up right next to the glass. We observed the effects mostly within 1’ of the glass, although it may spread as wide as 3’ near the ceiling. This was most pronounced on the right side where the glass is larger. It tends to push models off the glass and away.

After the time spent sealing the drafts and holes, the air is remarkably free of turbulence. Close to the hangar doors, there is some due to the convection current.

There is drift. We noticed that right at the ceiling, it goes one way, but 3-5’ below the ceiling, it goes in the opposite direction. In my many F1D flights, I used my pole more for retrieval due to hang-ups than for steering. In nearly every flight steering was not necessary. Balloons may be useful, however I suspect poles are more practical here.

This is the pole I have. I remove the two smallest rods for steering here.

Higher windows facing NW

Higher windows facing NW

View along the windows from lift

View along the windows from lift

Midday, sunlight isn’t as much of a factor.

Later in the afternoon, sunlight starts to come in from the north-western windows. In the corner (right side in the photo) Leo commented that the air seems to be more buoyant. As the windows are much smaller, no convection current was noted.

Adapted from blueprints on Village of Rantoul websiteElevation is 735’ above sea level. Red dot indicates approximate location where temperature readings were taken.

The ceiling height is 44’ 4”. There is a gap between the ceiling and the red beams, seems to be about six inches. Below that is the sprinkler pipe, hanging down a bit more. The sprinkler pipe is where more hangups occur. Additionally, there are lights that hang down.

Safe no touch flights are 40′. The sprinkler pipe is about two feet above this.



I took the above pictures at the ceiling after sealing the last large hole. Above the insulation panels, the temperatures were significantly higher than below.

What is interesting though is how much the insulation moderates the temperatures inside.

Rantoul 5-21-16 temp dataSaturday was windy outside, gusting to over 20MPH. Inside was very steady, with some drift but no turbulence. Interestingly the temperatures were very even all day long, with very little difference between floor and ceiling.


I left the temperature logger plugged in overnight. As you can see, the insulation really moderates the temperatures inside the hangar. The concrete acts as a heat buffer I suspect, so the floor temp is more stable than the ceiling. After the sun goes down, the ceiling temps drop below the floor temp, and the 20′ to ceiling height becomes amazingly homogenous. In the morning on Sunday, I put up several F1D flights while the ceiling temps were below that of the floor. The inside air was still, with no hint of turbulence. Outside the wind was about 10MPH in the afternoon.

As the temperature rises more than Saturday, the air temps slowly stratify a bit, with the ceiling a bit warmer than below, however the difference is still fairly minor. There is a bit more of a spread on Sunday than Saturday, which is likely explained by the higher outdoor temperatures and relatively light winds.

There are plenty more dates to fly this summer:

June 18-19 – Contest Flyer
(NATS July 19-24th )
August 13-14 – Fun Fly and Record Trials August Flyer
September 10-11 – Fun Fly and Record Trials Sept Flyer

October 1st/2nd – Contest

See the Bong Eagles Rantoul page for details.


Leo Pilachowski sent me the following e-mail and has given permission for posting it here.

I have a few comments on the convection cells and the air at Rantoul.

The fast ceiling drift and the slower opposite direction drift a bit below the ceiling are common results of the a flat ceiling building’s convection currents.  The air rises over the hot spot, in this case the concrete the sunlight falls upon.  Upon reaching the ceiling, this air needs to go somewhere so there is the drift away from the sunlit spot.  Since these spots are near the walls at Rantoul, the air closes off in the direction away from the wall.  The rising air needs to be replaced by other air so there is a lower height drift toward the sunlit spot.  The difference in speeds is caused by the hot air having a smaller cross section right up at the ceiling resulting in a faster speed to move the same volume of air as is sucked in below.  Obviously, there is a fast rising thermal at the sunlit spot.  The accompanying downdraft, however, is pretty much over the whole rest of the building, lessening a bit toward the far walls.  This is because the air along the ceiling continuously cools and then falls, completing the convection cell.  I noticed this most of the time during the event weekend.

What is neat about the Rantoul hanger is the thick concrete floor.  The sunlit spots get a warmer surface and heat the air.  However, when the sun stops shining upon the spot, say when a cloud blocks the sun or the sun is high in the sky around noon, the concrete surface cools back down to the internal temperature.  Then, the convection cell gets weaker and even stops.  This, I think, along with the evening after dusk, is the best time for flying at Rantoul.  The air is essentially driftless and there is no large area downdraft.  Even a minor downdraft can affect times and a plane does not stay over the hot spot long before drifting out.  Note that one inch per second downdraft is five feet per minute.  I found that my F1L descent rate last Saturday evening at Rantoul was fantastic and nearly as good just after lunch on Sunday.  I would rather have driftless air with no up and down drafts than convection cells as, unless one continually steers into the up draft, much more time is spent in the downdraft.  I think that the evening flying at the Rantoul Nats will be great.

Kibbie has a similar convection cell setup.  The fast ceiling drift is called the “jet stream” and will take a plane 300’ in a matter of minutes.  F1D’s near the Kibbie ceiling need to be steered (with hardly any room for a balloon under the ceiling tile clouds) many times during the high portion of the flight.


Tim Stone offers the following: The walls there are deadly. Slide down and you are almost guaranteed to get stuck. (Good advice – Steer before you get into trouble. Hangups can easily occur on the hangar door or below the windows. The area by the dock doors can be especially hard to steer out of.)

Bill Gowen offered the following observation: (Am paraphrasing)

On smaller models with droop booms, the sprinkler pipes are sure to be a problem.