2 stroke thread (with occasional F1 relevance!)

[Rodak]

Okay, forget the rocket pendulum stuff since your machine is a rigid body. So... to deflect the flight path you need to swivel the motors driving your machine and generate a thrust vector. Why is this so hard for you to see? Every rocket motor in use today has the ability to generate a thrust vector in some way, either by pivoting or deflecting the gas stream with guide vanes. As far as aerodynamic stability, you have an unstable machine and the best pilot in the world will not be able to fly it.\\

Here's a pretty good basic discussion about stability. Worth a watch manolis.... this applies to you, despite your protests.

https://youtu.be/sCJ3XJmD0DA

[Tommy Cookers]

.... a bicycle ... in order to be controlled.
The explanation / proof / experiment is not mine; it comes from the famous Cambridge University.
The best points of the article about the usefulness, or not, of the gyroscopic rigidity in controlling / driving a bicycle, are:

• “The way we stay upright on a moving bike is by active control through steering.”

• "Why is it easier to hop (or pogo-stick) along a straight path than it is to stand still on the ball of one foot?" The reason is that we use each hop to generate correcting forces and also to put our foot down in a new place that is closer to where we need it to be in order to maintain our balance”.

M P and Prof. Hunt have misrepresented things ......
(and infamously Cambridge didn't teach anything engineering-related till after the modern bicycle was developed)

consider a motorcycle's 'gyroscopic' torque can be 100 times a bicycle's
the (motorcycle's) angular velocity in roll is high and the speed and wheel rpm are very high
and the 'gyro' torque(s) seem to be regardable as damping some motion

though with propellers such torques can be large even at low vehicle speed .....
aircraft designers often didn't/don't today cancel significant 'gyroscopic' torque ....
particularly in tailwheel aircraft their effects appear (independent of other effects)

the cancellation in the P F design of these (as well as other torques) seems somewhere between important and vital

[nzjrs]

Similar is the case of the “Pendulum Rocket Fallacy” article.
I know only two members in this forum who really got it.
The others either don’t talk, or snob it / laugh at it (read the comments) refusing to accept what the Newton’s laws (who, by the way, was a professor in the Cambridge University) say / predict.

Absolute insanity.

The pendulum rocket fallacy speaks to the (misunderstanding) that static stability of a rigid body is influenced by the relative position of its center of thrust relative to its center of mass. At my engineering school the pendulum rocket fallacy wasn't even introduced specifically! It was joked about because it naturally comes out as obvious once one has done enough force-body diagrams!

What I continue to care about is what one learns In the second lecture of an aeronautical or control systems degree when one learns about dynamic stability and controllabiliy (using the control theory definition of that word). This is the part in the course where one can not learn anything from looking at pictures but needs to do actual modelling and analysis. What is the response of the body to a rotational torque, how does one generate that torque, what is the size of that relative to the rotational inertia. Follow up questions then include what are the control surfaces of the craft and what is the coupling between axes. Within the desired flight performance envelope, are the means for controlling the air-frame adequate.

As my control systems lecturer once said "you can turn a cruise ship by pissing off one side for long enough, but it doesn't make it controllable".



The third lecture is of course on aerodynamic stability and stability of a lift generating body, the relative positions of control and aerodynamic surfaces wrt. centre of mass.

manolis: do you not perform thorough and quantitative modelling of the controllability and stability of the portable flyer because you are unable to (software missing) or unwilling to (because you have learned everything you need from photos)?

[Rodak]

nzj, well written. As I've posted previously I like some of manolis motor ideas and hope he can produce a working engine, but I am surprised at his blanket rejection of comments and suggestions about control. I await a video of the first test flight......

I participate on the PPRUNE aviation site and have thought about posting some of manolis' flyer pictures and comments to see what sort of technical feedback results but haven't done so. Perhaps, manolis, you might post there yourself on the Tech Log page and ask for input and discussion. There are a lot of very good brains there with vast experience and expertise in both fixed and rotary wing aircraft, including design type persons.......

https://www.pprune.org/tech-log-15/

[manolis]

Hello Gruntguru.

You write:
“For stability in forward flight you want CG slightly forward of CP (CL in forward flight).”



Suppose the pilot of a military transport aircraft, full of armed soldiers, has to deliver them urgently somewhere. Every second of delay is vital.


Typical scenario:

The pilot follows the rules and distributes the soldiers (i.e. the load) so that the overall center of gravity of the aircraft to be adequately forward relative to the center of aerodynamic lift / pressure.

At horizontal cruise the back stabilizers apply a reverse lift of, say, 10% of the total weight of the aircraft (which means, the wings apply a lift equal to 110% of the total weight of the airplane).

Due to the overloading of the wings the AOA (angle of attack) increases, the stall speed increases, the aerodynamic drag increases and the cruising speed (for the same power) decreases.

However, the flight is safer and the stability is better.
In case of a stall:


(the drawing is from http://www.mpaviation.com/lesson7.htm )

the aircraft starts falling lowering its nose and gathering speed; the increase of the speed of the wings relative to the surrounding air makes the airplane controllable again.


Unconventional scenario:

The pilot distributes the soldiers so that the center of gravity coincides with the center of lift / pressure.

Now the wings need to provide a lift equal to 100% of the total weight of the airplane, while the rear stabilizers apply no reverse lift; the AOA of the wings lowers, the stall speed decreases, the drag lowers, the cruising speed increases (for the same power) and the time to go to the destination reduces.

However, at a stall things would be difficult / dangerous (case of “fixed” cargo);

However, the cargo is alive and can move / be re-distributed: the pilot can command the soldiers to move forwards leaving the back side of the plane empty. Their weight shifts the overall center of gravity forwards. The airplane falls lowering its nose and gathering speed. . . the rest as in the typical scenario.



The Portable Flyer has a living cargo: it is the body of the pilot and, and it reacts instantly (not as the soldiers above).
Depending on its pose, not only the center of gravity shifts forwards / backwards, but also the lift varies substantially (in size and “center” of application), while the external shape changes dramatically.

Thanks
Manolis Pattakos

[manolis]

Hello Tommy Cookers.

You wrote (a week ago):
"pendulum ....
what does a pendulum do if it's in or nearly in free fall ??"


Here is a similar question and the reply given in another discussion:

• Hello Kelpiecross
  
  You write:
  “Another "thought" experiment: a typical fireworks "skyrocket" - with the long wooden stick etc. A weight - like a fishing sinker - on the on the end of a piece of string 10 feet (or whatever) long tied to the end of the rocket's stick.
  The rocket is launched from a 10 feet-plus-a-bit ramp angled at 30 degrees to the ground - the rocket at the head of the ramp - the weight 10 feet behind. Off goes the rocket ("like a rocket" - so to speak) - travels in a more-or-less straight path for 10feet – then the weight comes off the end of the ramp - What happens? The weight immediately starts dropping towards the ground - surely pulling the end of the rocket stick to a more vertical position and tending to guide the rocket away from the Earth's CoG. Something must happen - but what? Surely the above is the most likely scenario?”
  
  
  The key point is that the weight and the rocket, both, undergo the same gravity acceleration.
  
  Immediately after the rocket leaves the ramp, it starts dropping relative to the ramp plane (which leans 30 degrees from horizontal).
  
  How fast the rocket is dropping?
  
  The upright (vertical) component of rocket's velocity (which, initially, is half in size than the total take-off velocity of the rocket: sin(30degrees)=0.5) is decreasing by 10m/sec per second (this is the gravity acceleration: 10m/sec2 = (10m/sec)/sec).
  
  Until the moment the weight leaves the ramp, the rocker has fallen a little relative to the ramp plane.
  
  Then both, the rocket and the weight are dropping relative to the ramp plane at the same rate.
  The vertical components of their velocities decrease, both, by 10m/sec per second, which means they both drop at the same rate, which means the weight cannot push downwards “the end of the rocket to a more vertical position”..
  
  Thanks
  Manolis Pattaos

Combine the above with the video:



wherein a Chinese teacher demonstrates a pendulum motion in space.

I suppose it is clearer now.


If the Portable Flyer were fixed on a frozen pilot, it could not be controllable. It would be like the rocket in the “Pendulum Rocket Fallacy” above.

However the pilot of the Portable Flyer is alive and can move his body reacting to the environment (i.e. to the signals his/her brain receives) .

The thrust from the proprotors of the Portable Flyer is displaceable relative to the body of the pilot (or, if you like, the body of the pilot is displaceable relative to the thrust from the propellers). The axis of the total thrust “plays” around the overall center of gravity, and this allows the continues active control (“feel and react to correct”).

There is a basic gimbal joint formed by the spinal cord of the pilot; compare it to the gimbal joint of the GEN-H-4:



This is the key point that allows control over the flight (weight displacement control), just like in the case of, say, the Mayman Jet Pack.
Then it comes the “aerodynamic control” that offer full control (including yaw).



You also write:

• “M P and Prof. Hunt have misrepresented things ......
  (and infamously Cambridge didn't teach anything engineering-related till after the modern bicycle was developed)”

Please be more specific.
Isn’t “crystal clear”, by the experiment of Hunt (Cambridge University), that you are not based on the gyroscopic rigidity when you drive a bicycle?

If we cannot agree on simple, “basic physics”, things like this, it is meaningless to talk about “flight control”.

Let me remind what Hunt writes:

• Bicyles are not held up by the gyroscopic effect
  
  How do we manage to stay up on a bike?
  
  Gyroscopic forces are not important for the stability of a bicycle - as you can see if you read on below - but they help us to control the bike when riding with no hands.
  
  More important than anything is "the trail"
  The front wheel makes contact with the pavement at a point that lies behind the point where the steering axis intersects with the pavement - and the distance between these is called the trail.
  The trail is not zero because the steering axis is tilted and the front fork is bent.
  The trail works to stabilize a bike in much the same way as castors work on a tea trolley.

Do you want to make things a little more interesting?

Hunt explains the controllability of a bicycle based on “the trail”.


So, how a unicycle can be driven?

At small speeds there is no significant gyroscopic rigidity, and there is no “trial”.

Here comes the “active control” based on the human brain and the human body: a continuous “feel and react to correct”. Enjoy it:



Flying with the Portable Flyer is far easier.

Thanks
Manolis Pattakos

[Rodak]

Still waiting for your explanation, with force diagrams, as to how one transitions from high speed forward flight to braking mode. Please include stability calculations and show why the transition could possibly work. Verbiage and pictures don't count.

[Tommy Cookers]

"pendulum ....

Isn’t “crystal clear”, by the experiment of Hunt (Cambridge University), that you are not based on the gyroscopic rigidity when you drive a bicycle?
If we cannot agree on simple, “basic physics”, things like this, it is meaningless to talk about “flight control”.

How do we manage to stay up on a bike?
Gyroscopic forces are not important for the stability of a bicycle - as you can see if you read on below - but they help us to control the bike when riding with no hands.
More important than anything is "the trail"
The trail is not zero because the steering axis is tilted and the front fork is bent.
The trail works to stabilize a bike in much the same way as castors work on a tea trolley.[/list]

no-one has said that 'gyro effects' were an issue (in bicycling)
and I have said that the cancellation in the PF design of 'gyro effects' is necessary

the bicycle is rideable because the steering axis tilt and fork bend are matched to each other
so that the bike doesn't rise or fall when the fork is steered
without this match the weight on the wheel would cause the steering to turn (one or the other way)

in principle trail is independent of match ..... but trail without match can make the bicycle .....
want to steer straight on but when forced to bank then turn more and more tightly (terminal instability) .... or ....
wander when meant to run straight but never want to turn more and more tightly (a wrong kind of stability)
EDIT these mismatches the USA now understands and calls high trail and low trail

this was discovered afaik after the first few years of the 'safety' bicycle ... eg and iirc ...
a 71 deg angle is matched by a 2.25" fork 'bend' (usually called rake but better called offset)
a 73 deg angle is matched by a 1.75" fork offset
old-style angles were eg 68 deg - hence their characteristic conspicuously large 'bend'

these days cars and many motorcycles have designed-in a mismatch that suits their (different) purposes
bicycles often have a small mismatch to ease manufacture and inventory ....
and mismatches can be large .....
extra offset relative to angle helps very small or very tall riders get foot clearance - but gives the 'wrong kind' of stability
but insufficient offset relative to angle cuts cost but the 'terminal instability' is dangerous - don't ride hands-off
this is now appearing eg in smaller 'women-specific' machines with shallow angles (because CF forks restrict offset choice)
RETRO-EDIT
a 'castor' vertical steering axis straight fork was used in some (early ?) ordinaries aka penny-farthings aka high-wheelers
giving no effect of weight on steering (when upright/straight ahead) - but no trail
but most ordinaries had slightly inclined steering axes and straight fork - extreme 'high trail' mismatch
tolerable as the weight effect on steering is (relatively) less with these very large wheel diameters
so intolerable and rapidly abandoned by the safety bicycle makers
the ordinaries greater 'gyro' effects might have been somewhat helpful

the 'high trail' weight effect (greatest with small wheels) prevents small-wheeled bikes ever having sufficient trail



again regarding PFs .....
I have never used 'pendulum' arguments

I can't find anywhere that rotor coning contributes stability only in ground effect

[manolis]

Hello Tommy Cookers

Thanks for all this info for bicycles and trails.

So, let’s summarize:

• If we completely cancel out the gyroscopic rigidity in a bicycle, it is still drivable / controllable.

and

• The most important characteristic when driving a bicycle is the trail.

However, in a unicycle, where there is neither “trail” nor gyroscopic rigidity (it is easy to test / confirm with the experiment of Hunt / Cambridge, and it is easier to just look at the video at what rhythm the single wheel / tire of the unicycle turns when the guy makes his tricks), the active control (“feel and react to correct”) makes “miracles”.

In comparison, a Portable Flyer in the open sky is easier to control because there is more time and more space to react and correct, than with the unicycle on the ground.

• As for the “rotor coning” you mention: in the open sky there is nothing around to “react” with the “rotor coning”; the only you actually have is the total thrust which is vectored at some direction; and here comes the “Pendulum Rocket Fallacy” claiming that, no matter how wide rotor coning” you utilize, you need dynamic control (i.e. continuous re-vectoring of the rotors) otherwise the Flying Device will soon turn and fall to the ground.

And while in a bicycle some gyroscopic rigidity may be beneficial at some conditions,
in a Personal Flying Device having non zero gyroscopic rigidity, every time the pilot would try (by his/her body movements) to re-vector the thrust, a lot of time and a lot of non-intuitive effort would be required, which means slow / poor / bad / dangerous control.


The bicycle is known for nearly two centuries, and its dynamics are not yet completely explained / understood (see this discussion or at the Internet).

The control over a unicycle is way more difficult to explain, because there is nothing to “lean on”: neither a “trail”, nor a “gyroscopic rigidity”; just a small contract area between the tire and the ground, a brain and a body.


By the way,

I was thinking of communicating with the guy who makes the tricks with his unicycle in the video, asking his “explanation, with force diagrams, as to how one transitions from "idling to one-foot-idling, to bunny-hop, to leg-around, to wheel-walk, to one-foot-unicycling . . .”.
I would explain him that without a gyroscopic rigidity, without a “trail”, and most importantly, without “force diagrams”, he is violating the natural laws when he is “balancing on his unicycle”. . .

Then I decided that it was easier to ask a baby-boy (who, these days, is making / enjoying his first steps standing on the floor)



about his “explanation, with force diagrams, as to how one transitions from one foot to the other without falling”; but because he understands nothing about “force diagrams” and “transitions” (actually he is not talking yet) I left him alone in his ignorance. . .

Thanks.
Manolis Pattakos

[Rodak]

Still waiting for your explanation, with force diagrams, as to how one transitions from high speed forward flight to braking mode. Please include stability calculations and show why the transition could possibly work. Verbiage and pictures don't count.

Still waiting manolis. Just because a baby can learn to walk doesn't mean your flyer is stable.

[manolis]

Hello all.

Here they are shown some cast Inlet Manifolds for the OPRE Tilting engines (Portable Flyer, Broom Flyer, Small Airplanes, paragliders etc):



The same, stereoscopically (for those who can see this way)



Thanks
Manolis Pattakos

[nzjrs]

By the way,

I was thinking of communicating with the guy who makes the tricks with his unicycle in the video, asking his “explanation, with force diagrams, as to how one transitions from "idling to one-foot-idling, to bunny-hop, to leg-around, to wheel-walk, to one-foot-unicycling . . .”.
I would explain him that without a gyroscopic rigidity, without a “trail”, and most importantly, without “force diagrams”, he is violating the natural laws when he is “balancing on his unicycle”. . .

Then I decided that it was easier to ask a baby-boy (who, these days, is making / enjoying his first steps standing on the floor)

Hilarious.

Here is a nice paper on the modelling and control of a unicycle. https://royalsocietypublishing.org/doi/ ... .2009.0559

Figures 9-11 (lane change maneuver) is something you should do for the PF. They show the torques and displacement for this action. I would expect a similar thing for at least the transition between modes of PF flight.

[Rodak]

"You can turn a cruise ship by pissing off one side for long enough, but it doesn't make it controllable".

Good one nz! And this is really the nub of the matter. Let's all agree that a pilot can practice control movements until they are instinctual, that babies can learn to walk, and that unicycles are ride-able. I can picture Pete standing on his special pissing stand at the bow of a 50,000 ton (or tonne is you wish) cruise ship, having practiced and practiced until all his reactions are automatic. The captain orders hard aport, so Pete steps up on the starboard stand, adopts his wide stance, unzips and fires away. Pete is in perfect control but unfortunately the ship is still uncontrollable - and that's the important part. Pete, no matter how well he acts just doesn't have enough rocket force to have any impact on the ship's course.

And that, dear manolis, is the problem. It's not a matter of learning how to control the flyer, it's whether it can be controlled. Let's look at one case, transition from vertical to horizontal flight. Pilot Pete decides he wants to fly instead of steer ships, so he purchases a 'Portable Flyer' and learns all the actions he should take to control it. Pete straps the motor unit on, starts the engine (which runs great) and launches himself vertically, slowly climbing. Now Pete wants to actually go somewhere but how does he do that? He has to pitch the flyer into some path with a horizontal component. There are two ways to do that. First, the propeller might be swiveled to produce a thrust vector, similar to a gimballed rocket motor or, effectively, a helicopter rotor, but the motor unit is rigidly mounted to the pilot's torso so that's not possible

The second way to produce pitch is to rotate the flyer about its c.g. by applying a moment to the flyer; this is Pete's only option and the only way he can produce this moment is by moving is legs forward into the propeller air stream, generating an action similar to the elevator of a plane. Unfortunately Pete's legs bend back at the knees, so he can't just bend them,; he'd be pitching the wrong way. Pete, keeping his legs straight, has to bend his legs up from the waist, his only pivot point. But Pete finds that this is a very difficult thing to do, there is not much range, and it is very tiring after only a few moments. Pete also finds that not only is he lifting his legs up but that the propeller steam is apply considerable force against his lift, making it even more difficult to keep his legs up. Perhaps the best thing Pete could do is wear very large swim fins.....

We could go on with Pete and other scenarios, but I see no effective way moments can be applied in pitch, yaw, or roll. There is also the question of stability; once Pete is flying horizontally, how stable is that flight? That depends, of course, on some basic stuff we have discussed previously with no actual answers from manolis. I don't believe, like the cruise ship, this is controllable.

[gruntguru]

The portable flyer concept is perfectly controllable if provided with handlebars and a 2 DOF pivot (flexure perhaps?) at or above the pilot's shoulders. The end result is equivalent to the GEN H-4.

[Rodak]

Yep.