Thursday, November 12, 2015

Jet Packs... aerodynamics questions and thoughts....

If you follow such things the Jet pack from http://jetpackaviation.com/  was subject to all geek discussion last week. Today I say an article here:http://www.gizmag.com/jetpack-aviation-new-york-flight/40286/

This describes the controls of the jet pack.....

Notice that the only video we have seen is of it flying somewhat low over water. This means in the mind of the designer its not safe over anything hard.

There is pitch and yaw control, but no roll control other than weight shift.
To me this means that the output from the two jet engines must be really closely matched.
Loose either  engine to something as simple as an air bubble in a fuel line and your spinning wildly out of control.

So how would you build a safe useful jet pack?

Safe:
Minor expected faults won't kill you.
Single engine, sensor or actuator failures wont kill you.

Useful:
Able to fly at whatever altitude fuel will allow over any surface.


In my mind a safe useful jetpack would have the following:

Flyby wire artificial stability.
Engine out failure recovery from any altitude.
Safe escape from and single engine,sensor  battery or actuator failure.

This implies to me multiple engines with some kind of thrust vector control.
You need enough  control authority to maintain stability with one engine out.
You need enough control authority to maintain stability with one actuator or motor in hard over failure.

You don't have to always land, you can have failure induce a rapid climb to an altitude that ballistic parachute can deploy. (300 ft or so)


Rather than two motors I would think at least two per side. This means that if you loose an engine on one side all of the the other engines must have enough control authority to keep thrusting through the center of mass.  If the control authority point is above the CG (like their jetpack) then the gimbal on the good side will have to point toward the passengers legs, probably frying them.  To me this imples that the jet point of thrust vector control must be below the CG. IE the jets must move down on the human.  Then the thrust vector to compensate for rolling (or pitching) moment from a failure on the other side would point away from the pilot.  It also implies that each side have enough thrust so that if 1 motor fails it can hold up its side......

So I see something like 3, 4, or 5  smaller motors per side.

With 5 motors a side I believe you might be able to control the vehicle with just thrust throttling...

If you need thrust vectoring then a flap that you can deploy into the exhaust stream that deflects it in the  direction of the semi circle away from the pilot.

In thinking about things one of the scariest failures would be bearing failure where the motor dumps all its built up momentum in the free flying vehicle as its thrust goes away.....

I just don't see how the Jetpackaviation jet pack we have all recently seen could be made "safe" for flight over hard surfaces.

Some random observations:

From their images JB-8 was more of a flying sled. Its real clear that the JB-8 used two TT-100 247lb thrust jets, the same as the sonix jet uses.  These are about 50K each.
(See http://www.pbsvb.com/customer-industries/aerospace/aircraft-engines/tj-100-turbojet-engine)

Its also clear that the JB-9 uses two smaller engines... AMT Nike motors.(24K Euro each)  I wonder if the motors  counter-rotate, or if they don't how much rolling moment pitch changes make given the gyro coupling.... since the  designer and test pilot are both helicopter pilots they will be very familiar with the 90 gyro force motion phase shift....

In looking at the middle of their three videos the "Gopro" one that they are using standard box stock RC turbine ECU displays for the engines mounted on the stick grips, these displays are on the tether flight video, but not the NY harbor one... or the promotional jet pack pictures.

On the tether flight video one can clearly see a large LIPO battery pack connected to the engine ECU under the cover.

Also in the NY harbor video there looks like an added frame to keep the tether training cable from getting wound around the pilots head....

Needless to say I want one...


P.S. I want to see their blooper reel...
Edited to add info on what motors they are using.


12 comments:

Monroe said...

They don't seem to fly out of ground effect. Another reason to fly over water is fire. Why not have a stand on pad that flies? That way you don't ever get the pilot in the thrust vector? Well yeah it might not be as cool I suppose? But safer by far I would think.

Doug Jones said...

Ground effect reaches about as high as the span of the wing. No wing? The nozzle diameter, then- so they are certainly not in ground effect.

I would use 4-5 engines feeding a common duct to a wye, no gimbals, no fly by wire, simply enough excess power to fly on three. KISS.

Paul Breed said...

They are using amt nike motors.....

If you fed common duct with miltiple motors you could point them in opposite directions for gyro cancelation... you could even have a single motor with an oh-shit solid that would get you to parachute altitude...their first one had such ducts......

Robert Clark said...

OK. But still using 20th century tech. True 21st century flying cars and jet packs in the offing.
Watch this space:

http://exoscientist.blogspot.com

Bob Clark

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