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By manolis
Hello all.

From the Flyer (June 25 2018) : "UK team among ten Boeing GoFly Prize winners".

On Ausust 1, 2018 GoFly / Herox / BOEING (the big sponsor) deleted the Forum wherein the contestants of the GoFly / BOEING contest presented their opinions and their complaints.

GoFly’s / BOEING’s best response to contestants’ questions, was to shut down the annoying forum . . .

Each one of the contestants has paid 250 or 500 USD to GoFly / BOEING.
In return, all they took from GoFly / BOEING is the list with the ten winners.
No scoring, neither ranking, neither justification from the "97 judges"; just the ten winners alphabetically.

Take a look at wherein some of the Open Forum Discussions have been “partially” saved and presented:


20 minutes OR 20 miles? - Page 1 : ... miles.html

20 minutes OR 20 miles? - Page 2 : ... age_2.html

20 minutes OR 20 miles? - Page 3 : ... age_3.html

Note: the “20 minutes OR 20 miles” thread had in total 11 pages.

Design Discussion - Page 3 : ... 3Page.html

Design Discussion - Page 4 : ... Page4.html

Design Discussion - Page 5 : ... age_5.html

Debating Results of Phase I : ... ase_I.html

The above discussions are indicative and show the appreciation, the trust and the confidence of the contestants for the GoFly / Herox / BOEING contest.

When you have time to spend, read the and give your comments / questions.



Manolis Pattakos
Hmm. I see three photographs of designs that seem to be manufactured and flying, and one set of very rough CAD drawings of something that looks a little bit too likely to chop a limb off...!
The Admin Team wrote:They're here, as you know.

They don't explicitly say anything about axe-grinding, either. Perhaps that's an omission.

Just in non-aviation,then?#GA exempt?

Non Aviation
Non aviation content. Play nice – No religion, no politics and no axe grinding please

Hello Russ_H

You write:
"No offence but the performance claims for your design are collectively impossible."


The first OPRE Tilting functional "proof-of-concept" prototype weighs ~8.5Kg (19lb). See at .


This makes the 44lb (20Kg) total dry weight of the PORTABLE FLYER attainable.


In the DEVICE TECHNICAL REPORT (PDF file at ) the final speed, with fixed pitch propellers, is only 100kts / 115mph.

The 200mph in the table with the three existing JetPacks (JB11 Mayman, FlyBoard-Air Zapata, Daedalus of Browning) is for variable pitch propellers.

Is the 200mph top speed too high?
In comparison to a motorcycle running on the highway, the frontal area is several times smaller (the pilot is at supine position (like lying on the air near horizontal), which means the same top speed needs several times smaller power.


Differently speaking:
at a free fall, a skydiver easily exceeds 200mph (see the award winning video:

Suppose the weight of the skydiver is 155lb (70Kg, 700N).
The skydiver is powered by his own descending weight, and nothing else.
The calculation of the power produced by the descending weight of the skydiver is simple:
700N * 90m/sec = 63kW = 85bhp

So, 200mph top speed is attainable, too.


The overall BTE of the jet-turbines used by Yves Rossy (Jetman) Delta Wing (see in the Internet his youtube video flying over Dubai), and the BTE of the jet turbines used by the abovementioned JetPacks (Mayman, Zapata, Browing) is ~2% (not difficult to check).

Multiply by 20 the above BTE.
What this means?
That the flight duration and the range of the PORTABLE FLYER are 20 times longer (because for the same power output, the fuel is burnt at a 20 times slower rate).
From the claimed 20 miles of the JB1 of Mayman, you go to 400miles of the PORTABLE FLYER.


For those who think to participate in the GoFly / BOEING contest:

Quote from (wherein they are saved my posts (from 15 June 2018, to 1 August 2018) to the deleted forum of GoFly):

Hello Mokren. (9 July 2018)

To participate in the rest phases?
Only when BOEING will decide to take over and to turn this challenge to a decent and transparent one.

The credibility GoFly earned so far is for laugh.
They pretend their goal / vision is to foster thinkers and tinkers make people fly.

The 250 USD I paid them is nothing, but make me a sucker: that some guys, hiding behind a big name (BOEING), have deceived me is frustrating.

It is a shame for BOEING to continue as the big sponsor of GoFly.

To put it simply, what I purchased with my 250 USD is the following info:
“You are not in the ten winners of the Phase I. Period.”

Neither scoring, nor ranking, nor justification of the decision, nor a clue for the weak points the judges find in my solution, nothing at all.
Just that I am not in the ten winners of phase I. . .

And now they try to trap again the contestants with cheap “tricks” of the kind:

"Dear contestant, in order to get a REVIEW of your Phase I submission, you have first to register to phase II; and in order to register to phase II, you have, among others, to create a company, to pay insurance fee, to pay “team” fee and to give equity rights to GoFly.”

The trick / the cheating is double.
By “REVIEW” they do not mean the “fully justified resolution / verdict / decision of the judges”.
They just mean “a” review from “some” guy who is nominated as a “mentor”.

As you remember, they promised to give the scoring and ranking, and they never deliver.

Would you characterize the guy who thought the above “double trick” as a “decent guy whose vision is to foster people fly”, or as a scam?

Manolis Pattakos

End of Quote
User avatar
By Rob P
Fascinating stuff (I expect) but doesn't this belong in "Technical Discussion" rather than GA?

Rob P
User avatar
By Lerk
Your speed calculation seems to miss the fact that as well as moving sidewards, the engine needs to lift ~150kg at the same time...

I'm still not entirely sure what this post is about - I gather there are some hydrated raisins that are past their best before date due to Boeing choosing actual working devices over a highly aspirational sketch!
Hello Lerk.

You write:
“Your speed calculation seems to miss the fact that as well as moving sidewards, the engine needs to lift ~150kg at the same time...”


In the following image the pilot is leaning some 30 degrees from the horizontal plane:


The weight of the pilot is W1

The weight of the propulsion unit (engines, propellers and frame) is W2

The total weight is W.

The resultant thrust force from the propulsion unit is F and comprises the F2 component that balances the weight W1 of the propulsion unit, and a component F1 that pulls horizontally the PORTABLE FLYER (i.e. the propulsion unit and the pilot).

What it is not shown in the figure is the aerodynamic lift force on the body of the pilot.

Quote from the submission to the GoFly / BOEING contest, page 14 ( )

“The more horizontal the long axis of the pilot becomes, the higher the aerodynamic lift from his body (the ski jumpers, at “leaning” angles around 10 to 20 degrees, have a lift to drag ratio above 1.0 as the (following) plot (shows):


End of Quote.

After the above “analysis”,
Can you, please, explain what your “the engine needs to lift ~150Kg at the same time...” does mean. A drawing would help.

Manolis Pattakos
At a time when there is a clear move towards electric propulsion it seems counter intuitive to develop a new ICE concept.

I cannot answer for Lerk but during take off and landing that would be about the weight that needs lifting innit? Pilot weight 70 kg, 2 engines 20kg, fuel for two hours, electrics, clothes, boots, helmet etc etc.

Have you tried a tethered proof of concept, or a scaled down electric version?
Hello Flyin’Dutch’

You write:
“At a time when there is a clear move towards electric propulsion it seems counter intuitive to develop a new ICE concept.”

If you have to curry on the air the batteries, things are more than tough.

The following are copy-paste from the

Quote from

The EJ-1 was a submission for Boeing’s GoFly prize, which built off of 11 years of development by Electric Jet Aircraft on a gas-powered VTOL. The jetpack has a base weight of 69 pounds (31 kg) and can lift a person for 2-14 minutes depending on their weight. It is a hybrid system, as roll is controlled and balanced by a computer, but forward, reverse, and yaw maneuvers are manually controlled. The EJ-1 comes in a quick-build kit, and costs $19,995.00


Construction Specifications:
Weight 69 lb / 31.3 kg
Height 43 in / 1.09 m
Length 46 in / 1.17 m
Width overall 99 in / 2.51 m
Duct Materials Carbon Fiber
Propeller Materials CF over wood core
Frame Materials Aluminum and Titanium
Batteries 2x 96V LiPo

Performance Specifications:
Maximum Thrust 394 lbs / 178.7 kg
Maximum Payload 300 lbs / 136.1 kg
Average Flight Time 2 minutes
Maximum Flight TIme 14 minutes
Estimated Top Speed 39.1 kts / 72.4 km/h
Noise Level at 50 feet <70 dB

End of Quote

. . .

I am trying to figure out some advantages of an electric Personal Flying Device as compared to a Personal Flying Device powered by a reciprocating piston engine or by a Wankel Rotary engine.

So far the electric seems the expensive one, the heavy one, the one having quite shorter flight duration, the one having quite smaller range, the one with the slower accelerations / decelerations (maneuverability), etc.

To put it differently:
Suppose you are the seller and I am a potential buyer; what are your arguments in order to persuade me to pick the electric one?

I can’t think a good argument.

For instance, take the EJ-1, the first electric JetPack, that, according its specifications, has an “average” flight duration of 2 minutes (I suppose this duration is for the “basic”, the “cheap” (only 20,000 USD) version).

With 2 minutes flight duration, imagine the range.

Anyway, suppose it takes off and lands a mile (or two) away.

Then what?

You have to transfer it back to home to recharge?

Or some support team will follow it with a charger to recharge the batteries?

The EJ-1 (more at ) shows the present reality.

When numbers and calculations get into the discussion, the discussion gets more serious.

Manolis Pattakos
User avatar
By Lerk
Silly me, to think you might need the help of Bernoulli... FD was right in his assumption. Your second image now shows a much larger angle of attack which will increase drag - does exponentially translate?

You might want to look at the power to weight ratios of missiles!
I think your overcomplicated two stroke might need tuning within an inch of its life to get close.

Just out of interest (morbid curiosity) what rpm do you plan on the engine running???

Oh yes, and when do you start asking us to fund your project? (Sceptical moi?) :D
I find the concept of the piston-skirt flap -valve to be quite intriguing. I presume this has the effect of variable crankcase padding? I knew a man who placed fixed "padding-blocks" In a Scott 2-stroke twin crankcase,to increase crankcase compression (they were inside the piston at BDC) ,unfortunately, this reduces crankcase volume and this odd valve would appear to solve both problems.....The Scott engine developments were part of the Silk-Scott project, I had the pleasure of sitting astride the prototype Silk, but had just committed to a mortgage,otherwise I'd have ordered the delicate,rapid machine there and then.

Good luck with this novel engine...It'll be a few years yet before the power/weight ratio of batteries is anywhere near fossil fuel.....and don't forget, electric power has to cart-round the depleted battery as well as the charge-holding portion, whereas the IC engine only carries the empty casing (tank) when the fuel within is, range per unit of fuel actually improves as you use it, as you're carrying less "dead" weight!
Hello Lerk.

You write:
“Silly me, to think you might need the help of Bernoulli... FD was right in his assumption. Your second image now shows a much larger angle of attack which will increase drag - does exponentially translate?”

Below they are two plots (links from the submission to the GoFly / BOEING contest).

The first shows the relation of the frontal area with the pose of the pilot:


The second shows the relation of the drag coefficient with the pose of the pilot:


The aerodynamic drag force is proportional to the frontal area, it is also proportional to the drag coefficient.

From the above plots, you can easily see that the drag coefficient drops more than 4 times from standing to supine pose, while the frontal area drops more than 5 times from standing to supine pose.

This means that the drag force drops more than 20 times when the long axis of the pilot gets horizontal.

So, to take-off and land with the pilot standing (in order to use his legs as the “landing gearing”) and then to accelerate until the pilot to turn to supine pose (wherein his long axis is almost horizontal) is a useful characteristic of the PORTABLE FLYER architecture.


Because this way, at high speeds (i.e. at cruising, which covers more than 98% of the total flight duration) the power required drops dozens of times.

In the following drawing, the long axis of the pilot of the PORTABLE FLYER leans about 10 degrees from horizontal (angle of attack: ~10 degrees), while the propeller axes leans some 20 degrees from horizontal (yes it can: the pilot by bending his spine can rearrange the direction of the propeller axes relative to his body).


This is, more or less, the pose the pilot and the engine will have at high speeds.

At such angles (say from 7 to 17 degrees; see the plot in the #1629249 post) the lift to drag coefficient maximizes.

The total thrust force F leans about 20 degrees from horizontal.
Its vertical component F2 bears the weight W2 of the propulsion unit (engines, propellers and frame).
Its horizontal component F1 pulls the PORTABLE FLYER forwards.

The weight W1 of the pilot is taken by the aerodynamic lift force FL.

Note that the aerodynamic forces increase with speed square:

At small cruising speeds the FL is not adequate to take the complete weight of the pilot; in such a case the leaning angle changes to less horizontal and the propellers provide the necessary upwards thrust (i.e. the F2 balances not only the weight of the propulsion unit, but also a part of the weight of the pilot).

When the horizontal cruising speed is too low or zero (hovering) then the thrust from the propeller must bear the complete weight of the PORTABLE FLYER (pilot included).

At high cruising speeds the FL (aerodynamic lift force) takes all the weight of the pilot; if it exceeds the weight of the pilot, then the PORTABLE FLYER is ascending in the air.

Imagine two friends wearing PORTABLE FLYERS and flying 3m (10ft) over the Aegean Sea from Crete to Icaria (as Daedalus with his son Icarus did, according the myth).
It is a distance of some 300Km (185miles).
The two friends start with vertical take-off from a shore of Crete island, then the pilots increase quickly their horizontal speed to the best fuel economy cruising speed (say 125mph / 200Km/h, wherein the pilot is nearly horizontal (quite small frontal area, quite small drag coefficient)).
They fly for some 1.5 hour until to arrive to a shore of Icaria island, then they turn their flight to hovering and land.
The wise Daedalus was flying at low height, safely.
The daredevil Icarus made the mistake to fly high and paid the price.

You also write:
“Just out of interest (morbid curiosity) what rpm do you plan on the engine running??? “

The submission ( ) to the GoFly BOEING contest has this information.

The OPRE Tilting engines will run at 7,000rpm (and partial load) at a quiet take-off (the mean piston speed is only 7m/sec), at 9,000rpm (9m/sec mean piston speed) and heavy load at an emergency landing (i.e. when the one engine is off), at 10,000rpm maximum.
Typically the engines will run at 9,000rpm and partial load (lean burn).
Even at 10,000rpm the short piston-stroke keeps the mean piston speed too low (10m/sec), quite important for the long term reliability (to take an idea, the modern giant 2-stroke marine engines run continuously for weeks at 9m/sec mean piston speed).

You also write:
“Oh yes, and when do you start asking us to fund your project? (Sceptical moi?)”

The only I asked you was to support what you wrote (something about 150Kg etc).

Manolis Pattakos