What is the speed of air forced directly behind a piper cherokee, cessna citation, boeing 747?

Question

What is the speed of air forced directly behind a piper cherokee, cessna citation, boeing 747?

Answer 1

it cant really be mesured in MPH

its more in LBS of thrust

Answer 2

Are you talking about propwash/jet blast coming from the engines?  If that's what you mean, it's hard to find an exact number because it depends on how high of a power setting the engine is running at, the altitude, and many other factors.

Just sitting on the ground at idle power at close range, the answers are, in order:

Cherokee: a noticeable breeze

Citation: enough wind that you don't want to walk through it too closely

747: enjoy your fall to the ground - you may eventually stop when you get far enough away.

Takeoff power is quite a different story for any jet engine.  They produce anywhere from about 3,000 pounds of thrust each on a smaller Citation to 65,000 pounds of thrust each on a 747 to 115,000 pound of thrust each on a 777.

Answer 3

I don't know of any publications that would give the air speed behind a Cherokee.

I used to have a book that gave the exhaust velocities for various turbofan and turbojet engines.  They range from about 900ft/s for a turbo jet to around 600ft/sec for a high bypass turbofan.  This is right at the nozzle though.  Behind the plane it would be less.

The Aeronautical Information Manual issued by Transport Canada does give safe distances for taxiing behind jet aircraft to give you a clue of the force.  An executive jet is 200ft for idle and 500ft for takeoff.  A jumbo jet is 600ft for idle and 1600ft for takeoff.

Also given are the wind velocities for turboprop aircraft.  80ft behind a large turboprop (Dash 8, Convair 580, Hercules) the wind velocity for leaving the parked area, taxi, and takeoff are 47 knots, 36 knots, and 60-70 knots respectively.

Also for jet engines at idle you are more likely to get burned than blown away.  A guy were I used to work decided it would be brilliant to watch a 727 start up at night from a scissor lift directly behind it (a few hundred feet away).  He got most of his hair singed and couldn't get the lift to move out of the way.

Answer 4

This called "wake turbulence" and it all depends on the lift generated by the wing of any particular aircraft, as for the Cherokee which I have over a 1000 hours in all types, not much vortex is produced by the wing in the landing configuration, the Cessna Citation is about on par with the MU-2 as it incorporates more hi-lift devices, the 747 does not produce as much wing vortex as the 727 because of it's clean wing (absence of engine nacelles). Boeing aircraft use Krueger type leading-edge devices that increase wing area and lift during approach, you would think with just the mere size of a 747 this would be the case, but in FAA studies it was found that the 727 was more instrumental in producing wake turbulence.  The speeds in which you refer to depend on the the lift generated by the wing during cruise, landing and take-off. If you cruise at 130 mph at 3500 feet in a PA-28, the speed of airflow at the trailing-edge is usually half that number.