Plane Ride: Newsletter Challenge (06/21/05)

Down draft.

Nope, not a down draft. It was a calm day, remember? The plane probably passed throught the wingtip vortex of a larger passenger airliner above and ahead of the small plane.

The pilot has flown into jet or propwash wich is upset turbulent spinning air left from another aircraft that has passed through the same area fairly recently.

I used to know this as "downwash". Are you referring to the same effect?

Yes I Beleive that, that is the term. I do alot of fishing for muskellunge and one of the trolling techniques is to place the lure right behind the motor in the "propwash" and I have confused the terms.

Are you guys talking about the vortices generated at the wing tips? I think these are stronger than the surge of air generated by the propellors or a jet's exhaust.

I don't know exactly what causes the draft, however I do know that wing tip vortices are a result of the object moving through the air, the reason that it is moving through the air is due to the thrust out the backs of the engines, so I would have to assume that, the amount, and the speed of the air exiting the engines is far greater than the all of the other contributing factors combined. But I am not an expert at this sort of thing, what I do know, I learned from watching the movie Top Gun. Here's a question: I have always wanted to make a Helicopter lawn sprinkler that hovers around my front lawn and is propelled by the water by means of jets in the tips of the rotors, my question is this: is there a way to figure out how much water and at what pressure would be required to provide the lift?

im sure the calculations would be relatively simple of mass of water/sec x velocity(vertical) - gravity * mass of system but what you're really going to consider is the weight of the water filled hose weighing down the system. You'd probably have more luck suspending a hose about 20' above your lawn and letting it flail around like a fire hose. woohoo

An out of control hose doesn't stop traffic or wow the neighbours the way a helicopter would.

then i would suggest a higher pressure light weight hose, maybe a poly pneumatic air hose. Of course, this weight savings is not even considering the counterweight needed to keep your helicopter upright.

I have actually gotten about that far, I made a set of rotors out of thin wall stainless steel tubing and I made a low friction swivel/tee fitting and silver solderied the tubing into the swivel. I then made a set of pine blades and mounted them to the stainess tubes. Coupled to a pvc pipe and using compressed air I got enough lift to actually bend the rigid pvc pipe as it lifted. But that was with air not water.

Rather than using a hose, you might want to consider copper pipe in a lever-fulcrum arrangement, with the propellor system at one end, a counterweight at the other end, and with the water input located at the fulcrum. Say you use a 12-foot copper pipe with the fulcrum point located 3 feet from the counterweight end, and say the fulcrum is mounted 4 feet above your grass. With the water turned on, the propellors would rise to about 10 feet off the ground, if the pipe swung up to a 45-degree angle with the ground. You could design it so the counterweight provides 80% of the force needed to lift the props, with the other 20% provided by the lift of the water. Since it uses rigid pipe it wouldn't crash if a heavy gust of wind came along, which might happen if you used a hose.

focused reverse thermal. When the ground heats on a nice clear day, the air near the surface heats proportionally. Just previous to the generation of an updraft of warm air, there is an opposition of cold air aloft and warm air below, searching for a 'weak spot' for the exchange to occur. This can, on occasion cause a focused downdraft as the weak point develops into a circulating thermal-induced breeze. Hence, a downdraft. Just kidding, it was an airliner.

Problem is surely caused by Clear Air Turbulance

As no rolling forces are mentioned, one would have to assume that clear air turbulence (a form of wind shear) is the cause of the upset. If there were any rolling motion, wake turbulence would be the first assumed cause, considering the proximity to the airport. Wingtip vortices are the greatest danger produced by large aircraft, and are most pronounced when the airplane is heavy, slow and clean (landing gear and flaps retracted). They are similar to small tornadoes and are produced by the pressure differences between the top and bottom of the wing.

It is due to a phenomenon called Air Pocket which is a low pressure zone caused by heated air. The incident is momementary since the momentum of the aircraft carries it forward to the normal pressure zone.

As written in the 6/28 issue of Specs & Techs from GlobalSpec:

Chances are very good that you hit a pocket of "wake turbulence" from a large aircraft that passed through the area recently. When any airplane is in flight, air spills out from under its wings and wraps around to the top. The aircraft, of course, continues to travel and leaves a stream of this swirling air behind. This stream of highly rotational air is called a wingtip vortex and can last for quite some time — on the order of minutes. (It's kind of like a "horizontal tornado.") The heaver the aircraft, the stronger and longer lasting the vortex. (See a wonderful shot of this here.)

Wake turbulence is the bumpiness you feel when flying through these vortices, and you shouldn't be surprised to encounter it — especially in small aircraft in the vicinity of a commercial airport. You may have noticed that sometimes you have to sit and wait for a few minutes before you can take off on a flight after a large jet has just taken off. This is often because air traffic control has made the judgment to give the vortices time to die out and/or clear the runway area. You may also have noticed that some airplanes have little "winglets" on the wingtips. (See an example.) These are intended to prevent some of the vortex shedding and reclaim some of the energy normally lost to the vortex.