2 stroke thread (with occasional F1 relevance!)

[Rodak]

Manolis, nzjr is correct with his comment about 'word salad'. I am not referring to your English, which is very good, but rather the endless stream of words.

This means that from the 66m/sec (200Km/h) you need 6.6seconds (v=a*t, i.e. t=v/a, wherein v is the velocity, a is the deceleration and t is the time) to stop (to hover), and the distance covered will be s=(1/2)*a*t2 = 220m .

If you add the aerodynamic drag to the braking force from the propellers the above numbers drop further.

I discussed slowing and reversing many pages back and asked you specific questions about how this would be accomplished which you never responded to. Please explain how the pilot ends up feet first and using the propellers to slow down. How does the pilot change position to end up feet first at speed? On one hand you show pictures of sky divers or wing suit flyers to demonstrate you ideas re control with arms and feet, etc. Then you show the pilot flying backwards slowing; how does the pilot achieve this reversal with the wind forces on his body? Do you claim he puts his legs up (or something) and this somehow flips him around? You can't have it both ways. And as he is traveling forward feet first how does arm and leg control now work? It seem that there would be huge forces attempting to pivot him back into the normal flight position and if he got the smallest bit off axis he would weather cock instantly.

[NathanE]

And here are some rough calculations:

With both engines running, you can have an upwards acceleration of 1g, or a horizontal acceleration - deceleration of 2g.

If you are at a low altitude you have to use a part of the power (propeller axis at an angle from horizon) to keep your height unchanged, so suppose only 1g (10 m/sec2) deceleration.

This means that from the 66m/sec (200Km/h) you need 6.6seconds (v=a*t, i.e. t=v/a, wherein v is the velocity, a is the deceleration and t is the time) to stop (to hover), and the distance covered will be s=(1/2)*a*t2 = 220m .

If you add the aerodynamic drag to the braking force from the propellers the above numbers drop further.

Thanks
Manolis Pattakos

Also: "Word Salad" love it, if you don't mind I'm going to use that the next time I'm forced to sit in a strategy meeting with a bunch of consultants.

[Rodak]

'Word salad®' is a registered trademark and in all uses must be attributed to Rodak under penalty of law.

[NathanE]

Even better I can quote it as a technical term developed by prof Rodak

I still have no idea how you get your feet stuck out front of the props to enable deceleration without dropping like a stone.

[Rodak]

When I posed this question to manolis a while back I explained my analysis of what would have to happen to get into that reversed position and came to the conclusion you would have to come to a full stop before reversing; so the braking effect seems not to exist - the pilot would have to slow down and gradually lose speed and rotate into vertical before going into reverse position.

I can't, unfortunately, take credit for 'word salad' as it's in common usage here......

[coaster]

Please bring a pal to film your flight, please share with us your 'kittyhawk' adventure.
Good luck.

[uniflow]

I guess Manolis, the difference is my engine is doing the business, out there flying, for all its apparent compromises.

It's a pitty you cant relax a little manolis and just accept everything is a compromise, not all black and white.
The good designers are the ones that can juggle these compromises to get an outcome they desire. You do have some good ideas, but never the less still a compromise. Thermal loading for example.

[manolis]

Hello DrAculla.

You write:

• “Triple redundant flight controls - two wired systems, plus one wireless backup system.”
  “Zapata doesn't fly this thing with moving around his center of gravity alone, because that wouldn't work. Normally if he would lean forward, he would simply fall over. The board has a Fly-by-wire system for autonomous stabilization.”

Moving around the center of gravity is the way the Flying Platforms work.

Last version of Zapata’s JetPack:



has five turbines, with some of them having directable nozzles (yaw control).


Here is a previous version:
(quote from newsatlas)

• Bolted to the top of the eye-catching, angular platform are a pair of white and black ski-like boots, each flanked by a 70 mm JP electric ducted fan powered by a LiPo battery that helps with stability.
  
  
  
  A computer system running a proprietary algorithm helps keep the Flyboard Air stable by increasing or decreasing each jet automatically.
  "The problem is that you have to develop an algorithm that's able to keep you stable, but doesn't conflict when it's time to move the machine," he told us. "So the machine has to understand when you want to move or when it's an uncontrollable movement." And that's about all he would tell us about the computer control system.

In the following video Zapata explains the control of his JetPack:



With several engines, with directable nozzles at the bottom of some turbines for yaw control, with the need to stabilize the thrust when some of the (eccentric) engines stall , the electronic control is a must.
Then based on the electronic control, it can fly unmanned or with untrained pilots, and so on.


The Portable Flyer has two independent propulsion units, each providing “central” thrust; i.e. at an engine stall, the thrust remains at its original position and direction; the only necessary reaction from the pilot is to increase the throttle to compensate for the loss of thrust from the other engine.

Thanks
Manolis Pattakos

[Rodak]

Word salad. Please answer the question re transitioning from high speed forward flight to deceleration orientation and how that is achieved using arms and legs in the air stream; please address stability considering center of pressure and center of gravity during the reversal.

[manolis]

Hello Rodak.

You write:

• “Please explain how the pilot ends up feet first and using the propellers to slow down. How does the pilot change position to end up feet first at speed? On one hand you show pictures of sky divers or wing suit flyers to demonstrate you ideas re control with arms and feet, etc. Then you show the pilot flying backwards slowing; how does the pilot achieve this reversal with the wind forces on his body? Do you claim he puts his legs up (or something) and this somehow flips him around? You can't have it both ways. “

The resistance of the "Portable Flyer / Pilot" assembly to rotate about any axis is small.

• Why? Go at https://www.pattakon.com/pattakonFly.htm and read the first paragraph.
  
  The following video is from there:
  
  
  
  and demonstrates the gyroscopic rigidity of a set of parallel flywheels; spot on the almost zero resistance of the set of the counter-rotating flywheels to change their direction of rotation and compare it to the other pair of rotating at the same direction flywheels.
  
  It is like day with night.

By applying a torque (pair of forces) on the Portable Flyer / pilot assembly , it starts rotating about an axis passing from its overall center of gravity, with the rotation axis being parallel to the “vector” of the torque applied.

If you change the direction of the torque, the rotation axis changes direction.

The moment of inertia of the assembly and the applied torque define how quickly the assembly will change direction.


When the pilot needs to brake from high speed, he can re-pose his head and legs so that the aerodynamic drag on his head to increase and the aerodynamic drag on his legs to decrease (say, he bends his neck backwards, he also bends his knees so that his feet to go higher).
The pair of forces force the assembly to rotate about a horizontal axis normal to the velocity.
The above aerodynamic torque forces the Portable Flyer (and the pilot) to pitch with the propellers looking more upwards, and continuous to pitch until the propellers look adequately backwards.
Then the pilot reposes his body to cancel out the above aerodynamic torque and completes the transition to hovering.

The duration and the distance covered until hovering are as described in the post to NathanE.


If the overall gyroscopic rigidity was not tiny, this maneuver would take a lot of time and a lot of effort (like making aerobatics) by the pilot.


To put it differently:
Either with the engines running and the propellers rotating, or with the engines / propellers stall, the pilot wearing the Portable Flyer is like the “wind tunnel dancer” and can change his orientation easily and quickly at any direction.

If you get how easily the direction of the Portable Flyer changes, you got it all.

Thanks
Manolis Pattakos

[manolis]

Hello all.

After reading my last post, please watch this video of Yves Rossy (the JetMan) who explains the same things.

https://www.ted.com/talks/yves_rossy_fl ... =tedspread

He knows what he says.
He is an ex pilot of military airplanes.


From the video:

• Many of the test are conducted while Yves is strapped onto the wing, because Yves’ body is an integral part of the aircraft.
  
  The wing has no steering controls, no flaps, no rudder.
  
  Yves uses his body to steer the wing.
  
  He turns by just putting his head on one or the other side.
  And sometimes he assists that with his hands, sometimes even with his leg.
  He is acting as a human fuselage, so to say.
  
  When he arches his back, he gains altitude.
  When he pushes his shoulders forward, he goes into a dive.
  
  
  ROSSY, LIKE A BIRD:
  
  I don’t have feathers. But I feel like a bird sometimes. It is really an unreal feeling.
  Because normally you have a big thing, a plane, around you, and when I strap just this little harness, this little wing, I really have the feeling of being a bird.
  
  
  ROSSY, FREE FALL:
  
  I started 20 years ago when I discovered free falling.
  When you go out of an airplane, you are almost naked.
  You take a position like that.
  And especially when you take a tracking position, you have the feeling that you are flying.
  And that’s the nearest thing to the dream.
  You have no machine around you. You are just In the element.
  It is very short and only in one direction.
  
  
  So the idea was:
  OK, keep that feeling of freedom, but change the vector and increase the time.
  
  
  Equipment: 55Kg full with kerosene
  
  
  JOURNALIST:
  
  And you are not piloting?
  There is no handle, no steering, nothing?
  It is purely your body, and the wings become part of the body and vice versa?
  
  
  ROSSY:
  
  That’s really the goal.
  Because if you put in steering, then you reinvent airplane.
  And I wanted to keep this feeling of freedom of movement.
  
  And it is really like the kid playing the airplane:
  I want to go down like that.
  And up I climb.
  I turn.
  It is really pure flying.
  It’s not steering, it’s flight.

Thanks
Manolis Pattakos

[nzjrs]

I've already written more or less this exact reply to you once before, but lets try again.

By applying a torque (pair of forces) on the Portable Flyer / pilot assembly , it starts rotating about an axis passing from its overall center of gravity, with the rotation axis being parallel to the “vector” of the torque applied.

If you change the direction of the torque, the rotation axis changes direction.

The moment of inertia of the assembly and the applied torque define how quickly the assembly will change direction.

Yes, this is an explanation of how forces and torques act on a rigid object. No one disputes this. We all understand you believe this is the only thing that matters.

When the pilot needs to brake from high speed, he can re-pose his head and legs so that the aerodynamic drag on his head to increase and the aerodynamic drag on his legs to decrease (say, he bends his neck backwards, he also bends his knees so that his feet to go higher).
The pair of forces force the assembly to rotate about a horizontal axis normal to the velocity.
The above aerodynamic torque forces the Portable Flyer (and the pilot) to pitch with the propellers looking more upwards, and continuous to pitch until the propellers look adequately backwards.
Then the pilot reposes his body to cancel out the above aerodynamic torque and completes the transition to hovering.

What is your measured or estimated forces/torques for the airstream deflection by the limbs, and similarly the forces from the knee bending moment? Given these and the mass and inertia of the personal flyer, what is dynamic behaviour of the PF over time?

If you can't do the dynamic stabilty calculations or modelling then if you provide the estimates of these and other values here, perhaps someone else can be bothered. The PDF I posed of the H4 modelleing is a nice introduction and example to what is considered normal for this sort of thing.

Again, stability and controllability is a dynamic question that should not, if one is responsible, just be waved away and every time we bring it up you just revert back to static explanations.

The duration and the distance covered until hovering are as described in the post to NathanE.

Congratulations, I mean that honestly, these were the first quantititive numbers I have ever seen you post or estimate. Its a good first step!

If the overall gyroscopic rigidity was not tiny, this maneuver would take a lot of time and a lot of effort (like making aerobatics) by the pilot.

To put it differently:
Either with the engines running and the propellers rotating, or with the engines / propellers stall, the pilot wearing the Portable Flyer is like the “wind tunnel dancer” and can change his orientation easily and quickly at any direction.

If you get how easily the direction of the Portable Flyer changes, you got it all.

You are almost there...

• The PF is stable and suitable for beginners (I'll just take your singular example of what matters in a dynamic stability sense - 'was not/ 'is' tiny gyroscopic rigidiy', whatever)
  • kind of like pissing of the side of a cruise ship, but for some definitions of the word, dynamically stable. Yay!
• The PF is easily manoverable (huh, but above?, YAY!?)
  • Just pop your foot or and into the airstream whip it around for corrections
  • Just lift those knees to come to a stop in a couple hundred meters

So, it sounds like the PF is just perfect, because you believe it to have these mutually exclusive properties Compromises are for worse engineers than you

But, I'm a serious engineer so I'm not just going to ridicule you for believing these things on the basis of animal and baby photos. I know that everything is a compromise.

The way you work out where on the spectrum of stability vs controllabiliy your flyer sits - and this depends on its state of flight - is to do the dyamic modelling of performing these manouvers and then you can know, when moving from one configuration to another (forward -> backward, backward -> hover, dusturbance -> correction -> hover) what is needed as a control input and what the bounds of what the airframe will do in response.

You almost get this - you say yourself - one of the few things it seems you have recognised as relevant, the gyroscopic rigidy is 'was not / is tiny'. 'Was not / is tiny' is a qualitative description in relation to a control input, like foot waggle in the airstream or a knee bend, and you are using this sentence fragment to assert stability or controllability. Do you know how you move from a 'was not / is tiny' assertion to a true understanding of stability and controllability? You do the modelling, or AT LEAST you run these 'was not / is tiny' thought experiments for many different values of control input (with numbers) and then pretty soon you have learnt something of value of where your PF sits on the spectrum in different flight states.

YOU CAN NOT SAY ANYTHING GENERALISABLE OR SIGNIFIGANT OF VALUE ABOUT THE DYNAMIC STABILITY OR CONTROLABILITY OF AN AERODYNAMIC BODY IN FLIGHT ON THE BASIS OF SIMPLE STATIC HAND WAVING OR BY COMPARISON WITH OTHER AIRCRAFT (ROSSY) WITH DIFFERENT PROPERTIES (OR BIRDS OR BABIES).

This is literally my point. I used to see undergraduate students that would assert that their widget was stable and controllable without needing to do the useless maths and modelling. Those students did not become good aircraft or control engineers.

[manolis]

Hello all.

My last two posts are about braking with the Portable Flyer and about Rossy’s JetPack.

Excluding the Delta Wing of Rossy’s JetPack, the architecture of the above two Flying Devices is the same.

Why?

• Because in the JetPack of Rossy and in the Portable Flyer:
  Everything is controlled by pilot’s body.
  The engines are secured on pilots body (no gimbal joints or other joints).
  The steering is nothing else than pilot’s head and limbs.
  No handle, no steering.
  The only controller is pilot’s brain; sensors are pilot’s eyes, otoliths, ears and skin; servomechanisms are pilot’s bones and muscles.

Rossy flies several years with his JetPack (younger he was a Mirage III pilot) proving that the above architecture works perfectly.
And he flies faster than any other JetPack can:



Follow his head / limbs motion when he performs his aerobatics.


Rossy's JetPack uses four jet turbines, each providing 220N (~50lb) thrust, the Portable Flyer uses two OPRE Tilting engines, each driving its own pair of counter-rotating propellers for about three times larger overall thrust.

Provided Rossy’s turbines counter-rotate, both Flying Devices have zero gyroscopic rigidity and zero reaction torque.

All engines (turbines and OPRE Tilting) are vibration free.

While the dry weight of the Flying devices is similar, Rossy’s turbines require some 20 times more fuel for the same range (or fly duration).

The thrust of Rossy’s turbines is not adequate for vertical take-off / landing, nor for hovering (so he falls from a helicopter or airplane, and he lands using a parachute).

The estimated thrust of the Portable Flyer propellers is adequate for vertical take-off with 1g (10m/sec2 upwards acceleration), or for vertical take-off / landing and hovering with the pilot and a “passenger” on his back.

• With a Wing secured on the back of the Portable Flyer pilot (or with a pilot wearing a wingsuit) the two Flying Devices get almost the same.
  
  To put it differntly: substitute the two pairs of turbines on Rossy's Delta Wing by two OPRE Tilting Broom propulsion units:
  
  
  
  and you have a different Portable Flyer, say a "Rossy Portable Flyer".

It seems Yves Rossy has to prove to some “experts” “he is not an elephant”.

His web site is at https://yvesrossy.com/ , his email is contact@yvesrossy.com

I wish he would come to participate in the discussion.



PS.

Nzjrs wrote:
“But, I'm a serious engineer"

Of course you are.

Two advices:
1. To accuse unanimously, is not decent. So, either be more polite unanimously (you lose nothing to be polite), or sign by your real name.
2. Delete your “photo-shop” at page 209; the guy shown, like Lilienthal, killed trying to prove something. We should respect the dead.

Thanks
Manolis Pattakos

[Rodak]

Manolis, you seem to be constantly ignoring the fact that your 'flyer' is moving at 200 kph when you attempt to go into braking mode. The air stream, per you, acts as a stabilizing force to give directional stability. That force would need to be overcome when rotating the 'flyer' at speed because of the weather cocking effect. You could try a simple experiment; hang a mass with a long trailing section (the pilots body) through its center of gravity on a rod. Stick it out a car window. Drive 100 kph. If the center of pressure is aft of the c.g. it will be stable. Attach some material to simulate the pilots folded legs. Hang it in the air stream again and see what happens. It won't flip around, the angle of attack will just change.

[nzjrs]

Nzjrs wrote:
“But, I'm a serious engineer"

Of course you are.

Two advices:
1. To accuse unanimously, is not decent. So, either be more polite unanimously (you lose nothing to be polite), or sign by your real name.
2. Delete your “photo-shop” at page 209; the guy shown, like Lilienthal, killed trying to prove something. We should respect the dead.

1. I'm sorry, but I've got many decades left on this earth and the current trend is that permanent archival of one's life online does not work out in the end.

I'm very direct and serious about safety (that's the serious engineer part, we have responsibility to safety). I've carefully not accused you of anything nefarious, just and unwillingness to do the necessary steps in preparing the PF for safe flight. By the way you have not answered why you have not performed any of the suggested calculations, model making or testing I or others have suggested.

2. I reported my post to the mods. I can no longer edit it. I think the Franz Reichelt story is a good lesson, we learnt about him at university in order to not repeat his mistakes.