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May 18, 2020, 11:22 PM
U down with EPP? Ya U know me!
Aerogance's Avatar
Thread OP

Wrap your head around this!

I have been thinking about this for a while. We fly our DS laps in order to accelerate. But all the actual acceleration is in the Z direction.

Imagine being a pilot in a DS plane traveling 500mph in a 75mph wind. You would experience around 80g's of acceleration vertically from the seat through your helmut the entire flight. But you would also experience decelleration, non-stop deceleration in the forward direction as if you will slamming the brakes in a car. This deceleration would be equal to the scrubbing off the wind speed twice per lap in the duration of the lap.

So if you are doing a 2 second lap at 500 mph steady in 75mph wind, you will be going from 150mph to zero ever 2 seconds in the x axis. This calculates as 3.42 G's. A 200 lb pilot would have 680 pounds of force against the seat harness. Your head and body would be shoved toward the console and your ass would be through the bottom of the seat. You would also be dead, but if we ignore those pesky biological limits it would be a very uncomfortable flight!

If I got any of the above correct, hooray!
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May 19, 2020, 12:32 AM
bjaffee's Avatar
FWIW, Formula 1 drivers experience around 5g's while braking. Of course, they aren't doing it every couple seconds.
May 19, 2020, 12:49 AM
Registered User
Sounds like a terrible idea, this DS business :-))
May 19, 2020, 12:57 AM
Remember... Fly for fun!
I thought I had coaxed my friend out of a DS circuit in his full size sailplane, but i was wrong. However, his experience lead to him not ever wanting to do it again (he and glider survived BTW)
May 19, 2020, 04:23 AM
launch low, fly high
About 15 years ago, I was a consultant to a project at the flight test center at Edwards Air Force Base. The project lead contacted me, he wanted to do his masters thesis on full scale dynamic soaring, and wanted to emulate what the rc planes were doing... that was an interesting conversation, to say the least. I admired his bravery, although I suggested an alternative series of flight tests that could show the promise for dynamic soaring while still living to make the next flight. The result was SENIOR ShWOOPIN... you can find the result via a google search.
May 21, 2020, 10:27 PM
Heavy's good, heavy's reliable
BDK74's Avatar

It’s like a trip to,the store with my wife driving.
May 22, 2020, 01:29 AM
Registered User
I am wrapping ��

The circle flown should produce centrifugal force on the pilot and 80g at 550mph (245m/s) is a circle of 160m which is very small for a full scale but reasonable for an RC.

To survive, the circle done by a full scale would need to be a mile resulting in a g-load of 7.5g so only skilled pilots in g-suits would be able to try it.

So far so good!

I never thought about the forces when hitting the shear lines but since the plane increases or keeps its speed relative to the ground (and the circle flown can be assumed to be fixed relative to the ground or the wind) there should not be any constant deceleration, if there were the plane would just stop and fall out of the sky.

The wind speed is interesting but matters little, as the longitudinal acceleration is dependent on inertia and wind resistance only and the ride would be bumpy in the longitudinal axis but never reach high g values.

Imagine riding your car at 250 km/h and just putting the stick in neutral, it would slow but not hit a wall. The car is not aerodynamically slippery compared to an airplane. The F1 car mentioned typically decelerates at about 1g in the same situation due to the drag from all the wings.

A plane hitting the shear line would probably be perceived as a quick dab on the brakes followed by acceleration as BDK74 noted.

The change in force when crossing the shear line could be calculated knowing the drag coefficient and frontal area of the plane. Combined with speed and weight it should be possible to estimate the deceleration.
May 22, 2020, 09:31 AM
U down with EPP? Ya U know me!
Aerogance's Avatar
Thread OP
1 mile circle is too large to produce the result. To go fast the circle has to very small, limited by the performance of the plane and the shape of the shear zone. From onboard g sensors, we have found that wide and slow laps still average 11Gs. As the circles open up wider, once the G force reduces below 11 it becomes unlikely that momentum can be maintained.

All forces combined create a force vector which is direct straight up and slightly aft. Because the plane is rotating, the much larger z axis acceleration is constantly translated to x axis velocity, this is real active to the ground. Velocity through air is much simpler, as exiting the shear into a new headwind twice per lap cause airspeed increases twice per lap with zero actual acceleration on the airframe.

An oversimplified example: a person is sitting on a toilet. There is a rocket engine attached to the toilet. The engine is aimed straight down. The engine ignites and the toilet accelerates straight up. The potty pilot feels zero acceleration in the forward direction, only through the toilet seat. After the acceleration phase the toilet rotates 90 degrees so the potty pilot is facing straight into the apparent wind. Drag is huge because of the high speed, so the toilet slows down. The potty pilot experiences acceleration forces aimed rearward, or what we would call deceleration. The toilet engine fires up again. The toilet is still slowing down in the x axis as it hurls through the air but now the toilet is also accelerating in the z axis again. The toilet rotates again. Dynamic Soaring is this scenario but endlessly repeated.

The question is, would you need barf bags if DS’ing on a toilet?!
May 23, 2020, 08:32 AM
Screamin' Eagle's Avatar
I've seen shltty DS laps before, but this takes it to a whole new level.

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