Needing an explanation

Our club's site has an unusual occurance. One circle gets some "funny air" somtimes. I don't know how to exactly explian it, but a plane will just "drop" instantainiously about 4 or 5 feet. I have seen this happen a couple times, and experienced it myself once.(reason fo rthe refinish on the P-47)
Fliny inverted about 20 after the 1st pullout of the wingover and wham. No control input, nothing, just wham. Also saw this happen to an expert in the round loops. Same thing, wham.
Any explainations?
Thanks
Bill <><

I know nothing about the layout of your field, but it sounds like what happens when you fly through your own turbulence. If the air is calm (or blocked off with trees) this can happen. Best thing to do under these conditions is back up through maneuvers.

An other possibility is a down draft (caused by wind) flowing over threes or a building or some such.

What is our field like, and what were the conditions at the time that this happened (wind or no wind)?

Bill,

What are the surroundings like around the site? (Trees, wind direction at the time this happen, rolling hills, etc.) Sounds to me like you are getting rotors of trees when it blows from over them towards the circle. Thermal activity can also cause this sometime depending on how close trees or obtructions are.

We have the same type thing happen at one of the local airports I fly at. Ron King may have had some fun there before also!. They have cut the trees back now so not bad anymore.

A pic with a new digital cam would be nice to see it when it's warmer!

LAST EDITED ON Feb-01-04 AT 12:41 PM (CDT)
 
The same thing happens at Huntersville, but I think it's a product of the tress that are close to the circle. In a open field it's more than likely a down draft generated by sinking air after a thermal passes by.

Hey John,
Yep, the middle circle at Huntersville is the one I'm talking about.
Bill <><

When flying over ashpalt or cement, the sun gets the surface very warm, which warms the air and the motion of the model disturbs the air and up goes a bubble. In rushes cooler air and blam, down goes the model. This is different from the rolling air coming over the railroad fill at Dallas. Yet, it does the same thing. "Air pockets" they used to call them.
At some free flight contests in the 70's, we used to send a few younger boys up wind with jackets or small blankets or large towels and have them running circles and whirl the towels, etc over their heads. This would disturb the air enough to cause a thermal to break loose. You could see the whirling dust as it did and then we would try to catch the thermal with a good launch. Naturally the darn engine would not start until it had passed.
Also trees give off tons of water vapor and this causes a cooling effect in the area and cool air settles until heated and the cycle goes on. There was a very interesting article in the 60's in MAN about why a single tree would "capture" so many free flights. Only tree in the area jokes are based on fact. Studying how weather is created sure made this all come clear.

Bill,

You can add Jimmy Paul to the list of middle circle victims at Huntersville. October year before last his Chipmunk suddenly dropped out of inverted flight and smacked the ground. Conditions were about 72 degrees and light gusty breezes. JP said it suddenly dropped about four feet before he could even throw in any control input.

>Our club's site has an unusual occurance. One circle gets
>some "funny air" somtimes. I don't know how to exactly
>explian it, but a plane will just "drop" instantainiously
>about 4 or 5 feet. I have seen this happen a couple times,
>and experienced it myself once.(reason fo rthe refinish on
>the P-47)
>Fliny inverted about 20 after the 1st pullout of the
>wingover and wham. No control input, nothing, just wham.
>Also saw this happen to an expert in the round loops. Same
>thing, wham.
>Any explainations?

The turbulence is one possibility - and certainly can happen. But there is a phenomenon that you sometimes get in still/hot/humid air that I really don't understand that acts exactly like you describe. It's like a little bit of vacuum you flew through - whatever way you were pointed and whichever way you are going, it just keeps going that way no matter what you do with the handle. I have no decent explanation - they are usually very small. apparently ~10-15 feet across. Obviously, it's not a real vacuum, and it may be just be some small-scale thermal-induced turbulence. But's it's certainly real, and I have felt this phenomenon lots of times when I lived in the Midwest.

LAST EDITED ON Feb-01-04 AT 03:09 PM (CDT)
 
Yeah Brett,
There just doesn't seem to be a definition of exactly what is happening, but it is happening as both of us can attest. Mine was flying inverted, and then silence. No time to react, no mechanical failures, etc. Same with Dave H. Come to think of it, it has been the June contest when these event's happened, and when the temps were climbing. Pretty sick feeling, though, because it is just "out of the blue". No way to plan for it.
Bill <><

Bill,
The middle circle has two issues in my opinion. Depending on wind direction you have the rotors coming off the tops of the trees, also the circle is on top of a small hill.

Bill,
We, too, occasionally have planes drop ten or twelve feet for
no apparent reason at one point of one circle. We go with the
theological explanation that the Ned Brown Ground Gods (NBGG) are
angered.

Interesting to read that there actually might be a scientific
explanation, though .

Could it be... SATAN??!!

I heard that Old Scratch doesn't like model airplanes and people having fun flying them. He's particularly naughty on contest days..

[photo not recovered: 3e69bb3870f12ad5.jpg]

"And God said: 'Let there be Satan, so people don't blame everything on me. And let there be lawyers, so people don't blame everything on Satan.'" -George Burns

LAST EDITED ON Feb-01-04 AT 08:54 PM (CDT)
 
I've had this happen twice, but for different reasons. The first, in still air I think is what we call in free flight the downside of a thermal, the cold air rushing in the fill the void left by the departing "bubble" of heated air. This happened on a paved circle and the model went in flat (I was flying straight and level) and drove the landing gear up through the wing. There was absolutely no change in attitude, the bottom fell out and the model was instantly on the deck.

The second time was completely different, at Brodak's circle #4 (I think), the one to the back of the field on top of the ridge. It was windy that day and coming out of the second inbside square. Had made the tranbsition to level flight and again, the bottom dropped out and the results were the same as before. This incident is easy to identify as the wind came up the hill on one side of the circle and down the other side, a huge venturi.

The worst thing about this is that there is no time to correct and infact it seems that sometimes there is no reaction to control inputs. I've had this happen in maneuvers and been able to recover if I was high enough and pointed up when it happened. My Veco Chief flew through a thermal and it was so violent it rolled over on its back and flew to the other side of the circle where I was able to regain tension and recover.

Thermals can be amazingly violent, even at ground level. I had an A-1 Nordic pulverized in flight at Glenview many moons ago, the most incredible thing I've ever seen flying models. It exploded and fluttered away in all different directions.

I'm curious... is your circle flat or domed?
I ask because years ago we flew on a site that had one domed circle. That circle had a tendency at times to exhibit the symptom you describe. I always wondered if it had something to do with the dome shape of the circle.
--jt

It closer to flat. But it sits on a leveled off section of an incline. One circle is below it and another one above it. There are also trees lining one side, and another tree line at the upper circle.
Bill <><

Hi Bill,

I hate that circle. All kinds of chinook type wind phenomenon there. I much prefer the lower circle or up on the asphalt section way up on top where everyone practices in the mornings. Just don't fly on that circle unless you have to at a contest!

Larry, don't believe everything the "Church lady" tells you. I think she is one of my lost aunts!

My backyard field is surrounded by 30ft plus trees, had to saw limbs to get a 135ft circle. Wicked turbulence, I can never do 3 consecutive loops, on the third one the controls won't respond. Crashed a banshee and a twister before I've learned where to anticipate the dead spots.