2 stroke thread (with occasional F1 relevance!)

[Tommy Cookers]

... I.e. are the turbines of Mayman spinning at the same direction, or are they counter-rotating? ....
If they spin at the same direction, the gyroscopic rigidity of the turbines is a significant problem. They may have lightweight shafts, with mass at small eccentricity, however they run at high rpm.

afaik
turbojets are never 'handed' (though larger turbojets have 2 spools, larger turbofans have 2 or even 3 spools)
and their gyroscopic torques are trivial relative to those of propellers
(the engine with prop has far more momentum - the large radius of gyration is the dominant factor)

yes some large turboprops are 'handed' (via the gearbox not the engine)
some small turbofans and now one medium turbofan have geared fans (but presumably not handed)

anyway for 60 years airliners etc have had automatic yaw control in flight (cancelling any inappropriate yaw)


the jetpack gyroscopic effects would seem to be predictable

propeller gyroscopic effects are apparent eg at takeoff in tailwheel aircraft of high power loading
(and a key factor eg in some types of spin and essential for some other manoeuvres)
for study, at times they can be separated from asymmetry effects

[manolis]

Hello Gruntguru

You write:
I agree with that analysis and always have.
One problem is the internal forces and moments required to support the mass of the flyer's power unit during horizontal flight when the lift is provided primarily by the pilot's body or wing suit.”


This drawing:



shows at right a “horizontal flight” (high speed cruise.
The power unit is not horizontal.
The thrust axis has a significant angle from the horizon.
A part of the thrust force “holds” the weight of the power unit (the same does the aerodynamic drag on the engines).



You also write:
Similarly if the system is hovering (or even standing on the ground) the upper mass (once displaced from a vertical axis passing through the point of articulation (the pilot's spine)) will tend to displace further under gravity ie unstable. This must be resisted by the pilot's musculoskeletal system. The simple fix is to move the point of articulation to the height of the upper mass or above.


On the ground

Tests on the ground show it is stable.
The frame keeps the engines so that they lean backwards (as in the above drawing).
You wear it for, say, half an hour walking, jumping, climbing stairs, bending forwards backwards to the sides etc, and it is OK and stable.
It is like wearing a back-pack, just higher.
The direct support of the unit on the back-torso is simple, lightweight and gives control leaving the limbs free (it has not handlebars, yet).


On the air

On the air the Portable Flyer together with the upper torso /back of the pilot comprise a “fixed body”.
If the thrust from the propellers passes through the center of gravity of this “fixed body”, there is no tendency this “fixed body” to turn at any direction (i.e. to be unstable, as you write). The engines fall, however the torso/back of the pilot fall the same exactly way.

It is what the Pendulum Rocket Fallacy says / explains.

The rest body parts (head, limbs) can shift (relative to the torso / back) to displace the “overall” center of gravity to the direction the Portable Flyer is desired to turn.

For instance, the pilot by bending his waist can move his legs to the right; as a result, the Portable Flyer (together with the pilot) starts rotating to the right.
Then the pilot is bending his waist oppositely (to stop the rotation) and finally the pilot straightens his body (it is supposed that during this process the pilot opens / closes properly the throttle).



You also write:
“A pivot located at the CG of the power unit. If the desired location is inaccessible, a system of linkages or flexures with instant centre at the desired location could be used.”

If the simplest way works fine . . .

Thanks
Manolis Pattakos

[manolis]

Hello Henry.

You write:
“I don’t think you understood my suggestion. The trunnions are part of a test rig not part of the control system. The pilot will use their limbs and joints to do the low speed hover learning you describe but within a rig that can secure them and which provides the opportunity to measure the parameters that will determine how to go to the next step of translational flight.”

Can you, please, make a drawing (by hand is fine) to help me get what you propose?


You also write:
“On another point, how will the pilot control the power of the motors? Given that their limbs are the control surfaces how do they operate a control device without impacting the aerodynamics?”

Yves Rossy controls the four turbines of his DeltaWing JetPack using a gas cable and a control in his hand (it takes two finger, leaving completely free his arm).

Similar is the way Frank Zapata controls the throttle of his JetPack:



In the above video Zapata flies high above the ground, holding with his left hand a "selfie" camera. With his right hand Zapata holds the throttle (and yaw) control; his right arm is completely free to move.


In a similar way, the gas cable arrives into the hand of the pilot of the Portable Flyer, who uses two-three fingers to control the throttle(s). The arm is completely free to move.

Thanks
Manolis Pattakos

[nzjrs]

On the ground

Tests on the ground show it is stable.
The frame keeps the engines so that they lean backwards (as in the above drawing).
You wear it for, say, half an hour walking, jumping, climbing stairs, bending forwards backwards to the sides etc, and it is OK and stable.
It is like wearing a back-pack, just higher.
The direct support of the unit on the back-torso is simple, lightweight and gives control leaving the limbs free (it has not handlebars, yet).

AFAIR there is only one picture of you from behind, shirtless, hands on hips, wearing the frame - so that is what I will refer to. Have you done more ground tests?

Unless you define stable another way, how is a frame sitting passively on your shoulders under force only the weight of gravity relevant to the frame secured to your torso and under thrust with you (at hover) hanging (somehow) underneath?

The true ability and magnitude of weight displacement is very coupled and limited by the attachment mechanism / frame / harness, and so it's hard to say anything true about the how much control authority you have in hover if we don't know how much you can move your weight - and even where the neutral hanging weight CG is!

Rodak (I believe), also with climbing experience, has also tried to emphasize the importance of the details of the attachment.

[manolis]

Hello Tommy Cookers.

You write:
"the jetpack gyroscopic effects would seem to be predictable"


Predictable, or not, a non insignificant gyroscopic rigidity makes the re-vectoring of the turbine shaft slow.



With the "shaft compressor and turbine" revving at, say, 80,000rpm, the gyroscopic rigidity is not insignificant. The diameter and the mass of the spinning parts is small, however the gyroscopic rigidity increases with revs square.

Each propeller of the Portable Flyer has, alone, a significant gyroscopic rigidity; however each pair of counter-rotating propellers of the Portable Flyer has zero overall gyroscopic rigidity: the re-vectoring of their axes (together) is as easy as with the propellers stopped.

Do the small-turbine makers provide clockwise and counter-clockwise turbines for each one of their models?

Thanks
Manolis Pattakos

[Tommy Cookers]

.... With the "shaft compressor and turbine" revving at, say, 80,000rpm, the gyroscopic rigidity is not insignificant. The diameter and the mass of the spinning parts is small, however the gyroscopic rigidity increases with revs square.

Do the small-turbine makers provide clockwise and counter-clockwise turbines for each one of their models?

isn't the angular momentum of the rotating parts proportional to their rpm ?
ie not to their rpm squared

'gyroscopic' torque being proportional to rate of change of angular momentum ?
(with angular velocity of axis of rotation)

for equal thrust ....
a small high-speed 'jet' will involve an engine etc of small radius (of gyration) and angular momentum
a large slow-speed 'jet' will involve an engine etc of large radius (of gyration) and angular momentum ....
angular momentum being proportional to radius of gyration squared .....
ie a propeller-produced 'jet' will involve much greater angular momentum (than the 'jet engine'-produced 'jet')
so much greater 'gyroscopic' torque

I believe that ....
no aviation turbine makers ever made clockwise and counter-clockwise versions
(though ship steam turbine makers did - (some) ships having 2 or 4 ('handed') propellers)

the gyroscopic effects were greatest in WW1 (aircraft engines then typically rotating with their propellers)
one such rotary engine design largely cancelled these effects .....
https://en.wikipedia.org/wiki/Siemens-Halske_Sh.III

[manolis]

Hello Tommy Cookers.

You write:
“isn't the angular momentum of the rotating parts proportional to their rpm ?
ie not to their rpm squared

'gyroscopic' torque being proportional to rate of change of angular momentum ?
(with angular velocity of axis of rotation)

for equal thrust ....
a small high-speed 'jet' will involve an engine etc of small radius (of gyration) and angular momentum
a large slow-speed 'jet' will involve an engine etc of large radius (of gyration) and angular momentum ....
angular momentum being proportional to radius of gyration squared .....
ie a propeller-produced 'jet' will involve much greater angular momentum (than the 'jet engine'-produced 'jet')
so much greater 'gyroscopic' torque

I believe that ....
no aviation turbine makers ever made clockwise and counter-clockwise versions
(though ship steam turbine makers did - (some) ships having 2 or 4 ('handed') propellers)

the gyroscopic effects were greatest in WW1 (aircraft engines then typically rotating with their propellers)
one such rotary engine design largely cancelled these effects .....
https://en.wikipedia.org/wiki/Siemens-Halske_Sh.III”


My wrong.

You are right: the gyroscopic rigidity increases with revs, not with revs square.

Despite the extreme revs of the turbines (say, 30 times higher than the revs of a propeller) their gyroscopic rigidity is smaller because their rotating mass is arrange at a substantially smaller eccentricity (the moment of inertia increases with radius (eccentricity) square).

For instance, if a propeller and a turbine have the same rotating mass, with the mass of the propeller arranged at a 7 times larger eccentricity (say, 70mm the turbine, 500mm the propeller), then the turbine has to rev 50 times faster in order to have the same gyroscopic rigidity with the propeller.

So, the turbines of the JetPacks have some (but not so critical) issues due to their gyroscopic rigidity.

Thanks
Manolis Pattakos

[uniflow]

without rotor pitch control you rely heavily on engine response for lift control. You've got that sorted?

[Rodak]

without rotor pitch control you rely heavily on engine response for lift control. You've got that sorted?

Heavily? More like totally........

Manolius, I'm not sure why you don't see walking around with a heavy pack as different to hanging from some device; forces are being applied in exactly opposite directions. The flier is going to carry you as an under slung load; you're not going to be carrying it as a weight on your shoulders...... I'm still curious how you plan to hang the pilot off the power unit.

Edited to add: I guess I'm going to start to call this the 'suspended pilot fallacy' or maybe the 'pilot pushing up with some unknown force while suspended by some sort of undefined dangling thing' fallacy. Got a better name? Or perhaps better, some rational explanation; but yeah, I know, word salad ®.

[gruntguru]

Hello Gruntguru

You write:
I agree with that analysis and always have.
One problem is the internal forces and moments required to support the mass of the flyer's power unit during horizontal flight when the lift is provided primarily by the pilot's body or wing suit.”

This drawing:

https://www.pattakon.com/Fly_files/Take ... ntal_1.png

shows at right a “horizontal flight” (high speed cruise.
The power unit is not horizontal.
The thrust axis has a significant angle from the horizon.
A part of the thrust force “holds” the weight of the power unit (the same does the aerodynamic drag on the engines). . .

If the thrust axis passes through the pivot point connecting the upper and lower masses, the full weight of the power unit is generating a moment about that pivot and must be reacted by the pilot's musculoskeletal system.

Imagine the pilot is lying face-down on a table. The table represents the lift on the pilot's body or wing suit. The power unit extends beyond the edge of the table. Its weight makes it tend to point down. Not desirable.

[manolis]

Hello all.

Here are the specifications of the JB10 and JB11 JetPacks of Mayman:



Spot on the dry weights.
Adding, say, 30 liters (24Kg) of kerosene / Diesel fuel (otherwise the flight duration will be shorter than 10 minutes), the total weight of the device at take-off becomes 59Kg (130lb) and 76Kg (167lb).

The JB11 has six turbines for safety (if one turbine fails, the functional ones allow it to fly and land safely). But this version (JB11) is too heavy. Only few people can carry such a weight on their back / legs without problem.

The JB10 is less heavy, but it cannot fly or land if the one turbine fails (the JetPack is no longer controllable: it will start spinning fast: with the center of gravity more than 0.2m away from the thrust axis (i.e. from the shaft of the functional turbine), it will make its first side loop (?) in 1.5 second, its second “side loop” will last substantially less than 1 second and so on. No case of safe landing or control.

In this photo / drawing:



Mayman (at right) wears his JB10 JetPack.

Compare the distance and location of the shafts of the two turbines with the distance and location of the rotation axes of the four propellers of the Portable Flyer (at left).

A difference between the two arrangements is the higher position of the Portable Flyer propulsion unit (higher center of gravity).

However at flight what does matter is the location of the overall thrust axis; and the thrust axis is, more or less, at the same position and passes near the center of gravity in both cases.
I.e. the big difference will be on the ground with the one unit being substantially more lightweight but arranged higher.

In both cases the “upper torso / back” of the pilot is secured to the propulsion unit. The turbines and the torso / back of the Mayman comprise a fixed body. The propulsion unit and the torso / back of Portable Flyer's pilot comprise a fixed body. Using their free body-parts the pilots instinctively control their flight in both cases.
As Mayman says, the average person needs three hours of tethered training before free low-height flights over water.

Mayman cannot use, at small / medium speeds, aerodynamic control because the air speed on his limbs / head is small (and he has to keep his limbs away from the supersonic hot exhaust gas), so he needs additional mechanisms for the yaw control. The Portable Flyer pilot has aerodynamic control from take off to landing for the yaw (and much more).

Another difference is the safety offered by the Portable Flyer: in case the one propulsion unit fails, the other with its two propellers continues to run independently, allowing both: controllable flight and safe landing.

There is no pitch control in either case.

The automatic pitch provided by the PatPitch propellers:



allows higher final speed and lower engine loads; more at https://www.pattakon.com/pattakonPitch.htm

Thanks
Manolis Pattakos

[manolis]

Hello Gruntguru

You write:
“If the thrust axis passes through the pivot point connecting the upper and lower masses, the full weight of the power unit is generating a moment which must be reacted by the pilot's musculoskeletal system.”




The pivot is at the center of gravity of the upper mass (comprising the power unit and the torso / back of the pilot), and the thrust force F passes permanently through the pivot.
No torque is created by the upper body.
If, say, you separate the “torso / back” (still secured with the power unit) from the rest body of the pilot, the assembly will fly without any tendency to turn (Pendulum Rocket Fallacy, again).



I.e. if you leave the upper-right assembly free, with the engines running, it will fly parallel to its initial orientation (at least until some disturbances to turn it slowly / randomly).

By adding the limbs / head, you give to the system “control”, because these masses (head / limbs) can displace to shift the overall center of gravity relative to the thrust axis.

The pilot has to displace his arms, legs and head relative to the torso / back, and this does require “some” effort.

The question is: how much effort, and for how long?

Please stand on one leg and move your other leg around, keeping it at, say, 45 degrees from vertical.
How difficult it is?
Its center of gravity is displaced some 70% than if it was lifted completely horizontal.

Do the same with your arms and head.

These are simple / easy motions every person performs several times per day, without noticing it.

For instance, when you take books for the top shelf of a tall bookcase, you perform all these motions, and more: say, you stand on your left leg, your right leg is shifted / extended sideways for equilibrium, you right arms goes as upwards as possible, your head / neck bends backwards to look at the books that your right hand is to catch.

Worth to mention: the corrections (“weight displacement control”) are applied from time to time; i.e. the displacement of the center of gravity away from the thrust axis lasts until the thrust axis is re-vectored towards the desirable direction; then the center of gravity gets back near the thrust axis, and the “effort” minimizes.



You also write:
“Imagine the pilot is lying face-down on a table. The table represents the lift on the pilot's body or wing suit. The power unit extends beyond the edge of the table. Its weight makes it tend to point down. Not desirable.”


This is not the case with the Portable Flyer.

The thrust force from the propellers lifts the power unit and pilot’s torso / back, not the muscles /bones supported on pilot’s lower body.

• Even if the propeller axes turn horizontal, the pilot can always recover to hovering (earlier posts).
  It is not too different than walking: you know that if you lean too much, you fall, so you keep your body leaning into the allowable / safe limits.

The same happens with pilot’s legs at horizontal cruising.

The high speed air keeps them near horizontal, not the muscles / bones connecting them to the upper body. They are like lying on an air mattress.
It is like the cloths / laundry hanged from a rope to dry. The wind keeps them away from vertical. The strong wind can keep them almost horizontal.

Thanks
Manolis Pattakos

[NathanE]

https://open.spotify.com/track/1tQ5TSr1 ... VA8CqMRzuA

[nzjrs]

It astounds me that manolis only explains things that do not require explanation yet always fails to explain things that do.

[Rodak]

It is like the cloths / laundry hanged from a rope to dry. The wind keeps them away from vertical. The strong wind can keep them almost horizontal.

No it's not. Hang a person by their shoulders on a clothes line, have a storm with 60 mph winds come through, and see how horizontal they get.... And let's not even think about the L/D ratio.