Hey, I was reading the forums and boom it came to me on designing a duct fan, would it be a possibility if a duct fan housing(the circular housing)would be in a airfoil shape to help produce lift capability, so you can use less wing space? I just was curious of this thought.
Hi Jon8088,
I have seen the drawings for the Jetpack, it is quite alarming and a very amature design, tho I am sure someone will try to kill themselves with one probably quite successfully. The fans are chain driven from a single engine, failure of one chain would spin the occupant into the ground before he realised there was a problem. If the engine fails this machine cannot autorotate like a helicopter, it will simply fall to the ground. If anyone does intend to fly one of these machines I would recommend an explosively deployed parachute. Even with a parachute an engine failure between 20ft and 300ft altitude would almost certainly be fatal. Having seen footage of the test flights for the origional jetpack, I would imagine serious injury or death would result long before anyone managed to master the control of this thing.
I saw that Mythbusters episode - the vibration alone would shake your fillings loose. They concluded that it did not generate enough lift to actually allow a man to fly. And during testing the safety concerns were enough for them to constrain the device should it ever actually lift off of the ground.
notwithstanding that it doesn't work, and would probably kill you if it did, who'd want to strap a 200+lb back pack on, particularly when the cg is probably 2' behind you ?
2 feet is a bit exaggerated, when you see him standing in profile he leans a bit forward, I 'd say a few inches.
Seems to me that, or the thing is actively stabilized, or you 'll need weeks of practice to learn how to fly it because it can not be naturally stable. Also a landing with more than 125 lb on your shoulders seems to me almost impossible to achieve. A chair or some other stand might have helped to take the weight of the pilot's shoulders.
Nevertheless, I think it looks nice and it would be fun to fly it although I would never do it.
Checking if it has enough power to lift 330 lb:
From the Rankine-Froude Momentum Theory of Propulsion :
1. Power = (Thrust) * (Air Speed of the accelerated air mass) :
P = T * (V + v)
2. Thrust = [Flow of accelerated air Mass] * (Final Air Velocity Increase of this mass flow) :
T = [? * A * (V + v)] * (2 * v)
Note: a. The air mass being accelerated by the fan disk will increase speed before passing the fan disk,
when it reaches the disk the speed increase is v, then it continues to increase behind the disk until it reaches a total increase of = 2*v.
b. In hovering conditions V = 0.
When we consider hovering conditions, the Thrust (T) equals the aircraft Weight (W), and using this in the 2 above equations by eliminating v you get:
For T = W, P = SQrt((W^3)/(2*?*A))
In reality P will be higher (for a ducted fan, probably about 10%) because in the above equations there are no losses considered like for example the energy lost in rotational momentum.
Doing the calculation P necessary for hovering at sea level is about 51 HP. With all losses included let's say about 20% more is about 61 HP. In other words, he can not even lift 330 lbs.
Maybe it is a bit insane, but I like it anyway, and it could undoubtedly find useful utilities, maybe in the army or so.
"Maybe it is a bit insane, but I like it anyway, and it could undoubtedly find useful utilities, maybe in the army or so."
The U.S. Army's combat VTOL weapon of choice is the AH-64 Apache. It only carries a pilot, co-pilot-gunner, and their armaments. Yet it still requires two 1900hp turboshaft engines. That's reality. A 61hp one-man vehicle doesn't seem very practical in comparison.
I’m sure the US Army does more than just fly around and shoot down enemies with AH-64 Apache Helicopters.
Also when I am saying that I like it, I really mean that I like it when people are trying to invent and make new things work. They may not be perfect and in desperate need of improvement, but I think you should not a priory condemn them.
OK GregLocock I was a bit fast with my assumption of 10% losses.
Realizing now that the fan has lots of blades, I must admit that the efficiency will be lower compared to a propeller with the same diameter and much less blades.
GregLocock, where could I find information on fan efficiencies? Specifically speaking fan blades/impellers. I’ve purchased the book mentioned in this thread (Marc de Piolenc) and gives great information, but I would like to see graphs.
I have lots of info on DF’s since my master’s thesis was related to them. But in it I’ve always treated impellers as another component that only reduced efficiency and never looked at it. I was evaluating the losses due to obstructions inside high speed air ducts.
It would be nice to see a graph with ‘x’ number of blades, ‘y’ fan swept area and ‘z’ fluid speed (air). Maybe throw in ID and OD, RPM and or blade angle also. Any combination would be interesting to see.
