Hi Manolis.
Anonymity (for those who wish it) is one of the founding stones of a forum like this.
I myself have an alias here and hence operate anonymously, and so do most users.
@nzjrs: Wish granted, the post is now “soft deleted”, that is, invisible to users.
Hats off for taking the high ground here.
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.
There is no problem with Manolis' analysis of transition from forward flight to hover. An expert pilot would be able to execute this manoeuvre quite quickly.
1. Forward flight at high speed using the AOA of the pilot's body for lift.
2. Throttle back and push forward on the handlebars ( ) to to increase AOA and maintain altitude as speed reduces.
3. Increase throttle as flyer stalls and continue rotating.
4. At some point the pilot will be vertical but still travelling forward at significant speed. Altitude is constant throughout if throttle application is correct. Note that in this attitude there is significant drag force on all parts of the pilot's body (comparable to a skydiver in freefall with their body horizontal). This drag force acts through the aerodynamic centre of the pilot-flyer and is simply counteracting the deceleration experienced by the system.
5. Rotating further to an inclined position with feet-forward, the remaining forward velocity is now generating negative lift. This means to maintain altitude the flyer "rotor" will need to be generating more thrust than it would for "hover" alone. This extra thrust will be minimal since forward velocity will be much lower by this stage.
The above stages will definitely take longer than the (2g deceleration) estimate given by Manolis but somewhere in the 1 - 2g range seems achievable to me.
Maybe, but I'm have a big problem with stability. I can't see feet forward flight as stable; if the center of pressure is forward of the c.g. the device is unstable.
But that generates more upsetting moment. Really, this could be easily evaluated with some experiments. A wind tunnel would be ideal.....but there are lots of other ways to get some basic data before trying this with a human. Ideas are great but tests are real.
You write: "But that generates more upsetting moment. Really, this could be easily evaluated with some experiments. A wind tunnel would be ideal....."
But you have the wind tunnel "experiment" in front of you.
Please make me the favor and do see, once more, the following video:
The wind tunnel dancer flies – she literally flies – at a speed of 200Km/h (or so): the air around her flows upwards at ~200Km/h.
The only thrust force is her weight that acts permanently at her center of gravity.
Here are the facts:
She changes her body orientation, at all directions, almost instantly.
She has full control over her flight (to the left, to the right, forwards, backwards, upwards, downwards, spinning etc, etc).
She “brakes” almost instantly (it is when her altitude increases).
She rolls, yaws and pitches(?) rapidly, calmly and gracefully (if you put significant effort to do something, you cannot do it gracefully).
If you see Rossy in the video where he flies over Dubai, he does the same:
he not only controls his flight “perfectly” (when he flies in formation with his partner, he is not flying "randomly"),
he not only changes his direction “instantly”,
he is not only making aerobatics (that make the ordinary person to shudder from fear),
but he does all these easily, enjoying the flight.
The center of gravity is where it is.
The center of lift is where it is.
Neither Rossy, nor the wind dancer (nor the birds, nor the bats, nor the bugs) know (or care) where these centers are.
They all use their brain as the only control unit, they all use their eyes, otoliths, ears, skin as the only sensors, they all use their muscles / bones as the “servomechanisms” .
They all “feel and react to correct”. This is all. And the fact is that it works.
So, take another "fresh" look on the Portable Flyer project
Why do you refuse to test stuff? Words are great but mean nothing. I can think of many cheap easy tests to verify or refute your ideas re control. This is F1 Technical; test it and get numbers; discussion is just words.
Manolis doesnt need to test, because he is always right. Its all us dummies that dont know what we are talking about.
Dummy I might be but I can tell you there is no way I would strap that contraption on my back and attempt to fly it. When the engine goes quiet you would not want to be more than a foot off the ground or traveling at much more than walking pace, at a minimim you will get hurt, I fly small rotorcraft in continual autorotation and even that gets your attention when the noise stops.
2. Throttle back and push forward on the handlebars ( ) to to increase AOA and maintain altitude as speed reduces.
As I think you know from the smiley, there are no articulated handlebars on the pf and the suggestion that these might assist with control has been flatly rejected. If the handlebar, gimbal joint arrangement was adopted I think many concerns would be lifted.
In the absence of this kind of articulation the approach is "Throttle back, bend head back at the neck and feet back at the knees" and I guess wait for rotation to take place. I guess with really supreme control, it would be possible to progressively throttle back up to perfectly counter loss of aero lift, but again without calculations it is hard to intuitively whether or not this re-establishes forward acceleration disrupting the braking manouver. It is a very long time since I designed control feedback systems, but the kind of analysis we used to do would be really helpful for building confidence in the pf.
Being fair to Manolis I guess many radical new ideas had no calculations behind them, they were purely based on experimental trial and error. I'm totally open to being persuaded through experimental evidence, but not by word (or for that matter GIF) salad (registered to Rodak)
were tested on the roads for long, in various modes (from climbing slowly (with long gear in the gearbox) on steep uphill roads, to following city traffic, to “racing” on the highway (at 7,000rpm the first, at 9,000rpm the second, way above their factory red lines):
Surprisingly, these handmade VVA (Variable Valve Actuation) mechanisms proved (despite their bad manufacturing quality and their improper materials) the most reliable thing on the cars.
So, when the time comes, many-many tests will be performed.
But (hello Rodak) to make tests in order to prove that the laws of physics are correct . . .
You also write: “When the engine goes quiet you would not want to be more than a foot off the ground”
This is exactly the idea for long term reliability tests and for training:
To fly no more than a foot from the ground, or, better, no more than a few feet (say 1 to 10) above the water.
Imagine someone climbing on a tall / rough mountain, flying just 1ft above the rocks.
Or jumping from rock to rock with the Portable Flyer on his back taking the, say, 95% of his weight.
You also write: “You do have some good ideas, but never the less still a compromise. Thermal loading for example.”
Thanks.
Cooling Outside the engine:
The cooling fins of the OPRE Tilting engines of the Portable Flyer are around the combustion chamber at the middle of the engine.
The cooling fins are eccentric relative to the propeller rotation axes, i.e. the cooling fins are where the downstream of the propellers is near maximum.
The cooling fins can substantially increase in size without affecting the design.
Cooling inside the engine:
The fresh charge entering into the “crankcase” (which is nothing but the inner side of the piston) is cooling the (rid of wrist pin) backside of the piston crown (the wrist pin is at the other end of the piston).
Scuffing:
The pistons take the thrust loads at the cool side of the cylinder, away from the hot exhaust ports (where, typically, the scuffing starts).
Liquid cooling:
The water cooling, which is always an option, is easier but it adds complications and weight (the worst things for a wearable Personal Flying Device).
Come on manolis, the view is great. Get that thing finished and flying. Flew my 'compromise' again today, great fun, I especially love that twostroke crackle.
GP twostroke motors went away from direct air cooling fourty years ago, too many accidents with seizure, dead racers and low power density, thermal loading issues.
But I expect you do know better than those designers so I am reluctant to mention it.
It would seem the only place for an air cooled aero engine is if you are being shot at, the war. Ive seen a picture of a radial engine that had a whole cylinder shot off and they still flew home. Water cooled a completly differnt case, one little hole in the coolant circuit and you are done for.
Yet the water cooled engine had a much higher power density, thats why they perservered with Liquid Cooling. Thermal Loading.
Last edited by uniflow on 21 Sep 2020, 11:08, edited 2 times in total.
I suspect because the maths is probably not something he is familiar with.
This is not a snide comment or a criticism by the way - like I said above, some of the best inventions were developed ahead of the maths that explained them. Whilst the maths to do this modelling is well established, it probably remains well out of reach of most shed tinkerers.
It is tempting to continue try to help, but I think we know it won't make any difference. I think best not to get wound up by it, and sit back to wait for Manolis to show us the videos of his test flights with the PF. He has a working engine and the frame developed. Given the simplicity of the flyer it can't be long until it is flying. You never know we might all be pleasantly surprised.
I suspect because the maths is probably not something he is familiar with.
This is not a snide comment or a criticism by the way - like I said above, some of the best inventions were developed ahead of the maths that explained them. Whilst the maths to do this modelling is well established, it probably remains well out of reach of most shed tinkerers.
It is tempting to continue try to help, but I think we know it won't make any difference. I think best not to get wound up by it, and sit back to wait for Manolis to show us the videos of his test flights with the PF. He has a working engine and the frame developed. Given the simplicity of the flyer it can't be long until it is flying. You never know we might all be pleasantly surprised.
I agree with that actually. I do have safety worries and obligations as an engineer, but the PF is out of my hands.
I also posted a nice historical overview of scale model making for flight stability / control testing, which is a reasonable proxy for those more practical inclined.
(The third option is maths free software simulation, I wonder if you could mock this up in KSP or another free body physics flying game. I'm not a gamer so I don't know what's available here these days unfortunately)
) to to increase AOA and maintain altitude as speed reduces.
.