What type of transport works best for you

(stella wood) #1

(VĂ©ronique Langlands) #2

I rather fancy a palanquin or sedan chair.
My absolute favourite would be an electric microlight :slight_smile:

(anon53589976) #3

I have a petrol powered microlight. The dragging behind of the extension cable puts me off electric ones.

(stella wood) #4

Sadly, many of the real Elders of our commune are no longer with us… but I have listened to the tales of their youth, with rapt attention.

The amount of walking they did… kilometre after kilometre… and cycling eg…from Limoges to Perigueux to visit friends and vice versa … yikes…

(Jane Williamson) #5

I used to cycle 11 miles to our family farm and back again in my teens.
We are too far from shops etc now and have to use a car.

(anon53589976) #6

So did I and to school 16 miles each day in wind and rain etc, we didnt get taxied there in a range rover by mum in her pyjamas.

(VĂ©ronique Langlands) #7

“I have a petrol powered microlight.”
Isn’t it terribly noisy? I don’t actually have a clue about microlights, I just love the idea of flying about silently in a small thing (and it is hypothetical so if electric power won’t work something else would have to power it.)

(anon53589976) #8

Vero, it isnt anymore noisy than a passenger car. The strict noise regulations make it so. There is an electric powered light aircraft near where i work, and that makes a phenominal amount of noise on take off and cruise. The noise is not due to the motor but
the propeller (tips). Also like the cars, electric powered aircraft dont have any range, basically if you can still see the socket then thats ok.

(Paul Flinders) #9

You’d think that they’d work to reduce that as it almost certainly points at aerodynamic inefficiency - which you wouldn’t want in an electric powered craft.

Mind you - you’ve piqued my curiosity, I didn’t even know there regular “electric powered light aircraft” beyond ultra light research stuff.

(Mark Robbins) #10

I thought the noise was the propeller tips moving at supersonic speeds, each creating mini sonic booms. Might be wrong though…

(anon53589976) #11

Correct, if the tips move at more than 400 fps, they produce mini sonic bangs. So the onus is to keep the RPMs down so they remain under this limit.

(Paul Flinders) #12

Why only 400 ft per sec? - speed of sound in air is 343m/s or 1125.3 feet per sec - surely they have to go faster than that to generate a sonic boom.

That said I am mildly surprised to discover that we are talking about the possibility that the propeller tips on a fairly modest aircraft will or could be travelling at supersonic speeds.

Now I know nothing about flying so I’m only trying to educate myself here - google tells me that for a light aircraft (Cessna type thing) 2400rpm and a prop diameter just under 2m are good numbers for the back of this particular fag packet

So 1.85 x 3.14 x 40rps = 232m/s

Which is a good way from supersonic.

Edited to correct my maths goof (and content which then no longer made sense).

(anon53589976) #13

My apologies, it should read 400 mph. This is taking into consideration the air temp and humidity and a couple of other factors. Normally they will be kept under 0.92 mach.

(anon53589976) #14

Cessna 172 with a standard Engine power of approx. 200 hp, propeller diameter of 80 “and max take off RPM 2700.

At take off the tips of the prop are moving at around 643 mph or 938 fps. At cruise rpm’s of 2300, the tips are moving at 540 mph, or 790 fps.

Calculation based on a temperature of standard 20°C, and 1013 hPa pressure.

That is why the spam cans make so much noise. Around 85 – 90 dba and ultralights at around 70 to 75 dba.

(Paul Flinders) #15

So, well under supersonic

I made an error above - 2400rpm is, of course 40rps (corrected).

(carl tunnicliffe) #16

Speaking as a private pilot myself, you really dont want your propeller tips going supersonic - highly inefficient. The only way a propeller aircraft moves forward through the air is the action of the propeller turning and the difference in pressure between the forward facing and rearward facing sides of the blades producing lift (much like the wing but in a vertical plane as opposed to horizontal). Introduce supersonic shock waves into the equation and this affects the lift which affects the thrust (pull of the propeller through the air) and causes damage to the propeller. It has been known for a prop to disintegrate in flight as a result of shockwave damage.

The primary reason a light aircraft makes so much noise when flying is that, until recently, nearly all these aircraft had no silencers on their exhausts. Extra, unwanted, weight. As the majority of light aircraft flying today are quite old in comparison (the Cessnas and Pipers of this world) the modern legislation has not caught up with them.

And as for electric aircraft - again, it is a weight problem. Batteries are very heavy and the more batteries you have, the less of anything else your aircraft can carry. Hence the only electrically driven aircraft that have seemed to have flown successfully are experimental types covered in solar cells.

(Mat Davies) #17

That gets a like just for the application of maths in the real world!

(Paul Flinders) #18

I’m glad you said that - it would sure be my gut feeling.

I’m not a pilot, would love to learn to fly but sadly lack of binocular vision is a bit of a show stopper.

Don’t know if Roger was also a pilot - he’s not around to ask now, I presume when he said 400mph he was actually talking about airspeed rather than the actual speed of the propellor tip (i.e over 343m/s or 768mph).

Again I’d have certainly thought so - taking the Cessna 172 that Roger mentioned, Wikipedia tells me that it has a Lycoming IO-360-L2A 120kW engine and a fuel capacity of 212l so engine plus fuel weigh in at maybe 330kg, compare that with a Tesla 100kWh battery which weighs in at about 540kg. You could probably build  a plane around that but even if you made enough weight savings elsewhere (presumably not easy as a whole Cessna 172 weighs only 767kg) so that the whole thing weighs the same as the Cessna the result would fly for maybe 90 minutes1 compared with 5-6 hours for the petrol-engined plane. The range would be uselessly tiny.

1] In case anyone points out that 100/120 is not 1.5 - you don't need full power output all the time.

(VĂ©ronique Langlands) #19

So is it Bernoulli’s principle at work with propellers as well as wings?

(Paul Flinders) #20

It’s questionable how much Bernoulli contributes to lift generated by aerofoils in real world conditions.