2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello JAW.

You write:

• “As for your 'wire' flex-pitch prop design, ~1/2 a century ago
  BSA/Triumph motorcycles went to the use of 'wire' mounting styled
  front mudguard & headlight supports, but found that even rubber damped
  wire would resonate, & eventually then fatigue-fracture due to vibration
  frequency inputs from both engine & road effects.”

The “wire mounting” of the headlight :



is different than the “wire mounting” of the blades of the PatPitch propeller.


The resonance frequency is reverse proportional to the mass supported by the wire, is proportional to the tension / preloading of the wire and is reverse proportional to the “vibrating length” of the wire.


Take the strings of a guitar:

• The heavier (thicker) a string, the lower its resonance frequency and the deeper the sound it produces,
  
  and
  
  the weaker the “preloading” of the string (applied by turning its tuning key) the lower its resonance frequency and the deeper (lower) the sound it gives.
  
  and
  
  the shorter the active (or vibrating) length of the string (it is defined by the finger that pushes the string below a fret), the higher the frequency it vibrates and the higher the sound it makes.

The headlight of the Triumph is heavy, the “support” wire is thin and not preloaded, and the “vibrating length” of the wire is long.

In comparison:

The blade of the PatPitch propeller is quite lightweight,
the wire mounting the blade is thick and heavily preloaded (we talk for nearly one ton of force "centrifugal" preloading at 4000rpm of the propeller, for a blade having only 0.2Kg mass),
and the “vibrating length” of the wire is short.


If it's not yet clear, please let me know to further explain.

Thanks
Manolis Pattakos

[manolis]

Hello Tommy Cookers.

You write:

• 
  it is not this force that varies the pitch
  
  whatever the type of structural linkage (tetrahedral or leaf spring/flexure) the PP uses to handle the centrifugal force ..
  the pitch is varied by the components (of linkage forces) in the plane of rotation about the blade-mounting spindles
  said force components producing rotation in pitch are small relative to the centrifugal force
  eg if a stationary blade had 1950 lbf applied (by external cable) wouldn't the PP be perceptibly compliant ?
  
  so oscillation in pitch including possible resonance and aero effects isn't clearly implausible

Quote from another discussion:

“With 80N force acting at an eccentricity (from the long axis of the blade) of, say, 25mm / 1”, the moment that tends to turn the blade about its long axis is 80N*0.025m=2mN.

The inertia forces that load the two “heavily skew” arms (that hold the blade), are some 100 times heavier than the aerodynamic force acting on the blade.

*"heavily skew" in the sense that their "angle" (when looked from a point on their common normal line) is nearly 90 degrees and their distance is, say, more than 50mm.

From an "enertgy viewpoint":

Suppose that the weak (80N) eccentric (for 25mm) aerodynamic force does rotate for, say, 5 degrees the blade about its long axis (~0.175J of energy); 5 degrees turn of the blade causes some 5mm change of the eccentricity of the blade center of gravity from propeller's rotation axis, which means 8850N*0.005m= 44J of energy, which is some 250 times higher. “

End of Quote



Start with the arrangement / mounting of the blades of the PatPich propeller.

The variation of the “instant” pitch of a blade comes with a wide variation of the eccentricity of the blade from the propeller rotation axis, which requires hundreds of times more energy than what the eccentric (relative to the blade’s long axis) aerodynamic force can provide.


You write:

• “the pitch is varied by the components (of linkage forces) in the plane of rotation about the blade-mounting spindles”

The correct is:
“the pitch is varied by” the flexing (elastic deformation) of the “wire frame”, and this flexing is caused by the strong centrifugal force (which increases with revs square) acting on the blade.

What the blade “feels” is that its “basis” (say, the BD arm in the following drawing , wherein its innermost end is secured) turns for a few degrees about blade’s long axis as the revs vary.





As for the usefulness of the PatPitch for the Ehang 841:

if varying the revs by 500rpm the pitch increases (or decreases) by, say, 15”, the horizontal cruising speed can change a lot, and the hovering duration can change a lot, too.
Unless the currently fixed pitch propellers of the Ehang are ideal for all flight conditions.
Provided the weight and cost of the propellers remains, more or less, the same. . .

Thanks
Manolis Pattakos

[NathanE]

Hello Tommy Cookers

You write:

• “regarding the PatPitch propeller design ....
  
  the blades have some 'natural' frequency of oscillation pitchwise due to their compliant mounting on their spindles and ...
  cyclic variation of blade AoA/pitching moment as the PF's 'body attitude' is significantly different to its direction of motion
  (unlike other aircraft)
  so unless otherwise designed might develop excessive and/or sustained blade-on-spindle oscillation ?”

The blades of the Portable Flyer are short (~500mm), they are lightweight and inflexible.

All these increase a lot the natural frequency of pitchwise oscillations.

The mounting of each blade (last post) is “inflexible”, too.

Let me explain using some numbers / calculations:

Suppose each blade weighs 0.2Kg (carbon fibers) and its center of gravity has an eccentricity of 250mm (i.e. 0.25m) relative to the propeller rotation axis.

At 4,000 rpm (i.e. 4,000/60 = 67 rounds per second), the tetrahedron that holds the blade receives a centrifugal force of:

0.2Kg * ((0.25m*2*pi)*67/sec)^2 / 0.25m = 8,850N (885Kgf, 1,950lbf)

This “extreme” force (as compared to the weight of the blade) flexes the two “free” arms of the tetrahedron and varies the pitch.

At high speed cruising, the aerodynamic force (lift or thrust) on each blade (there are 8 or 12 blades, depending on the number of blades per propeller) is about 80Nt (8Kgf, 18lb) and acts at a small eccentricity relative to blade’s long axis; if you take under account the way the blade is mounted (tetrahedron) and the size of forces on blade’s mounts, the aerodynamic force on the blade cannot vary noticeably its angular displacement about its long axis (i.e. the pitch).



You also write:

• “also - isn't a lot of engine rpm change required ??
  (to give the chosen pitch change)”

The OPRE Tilting engines of the PortableFLyer are to operate:

from 2.4*2,500=6,000rpm (6m/sec mean piston speed) and partial load (slow take off, landing, hovering, loitering),

to 2.4*3,500=8,400rpm (8.4m/sec mean piston speed) in case of emergency landing with the healthy engine running at medium load,

to 2.4*4000=9,600rpm (9.6m/sec mean piston speed) to cruise horizontally at 250Km/h, with the engine running at medium load and the pilot not suffering from the air.

Occasionally the engines can rev up to 11,000rpm (11m/sec mean piston speed) to allow cruising speeds over 300Km/h.

https://www.pattakon.com/GoFly/Portable ... _small.png

The characteristics of the OPRE Tilting engine:

• short piston stroke,

• extra piston dwell at the combustion dead center for efficient combustion at higher revs,

• built-in tilting valves for the control of the air fuel mixture,

• perfect balance (vibration-free quality)

• etc

fit with the previously described operational range.

Thanks
Manolis Pattakos

Looking at your picture of flight modes I wondered if you had calculated the time required to transition between cruise at 250km/h and braking modes and distance travelled. It seems the effort required to overcome thrust and drag forces to go from head to foot first will be significant? The big difference in center of thrust centre of gravity and centre of drag make for great stability but possibly lower agility?

On similar lines have you estimated turn radius at different cruise speeds?

[manolis]

Hello NathanE

See the 4th post in the page 198 of this forum, wherefrom the following video / quote is:



Zapata is literally dancing on the air.

Why?

Because bending a little his legs, Zapata causes a big displacement of the thrust axis relative to his center of gravity.

See in the above video, from 1':52'' to 2':02'', Fraky Zapata accelerating and then decelerating with his FlyBoard Air.

Did you ever see Mayman doing, with his JetPack, something similar?

And do you see any reason for being the Zapata JetPack more stable or more controllable or more responsible than the Portable Flyer?

On the contrary, while Zapata is based exclusively on the “weight displacement control” (the pair of “control fins” at the exits of two of its jet-turbines is only for the yaw control), the pilot of the Portable Flyer besides the “weight displacement control” has also aerodynamic control over his flight, because he is inside the high speed downwash of the propellers.

Thanks
Manolis Pattakos

[NathanE]

Yes, I see that but the relative position of centres of thrust, drag and gravity make the flyboard inherently unstable enabling rapid control adjustments. The personal flyer is inherently stable making adjustments harder, or am I missing something?

[manolis]

Hello Nathan.

You write:

• Yes, I see that but the relative position of centres of thrust, drag and gravity make the flyboard inherently unstable enabling rapid control adjustments. The personal flyer is inherently stable making adjustments harder, or am I missing something?
  

The thrust above the pilot of the Portable Flyer does not provide better stability.
Go to page 192 and read the "Pendulum rocket fallacy".
Then read all the following pages (they reply to your questions).

As for the response / rapid control adjustment of the Portable Flyer, the pilot not only can instantly and effortlessly vector the thrust towards the desirable direction, but the pilot can also use his limbs / head as aerodynamic surfaces (ailerons) to deflect the high speed downstream of the propellers.

These two different controls (weight displacement control and aerodynamic control) give better / compete / full control over the flight as compared to Zapata's JetPack.

Thanks
Manolis Pattakos

[nzjrs]

As for the response / rapid control adjustment of the Portable Flyer, the pilot not only can instantly and effortlessly vector the thrust towards the desirable direction, but the pilot can also use his limbs / head as aerodynamic surfaces (ailerons) to deflect the high speed downstream of the propellers.

I think we have all read your pendulum rocket fallacy, but many of us just do not believe your interpretation of it is correct, or it's application to the portable flyer. And that is even ignoring the broomstick flyer for now.

As I said, I've flown one of the Zapata water Jet things. My feeling of riding it was

1) vector the thrust using your legs (ankles really).
2) this moves your centre of mass relative to the thrust (you also naturally lose altitude)
3) your whole body tilts over
4) you naturally straighten your ankles and you enter sideways flight

I really encourage you manolis to go and ride one of these, and then as others have suggested, built a mockup of the portable flyer where you hang from a tree or something, and see the difference when you wobble about.

[manolis]

Hello Nzjrs

You write:

• “I think we have all read your pendulum rocket fallacy, but many of us just do not believe your interpretation of it is correct, or it's application to the portable flyer. And that is even ignoring the broomstick flyer for now.”

The “Pendulum Rocket Fallacy” article is not mine.

And what it says is just what Newton’s laws predict for a rocket.


My “interpretation” (?) is wrong.

OK.

What is your (i.e. the correct) “interpretation” / application of the ”Pendulum Rocket Fallacy” in the Portable Flyer?

How the same “Pendulum Rocket Fallacy” is applicable in the Mayman, Zapata, Browning and Rossy JetPacks? They all fly.

What is the difference between the Portable Flyer and the above four JetPacks?

And what is wrong with the Broom Portable Flyer?



You also write:

• “As I said, I've flown one of the Zapata water Jet things. My feeling of riding it was
  
  1) vector the thrust using your legs (ankles really).
  2) this moves your centre of mass relative to the thrust (you also naturally lose altitude)
  3) your whole body tilts over
  4) you naturally straighten your ankles and you enter sideways flight
  
  I really encourage you manolis to go and ride one of these, and then as others have suggested, built a mockup of the portable flyer where you hang from a tree or something, and see the difference when you wobble about.”

The water Jet of Zapata has a difference from Zapata’s FlyBoard-Air.

The water jet is actually a flexible water pipe, at the end of which is a guy who vectors the thrust that propels the water pipe (with the guy on it).
The water in the flexible pipe is too heavy.

In the case of Zapata’s FlyBoard-Air and in the case of the Portable Flyer, the mass of the pilot / rider is the big percentage of the total mass, and this gives a different feeling / freedom / control.


Instead of “riding” a water Jet, you can “ride” a unicycle.

The basics of the control are similar: the brain receiving signals from otoliths / eyes / skin feels what is going on and reacts by commanding the various muscles in order not only to avoid falling but also to move to the desirable direction and at thedesirable speed.

Try it. It is cheap (less than half the priceof a bicycle) and you can find it (and try it) everywhere

It seems impossible at first, then the brain “learns” and gets confident, finally you do the control intuitively (without thinking) and effortlessly; then is when you really enjoy the unicycle.


Quote from the “Device Technical Report” (at https://www.pattakon.com/GoFly/DTR_1.pdf , at https://www.pattakon.com/GoFly/index.html as filed in the GoFly – Boeing contest):

• “Tethered tests and training
  
  The top end of the hollowed pipes can be used for the tethered tests of the PORTABLE FLYER and for the initial training of the pilot (the PORTABLE FLYER can be hanged from a roof (or from a tree branch etc) by ropes tighten on the top ends of the hollowed pipes).

End of quote


See how “Gravity” / Browning trains people on flying with their JetPacks:




After a couple of hours of “tethered” training, the trainee with the Portable Flyer on his back will continue flying un-tethered over water at low height (say 3m) for as long as it takes.
Then they will use the Portable Flyer as a transportation means.

Thanks
Manolis Pattakos

[nzjrs]

The water Jet of Zapata has a difference from Zapata’s FlyBoard-Air.

The water jet is actually a flexible water pipe, at the end of which is a guy who vectors the thrust that propels the water pipe (with the guy on it).

Why do you explain this to me as though I haven't ridden it?

Anyway, the guy at the (other) end of the pipe doesn't vector (direction) the thrust, he adjusts the magnitude infrequently. Most of the time he sits there doing nothing. When I was riding he was mostly playing with his phone.

The water in the flexible pipe is too heavy.

In the case of Zapata’s FlyBoard-Air and in the case of the Portable Flyer, the mass of the pilot / rider is the big percentage of the total mass, and this gives a different feeling / freedom / control.

Sure, well weight equal in both cases, the feeling is different when one strafes left to right on the water board vs forward and back due to the pivot of the attachment of the hose to the board.

You can also explore the influence of the center of mass (and its relation to the center of force) on the waterboard by flying higer or lower above the surface of the water, lower obviously changes the proportion of the mass the rider makes up and the distance between it and where torque is exerted by the thrust jet.

Instead of “riding” a water Jet, you can “ride” a unicycle.

I'm pretty average at riding a unicycle. Riding one at low speed reminds me of the dangling portable flyer guy waggling his arms about trying to exert any torque trying to change the orientation of the airframe.

[manolis]

Hello Nzjrs

You write:

• "I'm pretty average at riding a unicycle. Riding one at low speed reminds me of the dangling portable flyer guy waggling his arms about trying to exert any torque trying to change the orientation of the airframe."
  

How many hours (or days) of training (I mean until to have real control over it and move stable even at low speeds) did the unicycle take in your case?

Mayman says that the average person needs three hours of tethered training with Mayman's JetPack before free low-height flights over water.

For the human brain the control of the unicycle appears more difficult than the control of a JetPack (or of a Portable Flyer).


By the way,
I think that a strictly technical question can be answered only by a strictly technical answer.

Thanks
Manolis Pattakos

[nzjrs]

How many hours (or days) of training (I mean until to have real control over it and move stable even at low speeds) did the unicycle take in your case?

So I am pretty bad at low speed, I can idle, but tend to drift left - similarly I can definitely turn better left than right. I probably spent maybe 20-50 hours playing I would say (I shared a house at university with a person who was very very good at riding).

By the way,
I think that a strictly technical question can be answered only by a strictly technical answer.

Fair enough, I mistakenly got a different impression of you from the photos you post. These seem to lack force balance equations and similar rigorous calculation seen in the aviation and controls fields.



I got the impression you were deriving the controllability of the fliers from youtube videos of jetpacks and hummingbirds.

In comparison to your engine drawings it seems less rigorous (and technical), but I can't really judge the engine side of things.

(Without wanting to reveal my personal identity, I was also very peripherally associated with, and know engineers who worked on, the Martin Jetpack, so I have some insight into the level of modelling and control engineering that went into one personal flying vehicle. I'm not completely full of --- on this topic)

[manolis]

Hello nzjrs

You write:

• "Without wanting to reveal my personal identity, I was also very peripherally associated with, and know engineers who worked on, the Martin Jetpack, so I have some insight into the level of modelling and control engineering that went into one personal flying vehicle. I'm not completely full of --- on this topic
  

Several years ago, when Martin was at his best, I wrote him explaining that a Flying Device must be lightweight. Not a "flying fortress". Never got any answer.

Martin JetPack empty weight: 200Kg (claimed).

Portable Flyer empty weight: 20Kg.

Thanks
Manolis Pattakos

[nzjrs]

Martin JetPack empty weight: 200Kg (claimed).

I hadn't seen this video before, one frame in the video shows 294Kg.

[PlatinumZealot]

@manolis

Looking at the tetrahedron mechanism how does it resist bending loads at the root of the propeller blades and keep the blades from exessive coning?

[Rodak]

I got the impression you were deriving the controllability of the fliers from youtube videos of jetpacks and hummingbirds.

In comparison to your engine drawings it seems less rigorous (and technical), but I can't really judge the engine side of things.

(Without wanting to reveal my personal identity, I was also very peripherally associated with, and know engineers who worked on, the Martin Jetpack, so I have some insight into the level of modelling and control engineering that went into one personal flying vehicle. I'm not completely full of --- on this topic)

Hey nzjrs, it would be interesting if you could submit manolis' 'portable flyer' to those engineers and ask their opinions on controllability, etc.