2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello Tomy Cookers.

You write:
“VTOL lift jets were designed 60 years ago with thrust/weight ratios of 40
jet exhaust velocity was presumably rather lower than the conventional”


In the case of the JetCat P550 turbine of the DeltaWing, the maximum thrust is 550N, while its weight (without fuel) is 4.9Kg = 49N, giving an impressive thrust to weight ratio of 11:1.



But this is the half truth.

Because the P550 needs a lot of fuel to operate for a “decent” duration of time, lowering substantially the “net” thrust-to-weight-ratio and the net take-off thrust.

Let’s make a few calculations:

For a 20 minutes flight at maximum thrust, the take-off weight (including the required fuel) of the P550 becomes:

49N (its own weight) + 1600ml/min * 0.008N/ml (fuel specific density) * 20min = 305N,

with the required fuel (for one turbine) being 32lit (fuel weight: 256N).

This leaves a net thrust of 550N – 305N = 245N ( ~25Kg, 55lb) at take-off, making the “net thrust to weight ratio” 1.8:1 at take-off.

If the flight duration halves (10 minutes), the “net thrust to weight ratio” (including only the turbine and the required fuel) increases to 3.1:1 at take-off.

If the duration doubles (40 minutes at maximum thrust), the take-off weight of the P550 (including the fuel) becomes:

49N (its own weight) + 1600ml/min * 0.008N/ml (fuel specific density) * 40min = 561N,

which means that even without lifting any other weight than its own weight and the weight of the fuel required for the flight, it cannot take-off at all (thrust to weight ratio 0.98:1).


From a different viewpoint.

Suppose the pilot weight plus the “dry” (no fuel included) DeltaWing weight is 120Kg / 1200N (only the 4 turbines weigh ~20Kg).

For a 20 minutes flight at maximum thrust you need, according the previous calculations, 4*256N = 1024N of fuel (or 128liters / 35 gallons of fuel), which gives a total take-off weight of 2224N, which is above the total thrust (2200N).

That is, even if pilot’s legs / back can bear the 220Kg total weight at take off, the DeltaWing cannot take off, at all.


That is, the impressive thrust to weight ratio of the jet turbines says the half truth; because the less than low “overall” fuel efficiency of the jet turbines requires a lot of fuel for a decent flight duration (or for a decent flight range).


Compare the above with an OPRE Tilting driving a pair of contra rotating propellers:



For short flight durations (say, less than 5 minutes), and forgetting the ownership and running cost, the extreme noise etc, the turbines appear a better choice.
But for longer flight durations, the piston engine driving propellers is better, or far better, or the only solution.

Imagine a pair of such propulsion units replacing the JetCat P550 turbines of Rossy’s DeltaWing.

Thanks
Manolis Pattakos

[Tommy Cookers]

VTOL lift jets were designed 60 years ago with thrust/weight ratios of 40
jet exhaust velocity was presumably rather lower than the conventional

the Bristol Siddeley BS 59/7 (7800 lb thrust T/W ratio=14) had 1080 fps exhaust velocity
a 119 seat airliner intended to use 32 of these
the BS 59/8 (48"x51" 7850 lb thrust) had a BPR of 3.75 and 650 fps exhaust velocity)
the BS59s were never made

the Rolls-Royce RB 108 RB 162 and many others were
14 RB 181s and 1 RB168R were to be used on each 'VTOL Starfighter' CL704

A.A. Griffith invented liftjets (after inventing fracture mechanics/metal fatigue and aerofoil turbine machine design)
with liftjets 'small is better' (up to a point of course)

https://www.secretprojects.co.uk/thread ... j-99.2069/

[J.A.W.]

VTOL lift jets were designed 60 years ago with thrust/weight ratios of 40
jet exhaust velocity was presumably rather lower than the conventional

the Bristol Siddeley BS 59/7 (7800 lb thrust T/W ratio=14) had 1080 fps exhaust velocity
a 119 seat airliner intended to use 32 of these
the BS 59/8 (48"x51" 7850 lb thrust) had a BPR of 3.75 and 650 fps exhaust velocity)
the BS59s were never made

the Rolls-Royce RB 108 RB 162 and many others were
14 RB 181s and 1 RB168R were to be used on each 'VTOL Starfighter' CL704

A.A. Griffith invented liftjets (after inventing metal fatigue and aerofoil turbine design)
with liftjets 'small is better'

https://www.secretprojects.co.uk/thread ... j-99.2069/

T-C, all these engines are too big for utilisation as a jet-pack though, & isn't it a fact that
gas turbines don't scale down so well, esp' in terms of SFC, as Manolis has pointed out?

As a fuel-fraction/endurance machine the small piston engine (esp' 2T, per thread subject)
is much more practicable, & with current electronics, there'd be no reason why a helmet-based
HUD could not provide essential operational data in a light compact package.

[manolis]

Hello Tommy Cookers.

With the weight of the fuel being several times the weight of the jet turbine engine in which the fuel is burnt, the “dry” weight is far from the “real” (fuel included) weight, and the “dry” thrust-to-weight ratio is far from the “real” thrust-to-weight ratio.


An example:

Suppose that instead of 4.9Kg, the weight of the JetCat P550 was only 490gr (the one tenth).

While this gives an impressive thrust-to-weight ratio of 112:1, in practice / in reality the difference is small:

Starting with a “dry” thrust-to-weight ratio 11:1, the “real” thrust-to-weight ratio at take-off, including the fuel required for a 20 minutes flight at full load, is 1.8:1 (calculation in previous post),

Starting with a “dry” thrust-to weight ratio 112:1, the “real” thrust-to-weight ratio at take off, including the fuel required for a 20 minutes flight at full load, is 2.1:1, i.e. almost the same.


A counter-example:

Take a piston engine that driving propellers provides the same 550N thrust with the P550 turbine.

With a total dry weight three times larger than the jet turbine (147N instead of 49N), giving a three times lower "dry" thrust-to-weight ratio 3.7:1, and with ten times less fuel than the turbine (26N instead of 256N) for the same 20 minutes of flight duration,
the “real” thrust-to-weight ratio of the piston engine is 550N / (147N + 26N) = 3.17:1
while
the “real” thrust-to-weight ratio of the turbine is 550N / ( 49N + 256N) = 1.8:1

Thanks
Manolis Pattakos

[Tommy Cookers]

the BS 59/8 had an exhaust stream velocity of 680 fps
ie not far from the 'propeller' stream velocity needed by the PF (without variable pitch)

the mission point where greater jet-flyer fuel overcomes lighter jet-flyer engine would be (very ?) different ...
using a modern lift-fanjet engine like a 'mini' BP 59/8'

the overall efficiency being engine thermodynamic efficiency x the stream propulsive aka inertial efficiency
as thrust comes only from stream momentum change
(so the '50% efficient' slow-speed marine Diesel is c.46% efficient at propelling a ship)

Rossy's 650 kW 'Exhaust Gas Power Output' - is this the rate of (momentum change + KE change) in stream ?
change of momentum in stream being 245 kW while fuel burn releases heat at 4100 kW
overall efficiency is 6% largely due to the 1940 fps stream velocity

changing design to give 680 fps stream velocity should bring overall efficiency to about 15%
(in hover small helicopters manage about 20%)
is there a modern mini lift-fanjet engine ? (maybe we've all gone electric)

[Dr. Acula]

I bet Yves Rossy does not agree with you.

Well, i'm pretty sure he does. Because he said that himself about his jet wing. It's meant as some kind of sporting equipment. Jet Wing flying as a sporty hobby for enthusiasts with the necessary financial backing. It's not meant to transport people from A to B.

[gruntguru]

I bet Yves Rossy does not agree with you.

Well, i'm pretty sure he does. Because he said that himself about his jet wing. It's meant as some kind of sporting equipment. Jet Wing flying as a sporty hobby for enthusiasts with the necessary financial backing. It's not meant to transport people from A to B.

To be honest, neither is the personal flyer. If Manolis can make it viable, the initial market would be sport and recreation. It would be a long time after that before it could become a prospect for transportation - if ever.

[manolis]

Hello Tommy Cookers

You write:

“Rossy's 650 kW 'Exhaust Gas Power Output' - is this the rate of (momentum change + KE change) in stream ?
change of momentum in stream being 245 kW while fuel burn releases heat at 4100 kW”

Pretty close but not exactly:

You can’t add momentum change and energy.
You can’t compare momentum change and power.
They are different things and are expressed in different units: N*sec the momentum change, N*m the energy, N*m/sec the power.


You also write:

“overall efficiency is 6% largely due to the 1940 fps stream velocity”

And largely due to the 360 fps (400Km/h) velocity of the DeltaWing / pilot.

If you take a more reasonable speed for the DeltaWing, say 225 fps (250Km/h), the already low overall efficiency of 6% drops a lot.

Thanks
Manolis Pattakos

[manolis]

Hello Acula.

You write:

“Well, i'm pretty sure he does. Because he said that himself about his jet wing. It's meant as some kind of sporting equipment. Jet Wing flying as a sporty hobby for enthusiasts with the necessary financial backing. It's not meant to transport people from A to B.”


Yves Rossy couldn’t say something different.

It is not a choice of his; it is a necessity that comes from its propulsion units.

His DeltaWing is so expensive to buy and run, has so small flight duration, emits so much CO2 and noise that it cannot be used to transport people from A to B.

If we could multiply by 5 the flight duration and the mileage of Rossy's DeltaWing, and if we could divide by 10 the ownership cost and the noise, then we would have a Personal Flying Device to transport people from A to B.

But can it be done?

Currently only the piston engine can do it.

The same is the case for the other three JetPacks (Zapata, Mayman, Browing):



Batteries have similar problems in terms of energy to weight ratio.


Any news from the GoFly - BOEING contest?

Thanks
Manolis Pattakos

[manolis]

Hello all.

Quote from https://www.herox.com/GoFly/update/3114#comments

“GoFly is thrilled to announce that Team teTra from Japan won the $100,000 Pratt & Whitney Disruptor Prize.
The $1,000,000 Grand Prize is still up for grabs so if you think you have what it takes to win it all, contact us at info@goflyprize.com.
. . .
Lighter (*** GoFly Founder and CEO) said that GoFly is deeply proud of all the teams from around the world that took up the challenge because “they – like us – believe that personal human flight is a key component of our future, not just for commuting to and from work and leisure activities, but for important needs like the delivery of medical care and disaster relief.”

At the moment, however, she explained that no team is able to meet the requirements for the Grand Prize, but “we are hopeful that teams may do so in the near future. In fact, we look forward to announcing the Grand Prize winner soon, and congratulate all of our teams on their innovation and inspiration.”
The GoFly Prize is supported by Grand Sponsor Boeing, Disruptor Award Sponsor Pratt & Whitney, as well as more than 20 national and international aviation and innovation organizations. “

End of Quote


Here is the Japanese “te Tra” Personal Flying Device, the winner of the US100,000$ “disruptive technology” award:



Enjoy its “flight” and “landings”.


Unless it is a “Top Secret”,
shouldn’t they explain what is the disruptiveness in this concept than won the only prize awarded?

Thanks
Manolis Pattakos

[Tommy Cookers]

overlapped proprotors seem to require more power than would independent (ie non-overlapped) proprotors
(btw is there something advantageous or disadvantageous in the convergent flow arrangement in the previous post ?)

eg in the hover ....
c. 40% more power is needed with 100% overlap (ie coaxial) proprotors using equal torque
c. 25% more is needed with 100% overlap when proprotor torques are matched to need according to relative position

so maybe 15% and 10% more with the (partial overlap) PF arrangement ??

and thrust efficiency falls greatly with the higher stream speeds needed for flight at the higher speeds intended

[J.A.W.]

Here below is a link to the 2020 2T Conference presentations,
some of which offer strong clues as to why future F1 is interested:

Manolis may also be interested in the SI opposed piston & BRP turbo items,
as they can pertain to his aviation usage (2T turbo - ground rated hp - to 5km altitude)

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

[manolis]

Hello Tommy Cookers

You write:
“overlapped proprotors seem to require more power than would independent (ie non-overlapped) proprotors
(btw is there something advantageous or disadvantageous in the convergent flow arrangement in the previous post ?)

eg in the hover ....
c. 40% more power is needed with 100% overlap (ie coaxial) proprotors using equal torque
c. 25% more is needed with 100% overlap when proprotor torques are matched to need according to relative position”



Quote from Wikipedia:

“Another benefit arising from a coaxial design includes increased payload for the same engine power; a tail rotor typically wastes some of the available engine power that would be fully devoted to lift and thrust with a coaxial design.

Reduced noise is the main advantage of the configuration; some of the loud "spanking" sound associated with conventional helicopters arises from interaction between the airflows from the main and tail rotors, which in some designs can be severe.

Also, helicopters using coaxial rotors tend to be more compact (with a smaller footprint on the ground), though at the price of increased height, and consequently have uses in areas where space is at a premium; several Kamov designs are used in naval roles, being capable of operating from confined spaces on the decks of ships, including ships other than aircraft carriers (an example being the Kara-class cruisers of the Russian navy, which carry a Ka-25 'Hormone' helicopter as part of their standard equipment).

Another benefit is increased safety on the ground; the absence of a tail rotor eliminates the major source of injuries and fatalities to ground crews and bystanders.”

. . .

Multirotor type Unmannes Aerial Vehicles exist in numerous configurations including duocopter, tricopter, quadcopter, hexacopter and octocopter. All of them can be upgraded to coaxial configuration in order to bring more stability and flight time while allowing carrying much more payload without gaining too much weight.”

End of Quote.


In practice, most electrical VTOL (manned or unmanned) use sets of coaxial contra-rotating rotors:



With their heavy batteries and lack of energy, any inefficiency of their rotors arrangement becomes crucial.


The same benefits exist in the case of the counter-rotating intermeshed rotors: like the Chinook CH47 where the propeller axes are parallel, or like the Flettner’s / Kaman K-MAX where the rotation axes of the intermeshed counter-rotating rotors form a significant angle:




The Portable Flyer combines both designs


Quote from https://www.pattakon.com/GoFly/DTR_1.pdf :

“Counter-rotating and contra-rotating propellers



The left upper and the left lower propellers compose a pair of “contra rotating” propellers (the one driven by the right engine, the other driven by the left engine, both rotatably mounted on the same pipe).

The right propellers comprise another pair of “coaxial” contra-rotating propellers.

The two engines operate independently from each other and can run at different revs if desired (to optimize the overall thrust and mileage).
For instance, if the lower propellers are similar (same diameter, same pitch, same design etc) to the upper ones, the lower engine may run at different rpm to align its propellers with the different air stream they “see” as compared to the air stream the top propellers “see”.

The set of the four propellers can be regarded as two “contra rotating” sets of counter-rotating propellers.
A common characteristic of both, of the contra-rotating propellers and of the counter-rotating propellers, is the higher thrust to power ratio.”

Thanks
Manolis Pattakos

[Tommy Cookers]

..... characteristic ...of contra-rotating propellers ... is the higher thrust to power ratio

how would that be ?? ....

regardless of the above ...
and regardless of snippets from Wikipedia (the famous aircraft designer) ....
my suggestion regarding effects on power requirement of interference came from ....

'Aerospace' paper 'Electric VTOL Configuration Comparison' by Bacchini & Cestino published 28.2.2019 and ...

NASA 1997 Technical Paper 'A Survey of Theoretical & Experimental Coaxial Rotor Aerodynamic Research' by C.P.Coleman

[gruntguru]

..... characteristic ...of contra-rotating propellers ... is the higher thrust to power ratio

how would that be ?? ....

Perhaps due to differences between hover thrust and thrust at speed?