2 stroke thread (with occasional F1 relevance!)

[NathanE]

Ok I looked at the PRF .

Big difference. Rigid structure, fixed axial thrust unit. The more relevant comparison would be of a semi rigid structure with at least three pivot joints perpendicular to the axis of thrust (arguably more if the spine is included). Control forces for these pivots are, in most cases an order of magnitude less than the thrust forces we are looking at.

There is a reason that not many people are able to compete on the gymnastic rings. This is a piece of equipment requiring exceptional physique and control to master based on professional commitment and time.

Try leaning 10 degrees off a vertical axis. Hard (unless you have really big feet!) but doable for most folks. Try holding your body 10 degrees off vertical suspended from your shoulders, almost impossible I reckon.

[manolis]

Hello NathanE

You write:

• “ The more relevant comparison would be of a semi rigid structure with at least three pivot joints perpendicular to the axis of thrust (arguably more if the spine is included). Control forces for these pivots are, in most cases an order of magnitude less than the thrust forces we are looking at.”

A single gimbal joint is OK.

For the Portable Flyer the most relevant comparison is the GEN-H4:



The Portable Flyer can be seen as a twin – symmetrical - compact GEN-H-4.
The POGO XFY1 is also quite relevant for comparison.
Then it is the Mayman JetPack.
And then the Zapata JetPack.



You also write:

• “ This is a piece of equipment requiring exceptional physique and control to master based on professional commitment and time.

No.
It is not necessarily more difficult than bicycling.
According to Mayman (JetPack), the ordinary person after three hours of tethered training is ready for free flights over water.


You also write:

• “Try leaning 10 degrees off a vertical axis. Hard (unless you have really big feet!) but doable for most folks. Try holding your body 10 degrees off vertical suspended from your shoulders, almost impossible I reckon.”

I don’t get your point.
Try remaining 1” (0.025m) above the ground (i.e. hovering) for 3 seconds without touching anything.
Nobody can.
So what?

Thanks
Manolis Pattakos

[Rodak]

Comparing your machine to a flyboard or whatever is a false comparison. The flyboard et al are effectively powered by a gimballed rocket motor(s) which can vector the thrust. Think of a rocket launch and how one can see the rocket motors pivoting to apply a vector to direct the rocket. Your machine does not do that, so a comparison is useless.

My question, posed previously, is still how do you transition from forward travel into braking mode? Imagine you are shooting along at your 200 mph and decide you need to stop. Magically, your machine is facing in the reverse direction, with the pilot's feet forward and is slowing to a stop. But how does the pilot make this maneuver? The aerodynamic forces at 200 mph are forcing the pilot's body back behind the propeller, to act as a wing by your claim. How, at 200 mph, does the pilot pivot in direction and then maintain that feet forward position whilst slowing? Really, please show by diagrams how this is accomplished and how he maintains stability and control in this position.

[gruntguru]

Re your transition pictures above, I'm curious how you plan to transition from high speed forward flight to braking mode. Seems like you would have to go through a zero speed interval to reverse the thrust as, per your comments, the human body is providing lift; braking then becomes unnecessary as you are already stopped. Please show how this transition is made and sustained without the wind stream forcing the flyer back into forward flight attitude (weather-cocking).

I think the transition is straightforward.

In horizontal flight the Angle of Attack is quite high. The pilot merely pushes forward on the handlebars as if to climb but eases back on the throttle if he wants to transition without gaining altitude. AoA continues to increase - as does drag. Horizontal thrust decreases because the thrust vector is becoming more vertical. At some point the pilot is vertical - still translating horizontally - lots of drag - thrust is near vertical but angled backwards therefore providing additional deceleration. He is now in a controllable hover.

[nzjrs]

Re your transition pictures above, I'm curious how you plan to transition from high speed forward flight to braking mode. Seems like you would have to go through a zero speed interval to reverse the thrust as, per your comments, the human body is providing lift; braking then becomes unnecessary as you are already stopped. Please show how this transition is made and sustained without the wind stream forcing the flyer back into forward flight attitude (weather-cocking).

I think the transition is straightforward.

In horizontal flight the Angle of Attack is quite high. The pilot merely pushes forward on the handlebars as if to climb but eases back on the throttle if he wants to transition without gaining altitude. AoA continues to increase - as does drag. Horizontal thrust decreases because the thrust vector is becoming more vertical. At some point the pilot is vertical - still translating horizontally - lots of drag - thrust is near vertical but angled backwards therefore providing additional deceleration. He is now in a controllable hover.

Earlier Rodak pointed out that the pogo stick portable flyer was an unvectorable and uncontrollable stick below a fan. I was surprised to see manolis concede this point. I was not aware the other portable flyer has handlebars to vector thrust, which would be logical and consistent with other portable fliers of helicopters origin. Perhaps manolis always intended to have a mechanism like this and he was just imprecise when describing it as easily controllable by simply shifting ones bodyweight, and drawing it as such.

At the moment from the PBF diagrams it seems like one starts to move forward using a simple and popular twerking dance, left and right turns are achieved by jive dancing to the left or the right, and stopping by a situp or knees to elbow (no dance example sorry). Of course this is all speculation because I haven't seen any evidence that these popular dance moves change the direction of thrust by any meaningful amount.

[Rodak]

An issue with braking mode is that if the center of lift is in front of the c.g. the aircraft is unstable. With the flyer in horizontal forward flight the c.g. must be ahead of the center of lift. When reversed, for braking, the center of lift, because the aircraft is reversed, is now ahead of the c.g. and the aircraft is unstable; but I suspect if the flyer is in braking mode there is no longer lift generated by the pilot so maybe this doesn't apply. But then, with the craft flying forward but in reverse I don't see how the pilot can apply any control forces; velocity winds would simply fold the pilots legs up, etc. Further, it should be noted that any lift from the prone pilot is not laminar but is rather kite lift, as the angle of attack would be past stall as the pilot is hanging from the frame and is only moved into a more horizontal position by weather cocking from velocity.

Edited to add: I really like what manolis is doing with his engine designs and am curious to see actual running. I do, obviously, have doubts about controllability of his flyer. Some of the other helicopter type flyers we have seen on this thread are interesting, particularly the design that flew successfully with a pivot of the power unit above the pilot which allowed simple torque application to control flight. The biggest issue I see in manolis 'control' system is the inability to apply torque to affect the direction of thrust; moving feet around in the air stream will not be effective. I think looking at how shifting center of gravity works with a hang glider might be a useful exercise......

[gruntguru]

Not sure what you mean by "reversed" but it doesn't sound like the braking I envisage. (The "braking" shown in Manolis' diagram is extreme and could not be applied at high speed) In the first stage of transitioning the braking is provided mostly by increased drag due to the pilot's increased AoA. Additionally you have reduced thrust due to the thrust vector aiming higher combined with thrust reduction to avoid climb. The whole process is simply a steadily increasing AoA until the pilot is vertical.

[Rodak]

Which is exactly what I posted previously when I commented that the flyer would have to go through a zero speed point in order to attain the attitude manolils shows. Then, braking would already have been finished and you're flying backwards.

[manolis]

Thanks Gruntgugu and Rodak.

There are other scenarios for braking.
For instance, being at high speed horizontal cruise the pilot closes the throttle (not completely), the propellers rev at lower revs not providing forwards thrust any longer (actually they provide braking); simultaneously the pilot bends his legs (and his head) upwards and opens widely his arms; the lack of lift from the legs, the increased lift / drag on the head / arms and the aerodynamic resistance on the engines / propellers turn the Portable Flyer upwards to hovering, causing a fast reduction of the horizontal speed (braking); progressively the pilot turns the propeller axes backwards opening the throttle to even reverse, if desirable, his horizontal velocity.

The flexibility of the human body and the human brain that controls it, do all these easily / intuitively.

This is what the “awarded” video (last page of this thread) with the “dancing” skydivers shows: in a couple of seconds the skydiver turns from high speed-head-down fall, to belly-down slow-fall, then to high-speed head-up fall, then to side sliding, then to spinning head-down (left or right), then to spinning head-up etc.

Likewise, Zapata utilizes his body flexibility to vector the thrust, and his brain to control everything.

Such capabilities of the human brain and body are better shown in the following video:



(see at 3:06 the transition from flying mode to flying mode).
The guys in the video are like the skydivers with the difference that they can “fly” for hours (while the skydivers have a couple of minutes, and need an airplane or helicopter to lift them at a high altitude prior to the free fall).

Secure a Portable Flyer on the shoulders – torso of one of these guys and let him fly in the open sky. After his training in the wind-tunnel, the flight with the Portable Flyer . . .


To Rodak:

Stop thinking of the human body as an airplane wherein the location of the center of gravity relative to the aerodynamic lift is crucial.
The human body (just like the bodies of the birds, bats and bugs) is flexible and has thousands of sensors (think of the skin) feeding the brain (which has huge processing capacity) with information.

Thanks
Manolis Pattakos

[gruntguru]

Thanks Gruntgugu and Rodak.

There are other scenarios for braking.
For instance, being at high speed horizontal cruise the pilot closes the throttle (not completely), the propellers rev at lower revs not providing forwards thrust any longer (actually they provide braking); simultaneously the pilot bends his legs (and his head) upwards and opens widely his arms; the lack of lift from the legs, the increased lift / drag on the head / arms and the aerodynamic resistance on the engines / propellers turn the Portable Flyer upwards to hovering, causing a fast reduction of the horizontal speed (braking); progressively the pilot turns the propeller axes backwards opening the throttle to even reverse, if desirable, his horizontal velocity.
Thanks
Manolis Pattakos

I have to agree with Rodak on this scenario. With the mass of the flyer above his head, the pilot/flyer is "nose heavy" and will tend to point in the direction of forward velocity (like a dart). It would be very difficult (impossible?) to execute this manoeuvre from high speed forward flight.

[Rodak]

Stop thinking of the human body as an airplane wherein the location of the center of gravity relative to the aerodynamic lift is crucial.

But you are constantly asserting that the human body (the pilot) is actually a wing providing lift. For stability, if the body is acting as a wing, the c.g. and center of lift locations are critical for stability. If the center of lift is aft of the c.g. the object will be stable, If the center of lift is forward of the c.g. the object will tumble. This is basic aerodynamics.

[nzjrs]

No one is saying that humans are not smart or cannot be taught to control (controllable and non under-actuated) complicated systems. We are disputing if the PF is controllable.

Even starting with the simplest case first, moving forward from a hover;

Likewise, Zapata utilizes his body flexibility to vector the thrust, and his brain to control everything.

He does this because the thrust is trivially able to be vectored on the flyboard using the flexible and strong lower body. The PF equivalent is asking a person hanging from a tree by his shoulders if twerking his bum to the back vectors the thrust rigidly attached to his shoulders enough to do anything.

[Tommy Cookers]

the PF could better get lift for horizontal etc flight from short chord 'ring wing' type 'cowlings' around/near the propellers
and the twin prop configuration naturally gives a corresponding near-ideal shape for a cowling-wing
consider - eg thick symmetrical aerofoils at these Reynolds numbers don't fully stall

and there's the Custer Channel Wing to consider https://en.wikipedia.org/wiki/Custer_Channel_Wing

a great read in part relevant to the above - see http:/www.esotec.org/hbird/HTML/Aero_F.html
that plane is designed to hover

[manolis]

Hello Gruntguru.

You write:
“I have to agree with Rodak on this scenario. With the mass of the flyer above his head, the pilot/flyer is "nose heavy" and will tend to point in the direction of forward velocity (like a dart). It would be very difficult (impossible?) to execute this manoeuvre from high speed forward flight.”


The distance of the legs from the chest (i.e. from the “gimbal joint”) , and the distance of the motors from the chest are more or less equal:



their weights are comparable (the legs are heavier), while the aerodynamic drag of the motors is higher due to their orientation relative to the direction of the flight.

Thanks
Manolis Pattakos

[manolis]

Hello Nzjrs

You write:
“The PF equivalent is asking a person hanging from a tree by his shoulders if twerking his bum to the back vectors the thrust rigidly attached to his shoulders enough to do anything.”


The Portable Flyer is different than what you describe.

Quote from the page 180 of this discussion:

The following image shows at its upper-right side the “fuselage”. It is the grey-colored assembly that includes the engines, the frame (side pipes and saddle) and the pilot’s upper torso (back, chest and shoulders). This assembly / fuselage is, more or less, a fixed / rigid body.



At the bottom-right side of the image they are shown (by light-brown color) all the “hinged” and “multi-hinged” parts (members) of the pilot; while the upper arms are directly hinged to the “fuselage” (as defined above), the lower arms and hands are “multi-hinged” as hinged to parts that are already hinged to the fuselage.


The question is:

What is gonna fall?

The engines and the upper torso, as a rigid body, will fall (or turn downwards) together, as a single rigid body.

The hinged parts (head, limbs, lower torso) also tend to “fall” (or turn) due to the moment of their weight about their respective hinge points.

Depending on their pose, the “parts / members / “fins” of the pilot provide some lift (not necessarily positive (i.e. upwards), but also negative if required) and drag. All these either with the pilot wearing a wing suit, or not.

If, at horizontal flight, pilot’s legs are completely horizontal (no lift), they balance with their weight the part of the moment from the engines not balanced by the “eccentric” horizontal aerodynamic drag on the engines.

If, at horizontal flight, the legs of the pilot are “leaning to the sky” (like the tail “horizontal stabilizers” of the airplanes, which provide negative lift for the sake of safety: to avoid an uncontrolled stall) they can provide additional aerodynamic moment that tends to turn the fuselage upwards.

End of Quote

Thanks
Manolis Pattakos