2 stroke thread (with occasional F1 relevance!)

[Rodak]

Well, manolis, build something and fly it; then we can talk. At least they got off the ground.

[manolis]

Hello Rodak

You write:
“Well, manolis, build something and fly it; then we can talk. “

Then it will be meaningless to talk.

Thanks
Manolis Pattakos

[manolis]

Hello Tommy Cookers

In your first reference (Aerospace / Electric VTOL Configuration Comparison' by Bacchini & Cestino) the Volocopter 2X (that uses single (not coaxial) rotors):



is classified as multirotor; in the same class (multirotor) is the E-Hang 184 (figure in my last post), that uses pairs of coaxial-contra-rotating rotors.

I.e. the efficiencies (thrust to power) mentioned have nothing to do with “single rotor vs coaxial rotors”, but with the efficiency of the various e-VTOL devices.

As expected, those having wings are by far more efficient at cruising, while the wing-less are more efficient at hovering.



Quote from your second reference (Coleman NASA’s paper: “A Survey of Theoretical and Experimental Coaxial Rotor Aerodynamic Research”)



Page 22

Conclusions
. . .
To some extend, these statements explain the increase in performance of a coaxial over an equivalent single rotor in hover (roughly 5% less power for same given thrust).
. . .
The great advantage of a coaxial helicopter in hover is its lack of a tailed rotor and the power that would require. As a result, coaxial helicopters are good choices for hovering platforms.
In forward flight experiments, the coaxial rotor required less power than an equivalent solidity single rotor (up to moderate adnvance ratios). . .The “hub drag” associated with the coaxial configuration will eventually cause the parasite drag to dominate at high advance ratios, thus giving the coaxial rotor a higher drag penalty than the equivalent single rotor.
. . .
Andrew . . . found that vertical spacing gave the greatest gains in performance up to H/D=0.05, with no practical thereafter. . . Most promising results were obtained for a 8% reduction in upper rotor radius.

End of Quote


In the above plot, the best thrust-to-power ratio is for the coaxial-contra-rotating rotors and the second best thrust-to-power ratio is for the counter-rotating-intermeshed rotors (tandem).

The Portable Flyer combines both arrangements (two pairs of counter-rotating-intermeshed rotors, with each rotor of the upper pair being coaxial-contra-rotating with its respective rotor of the lower pair).


According Coleman / NASA paper, the coaxial rotors efficiency drops at high advance ratios wherein the rotation axis of the rotors is near vertical (normal) to the speed of the vehicle.

In the case of the Portable Flyer, at high cruise speeds the rotation axes of the rotors are far from being vertical to the speed of the vehicle:



Thanks
Manolis Pattakos

[Rodak]

You're still not getting it. Where is your lift coming from while horizontal?

[tok-tokkie]

You're still not getting it. Where is your lift coming from while horizontal?

1) Resolve the propeller thrust into vertical & horizontal components.
2) The abdomen, legs & arms of the pilot creates drag which too can be resolved into vertical & horizontal components.
3) The sum of those 2 vertical components must match the weight of the pilot and personal flyer including its fuel.
4) The pilot adjusts the angle of both so he establishes the balance. At take off there is no horizontal component. Then the horizontal component is progressively increased.
5) If the pilot goes for 100% horizontal then there is no vertical component & the sink acceleration = gravity. If it goes below that then the sink rate is even higher. But the horizontal speed is increased. But that is just the same as a fixed wing aircraft.

[OO7]

[gruntguru]

A good idea. An axial cam engine with opposed pistons operating on a two stroke cycle with (Jumo stlye) uniflow scavenging. Probably been done a long time ago.

[Rodak]

You're still not getting it. Where is your lift coming from while horizontal?

1) Resolve the propeller thrust into vertical & horizontal components.
2) The abdomen, legs & arms of the pilot creates drag which too can be resolved into vertical & horizontal components.
3) The sum of those 2 vertical components must match the weight of the pilot and personal flyer including its fuel.
4) The pilot adjusts the angle of both so he establishes the balance. At take off there is no horizontal component. Then the horizontal component is progressively increased.
5) If the pilot goes for 100% horizontal then there is no vertical component & the sink acceleration = gravity. If it goes below that then the sink rate is even higher. But the horizontal speed is increased. But that is just the same as a fixed wing aircraft.

The discussion about the horizontal and vertical components and power requirements took place many pages back. A human body suspended from a structure attached to the upper body does not make a very good wing..... And, as always, to control the angle requires torque be applied; where is the leverage point?

[J.A.W.]

You're still not getting it. Where is your lift coming from while horizontal?

1) Resolve the propeller thrust into vertical & horizontal components.
2) The abdomen, legs & arms of the pilot creates drag which too can be resolved into vertical & horizontal components.
3) The sum of those 2 vertical components must match the weight of the pilot and personal flyer including its fuel.
4) The pilot adjusts the angle of both so he establishes the balance. At take off there is no horizontal component. Then the horizontal component is progressively increased.
5) If the pilot goes for 100% horizontal then there is no vertical component & the sink acceleration = gravity. If it goes below that then the sink rate is even higher. But the horizontal speed is increased. But that is just the same as a fixed wing aircraft.

The discussion about the horizontal and vertical components and power requirements took place many pages back. A human body suspended from a structure attached to the upper body does not make a very good wing..... And, as always, to control the angle requires torque be applied; where is the leverage point?

Rodak, have you never seen the 'X-Games' MX biker 'flying fools' do their stupendous 'air time'
stunts from massive jumps, inc' front 'n' back 180 degree flips,& using skilled/judicious bursts of
super-responsive 2T power, as pro-vectoring 'thrust/attitude' control?

One of those experts would be a natural for Manolis' machine, given their 4D proprioception...

[manolis]

A good idea. An axial cam engine with opposed pistons operating on a two stroke cycle with (Jumo stlye) uniflow scavenging. Probably been done a long time ago.

Hello Gruntguru

The Revetec engine with its contra-rotating tri-lobe-cams "has line-contact" between its track-rollers and its cams; it had a good BTE (Brake Thermal Efficiency) for the few minutes it managed to work under load before falling apart.

Unless I miss something, in the 2-stroke from Spain the contact between the track-rollers and the wave-cam-surfaces is "point contact".

Thanks
Manolis Pattakos

[manolis]

Hello all.

Pendulum rocket fallacy

Quote from https://web.archive.org/web/20091018232 ... drock.html

By Jim Bowery
Copyright 2001

Getting a rocket to lift off and fly straight up is a little like a juggler balancing a pool cue on his chin:



This balancing act is so difficult that many intelligent amateurs have tried building rockets that have their engines at the top.

This is one of the more intelligent amateur rocketeers, standing next to one of his first rockets:

• Notice the black engine is at the top and the heavy silver fuel tanks are near the bottom with long pipes going all the way to the top of the engine. He thought placing the engine ahead of the bulk of the rocket would keep the rocket stable as it pulled the rest of the rocket upward.
  
  PS: His name is Robert Goddard (more at https://www.nasa.gov/centers/goddard/ab ... ddard.html ). He invented liquid fueled rockets. He even did so when some respected authorities still thought rockets needed a medium against which to push. Newton's third law of motion already told them otherwise, but even respected authorities can be dumb, just like the rest of us. One liquid fueled rocket is the Saturn V you see in the picture of the balancing pool cue above, carrying 3 courageous men to the moon. They are in the tiny white triangular capsule at the top. Like most amateur rocketeers who are inventing new kinds of rockets, Robert Goddard was also courageous in his own way. A lesser risk of inventing things is that a few people who have low self-esteem, or who like things just the way they are, will try to find an excuse to ridicule your efforts. Don't worry about it. Rest assured you will provide them just the excuse they need. Explosions and governments are bigger risks.
  
  Live and learn, stay out of jail, make it work, and tell the tale.

Intuition says that when you lift something from the side, it's center of mass will swing below the point of support and stabilize, like a pendulum that eventually lines up right under your point of support after swinging back and forth a bit.

So far so good.

By putting the engine at the top of their rockets, some amateur rocketeers intend to keep them upright during lift off. They think their rockets won't fall over like a pool cue without the juggler, but will just wobble back and forth under the rocket engine's support, like a pendulum, until it is completely stable:



One of the two main places where intuition goes awry is in forgetting that a rocket engine is rigidly connected to the rest of the vehicle, so the engine's support of the vehicle changes direction along with the center of mass.

The other place where intuition goes awry is assuming that larger masses will fall faster than lighter masses. Galileo showed this false when he dropped two differing weights from the Leaning Tower of Piza and they both hit the ground at the same time. The top and bottom of a pendulum rocket will fall at the same rate so gravity doesn't stabilize it.


That means, if the rocket engine is just a tiny bit misaligned with where the center of mass is, the whole rocket will continue to turn:

Inverting the "pendulum" doesn't really matter.





In both (above) cases the center of mass is misaligned with the direction of thrust.


In outer space, such a rocket would just go round and round in a big circle. In Earth's gravity, however, the whole circle falls at 9.8meters/sec**2.

That means the "pendulum rocket" launch profile looks something like this:




If you are wondering how simple model rockets with fins manage to fly straight up, the short answer is, "Because that's the direction they were going when they got going fast enough that the wind on their fins made it hard to turn any other direction." The long answer can be found at Quantum Scientific's Model Rocket Stability page. If that long answer still isn't enough for you, and you have a serious desire to learn, check out The Guidance and Control Systems FAQ.


The author (Jim Bowery ) wishes to thank Henry Spencer for telling him three times that the pendulum rocket is fallacious and to Robert Goddard for inventing liquid fueled rockets (and for providing the author with a convenient excuse for being so obtuse).


Thanks
Manolis Pattakos

[OO7]

A good idea. An axial cam engine with opposed pistons operating on a two stroke cycle with (Jumo stlye) uniflow scavenging. Probably been done a long time ago.

Despite the title of the thread, whenever there's a discussion of information about a new engine, I always look for an F1 or racing relevance. Admittedly I don't know much about engines, so I'm always happy to be corrected and enlightened on the various subject matters and concepts involved.

The presenter in the following video (Spanish: ) suggests the design may be very capable as a range extender, but not as a main engine. Having worked on marine engines he believes the wear on the cams would be problematic. I'm also of the opinion that this would limit the operating speed of the engine and hence it's usefulness for racing applications.

It was also mentioned that at this stage in development, it isn't very efficient and likely requires significant cooling, judging by the size of the radiator on the test bench/rig. I wonder about the frictional losses of this cam design?

[J.A.W.]

^ OO7, as it happens - that Spanish "One stroke" engine was featured
recently among the 2020 2T Conference presentations - but other 2T's there
were more impressive, & thus likely of more interest for 'Future F1' potential.

Link to the 2020 2T Conference presentations, below:

https://drive.google.com/drive/folders/ ... wFXLrhjPH9

[manolis]

Hello all

More for the “Pendulum rocket fallacy”


Suppose a rocket is pivotally mounted – though a long rod secured at its base – on a fixed to the ground point (pivot point at the lower end of the long rod), with the rocket center of gravity exactly above the pivot point and with its thrust force axis aligned to pass through the pivot point.


Intuitively the standing rocket (with its upright looking thrust) will remain standing.


However, the slightest disturbance (say, a fly (insect) that comes and seats on the one side of the rocket, or the softest wind, or a tiny asymmetry of the exhaust gas “beam”) will turn the rocket (and the thrust which is “fixed” to the rocket) by a tiny angle.

Now the weight force is slightly offset relative to the pivot point, creating a tiny moment that further increases the leaning of the rocket.

The rocket cannot help turning aside slowly at first, and then more quickly until the rocket turns upside down as a pendulum.
After a number of oscillations the rocket stabilizes looking downwards.


In the above simple example, the fixed to the rocket thrust has not stabilizing action, at all.


Take the same rocket, switch-off its engines and balance it on the long rod that abuts on the ground. The unstable equilibrium: the slightest disturbance would spark the falling of the rocket.


The solution is to displace the thrust axis relative to the center of gravity.
And this is a dynamic process of the type: feel and react to correct.

This is what a bicycler, or a walking person, or a Portable Flyer pilot do uninterruptedly: they feel and react to correct.


In order to apply a corrective moment, Goddard would need to add a “control system” to vector the rocket relative to the rest vehicle.

This is what happens in the Gen_H-4:



If you think of the rotors / engine assembly as the “rocket”, with the “rest vehicle” hinged to the “rocket”.

In the GEN-H-4 the control system is the pilot who, displacing the relative position of the two assemblies, changes the thrust axis relative to the center of gravity (Weight Displacement Control).


This is also what happens in the Portable Flyer or in the Broom version of it:



The rocket is the OPRE Tilting engine with the two coaxial-contra-rotating propellers it drives, while the body of the pilot is the “rest of the vehicle”.

The pilot feels (otoliths, eyes, skin etc) and displaces the thrust around the center of gravity.

But in this case the pilot, who is in the downstream of the propellers (from taking-off to landing), has aerodynamic control, too: by using his head / limbs as ailerons he can control all: pitch, row and yaw.


Thanks
Manolis Pattakos

[Tommy Cookers]

again ....
the Gen H-4 has a lot of rotor coning
put there to contribute stability

try this ....
www.nar-associates.com>cruise_propeller_efficiency_screen.pdf
or try searching for Propeller Efficiency - NAR Associates
it contains a full propeller performance diagram
it seems to show how hover power requirement is high (for PF-type speed range without ideal pitch variability)
(btw it also shows that full power at the start of the takeoff of a fast plane is pointless)

page 7 of the Bacchini & Cestino paper iirc was my source re suggestions in previous posts
the argument over coaxial/intermeshing vs conventional rotor helicopters is unresolved after 60 years
and has little to do with the PF

PF credibility beyond this thread is not served by automatic dismissal alone (of aspects that I suggest in my posts)
or by dismissal of aspects that weren't suggested (as if they were)
I withdraw that remark with apologies if (I haven't seen any) other PF explanatory material exists