You're All Trash*, Part 234,567,988 - DarkAero

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BJC

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The Polen Special comes to mind, too.

https://en.wikipedia.org/wiki/Polen_Special

Yet these guys want to make us believe they can make an aircraft more than a hundred
kilos lighter with more surface area and drag. Wouldn't that seem a bit... optimistic? :)
IIRC, the over 250 MPH speeds of the Polen Special were with turbocharging. And it is a single seat.


BJC
 

Topaz

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Come on gentlemen.

There is literally an entire league of aircraft that break 250mph with O-200 engines....
Ummm. Racing airplanes. With race-tuned engines pushed to the limit, tiny uncomfortable cockpits, and the smallest possible wings and tails. Not general-purpose sportplanes, in which you can reasonably head off on a realistic cross-country flight. World of difference. I don't know of any sportplanes that are doing 250 mph on a genuine, stock, 100 hp O-200. Not even Klaus Savier's hyper-modified VariEze manages that.
 

BJC

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Ummm. Racing airplanes. With race-tuned engines pushed to the limit, tiny uncomfortable cockpits, and the smallest possible wings and tails. Not general-purpose sportplanes, in which you can reasonably head off on a realistic cross-country flight. World of difference. I don't know of any sportplanes that are doing 250 mph on a genuine, stock, 100 hp O-200. Not even Klaus Savier's hyper-modified VariEze manages that.
Klaus has a modified O-360 in his modified Long EZ.


BJC
 

12notes

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I'm not sure how speed scales with power in aircraft, but they're proposing an aircraft with similar power (200 vs. 210 hp) to the IO-290 powered Pipistrel Panthera with half the weight (750/1500 lbs empty/gross vs. 1700/2900 lbs). The Panthera cruises at 208mph.

*Wild speculation based on an attempt to remember a formula from long ago and based on automobiles follows*
I think drag varies with the square of velocity, so to go twice as fast takes 4 times the power. The Panthera has a power to weight ratio of 0.0724 hp/lb, the Darkaero proposal has a power to weight of 0.133. The Darkaero has 1.84 times the power to weight ratio. This could, in theory, assuming they design it exactly like the Panthera shape, mean it could go 1.35 times as fast, which would put it at 282mph.

In reality, they are two different designs, they have different goals, and different methods. And the theory probably only applies to spherical automobiles driving on an infinite frictionless plane. And I probably got it wrong anyway, but unless there's some physics that rules this out, I think it's in the range of possibility.

As to whether this particular group can actually do it, who knows? They might pull it off, I hope they do and wish them luck in their endeavor. If they don't hit these numbers, it can still be a really good aircraft even if it's slower, and they would not be the first (or even the thousandth), nor anywhere close to the most experienced team, to miss the performance goals of their design. Let's wait and see.
 

pictsidhe

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For a given plane, parasite drag varies with the square of velocity. Induced drag inversely with the square of velocity. The combined curve has a minimum at a certain speed which is where you get max L/D. By changing the span, wing area, weight you can move that L/D to a different velocity. Achievable L/D depends on quite a few things. I think some gliders are over 50, but they need insane aspect ratios to get there. Small planes 20 is a lot more realistic. If weight and L/D are fixed, the required power scales directly with speed. The shape of the plane will change to do that, though. If you take the same plane design and just bolt a bigger engine on, the parasitic power increases with the cube of speed.
 

Doggzilla

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Ummm. Racing airplanes. With race-tuned engines pushed to the limit, tiny uncomfortable cockpits, and the smallest possible wings and tails. Not general-purpose sportplanes, in which you can reasonably head off on a realistic cross-country flight. World of difference. I don't know of any sportplanes that are doing 250 mph on a genuine, stock, 100 hp O-200. Not even Klaus Savier's hyper-modified VariEze manages that.
Yes, but the aircraft in question is using a 200hp engine, not an O-200. If its possible with an O-200 its absolutely possible with a larger engine.

The additional power also allows for more room as well.

And lets be honest here, if you put 200hp on a sailplane it could do **** near the same speed. Arent there even sailplanes that break 100 knots with 35hp?

I dont see any reason to say this is impossible, or even unlikely.

The major restriction appears to be will power.
 

Doggzilla

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Ya, but nothing a split airbrake wont fix.

Airbrakes on prop aircraft are wonderful. They allow better control at any given airspeed, because they allow increased prop wash for a given speed. 70 knots with airbrakes and a high power setting is significantly more controllable than just flaps alone and a low power setting.

If people knew how much air brakes allowed better performance on prop aircraft, almost every aircraft would have them. They really make a huge improvement in low speed handling by allowing more prop wash over surfaces while youre going slow.
 

pictsidhe

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Ya, but nothing a split airbrake wont fix.

Airbrakes on prop aircraft are wonderful. They allow better control at any given airspeed, because they allow increased prop wash for a given speed. 70 knots with airbrakes and a high power setting is significantly more controllable than just flaps alone and a low power setting.

If people knew how much air brakes allowed better performance on prop aircraft, almost every aircraft would have them. They really make a huge improvement in low speed handling by allowing more prop wash over surfaces while youre going slow.
The problem is, that if you optimise for a high cruise speed, you need a high wing loading and have a small wing. That means a high stall and landing speed. Things get increasingly hairy. Or you need high lift add ons such as slotted flaps that multiply the complexity.
 

mcrae0104

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1500lb, L/D 10, 70% prop efficiency, 200mph needs 110hp
That is a true statement. However, while L/D of 10 doesn't sound that high, L/Dcruise is not usually as high as L/Dmax on a powered airplane. A 180hp Falco, for instance, is much closer to 8 than 10. Yes, a plane could be made slicker than that, but the skepticism of this new design is warranted. The darkaero promotional numbers could require L/Dcruise betwen 8.5 to 10.5, depending on prop efficiency and what they're calling a cruise throttle setting. Good for them if they can accomplish it; if not, it will seem foolish to have counted the chickens in public before they hatched.
 
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DangerZone

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I'm not sure how speed scales with power in aircraft, but they're proposing an aircraft with similar power (200 vs. 210 hp) to the IO-290 powered Pipistrel Panthera with half the weight (750/1500 lbs empty/gross vs. 1700/2900 lbs). The Panthera cruises at 208mph.

*Wild speculation based on an attempt to remember a formula from long ago and based on automobiles follows*
I think drag varies with the square of velocity, so to go twice as fast takes 4 times the power. The Panthera has a power to weight ratio of 0.0724 hp/lb, the Darkaero proposal has a power to weight of 0.133. The Darkaero has 1.84 times the power to weight ratio. This could, in theory, assuming they design it exactly like the Panthera shape, mean it could go 1.35 times as fast, which would put it at 282mph.

In reality, they are two different designs, they have different goals, and different methods. And the theory probably only applies to spherical automobiles driving on an infinite frictionless plane. And I probably got it wrong anyway, but unless there's some physics that rules this out, I think it's in the range of possibility.

As to whether this particular group can actually do it, who knows? They might pull it off, I hope they do and wish them luck in their endeavor. If they don't hit these numbers, it can still be a really good aircraft even if it's slower, and they would not be the first (or even the thousandth), nor anywhere close to the most experienced team, to miss the performance goals of their design. Let's wait and see.
In theory, this could be feasible.

However, let's be practical for a couple seconds. Let's check their numbers with a rule of thumb as we usually do to verify the Mass&Balance sheet of a new aircraft we'd intend to test fly.

The engine is 108kg, exhaust pipes a couple kilos more, prop&spinner with variable pitch about fifteen to twenty, dynafocal mount with screws and wirings up to ten... We're looking at 140kg firewall forward out of an 340kg airplane. Hence this leaves us 200kg for the rest of the aircraft. Minimalistically, let's make a list of required components to make this aircraft fly.
Horizontal and verticla tail - at least 10kg
Control push-pull rods or cables with stick and trim - up to 5kg
Seats - 5kg
Seatbelts - 1kg
Carburation (tanks, tubes, pumps, fuel indicators&electrics, etc) - 10kg
Dashboard and instruments (minimalist EFS with speedometer, anemometer, sltimiter and compass) - 5kg
Electrical wirings with battery - 5kg
Brakes, brake lines, discs and brake levers - 2kg
Pipt tube(s) with static ports and tubings - 1kg
Front landing gear with wheel and tire - 10kg
Main landing gear with wheels and tires - 20kg
Additional equipment including fire extinguisher, first aid kit, vests, etc - 25kg
TLB and other airplane&pilot books, licences, paperwork - 1kg
This comes tipically to a rough 100kg weight in ultralight aircraft, with lightest available components on the aviation market.

This leaves them 100kg for the wings, wing to main gear hardpoints, fuselage and interior. Since this aircraft does not have a wingspar to which a typical main landing gear would be attached, we can break down the following weights according to typical CF wings of similar lightest volume and required stregth.

Wings in CF with flaps, ailerons and main landing gear hardpoints - 75kg
Fuselage - 50kg
Interior - linings, mats, decoration, foams, ventilation, heating/cooling, lights - 15kg
Total - 140kg

Hence the Dark Aero dry weight might turn out to be around 380kg when they finish the aircraft, without oil and other fluids. This is if they make it with lightest aircraft components on the market and with good CF engineering skills so it can withstand cruise speeds of 275mph/240kts. The most common mistake when building in CF is that designers miss the estimated weights by 10% to 20%. We can mention the Millenium Master / Blackshape Prime and the Peregrine Tarragon two seat aircraft which were initially planned to be only 285kg but ended up more than 340kg heavy when finally finished. The Blackshape Prime aircraft can fly at 390km/h (210kts) with the 130HP EPA power turbocharged engine, and it has less frontal surface area. With 200HP and 380kg, the DarkAero has a chance of achieving the 240kts cruise at approximately 90% power (according to UL specs 2800rpm/180HP) and an average built pilot inside, we will see how much they will miss the empty weight when the aircraft is finished.

With a lighter fuselage and wings (<100kg), the pilot should be seated much further behind the CG of this aircraft (according to the bird view picture and engine specs on their website) or an additional dumbell weight should be positioned in the tail when flying solo. It would simply be too nose heavy, or would not meet the good weight&balance criteria for an efficient and economic cruise.

Conclusion:
In theory, it could fly a 240kts cruise. In reality, they will most probably make a heavier aircraft which cruises at a much lower speed when the throttle is set at 75% of power. At 150HP/2400rpm continuous and with good aerodynamics, don't be surprised if this aircraft eventually cruises 200kts. Beyond 220kts, the wings should be balanced, too. I am really curious how they will resolve the issue of wing flutter above 220kts with the light multiple spar wings. It will certainly be interesting to see their progress.

It reminds me of the Impulse microlight aircraft, but with tricycle gear.

[video=youtube;6K2n3sJL-NM]http://www.youtube.com/watch?v=6K2n3sJL-NM[/video]
 
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Topaz

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Yes, but the aircraft in question is using a 200hp engine, not an O-200. ...
In post #36, you said:

Come on gentlemen.

There is literally an entire league of aircraft that break 250mph with O-200 engines.

Where is everyone getting the idea that this is not possible? It's been possibly for 20+ years.
I was merely commenting on that post.
 

Tom Busey

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This is a really interesting discussion. Have folks followed the build for the last year? Any comments?
 

BJC

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They are young, eager, energetic, and overly optimistic in every aspect.

I wish them luck.


BJC
 

dino

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Empty weight of the final product will be the defining metric. It looks like they are well on their way to achieving 340kg. Anyone care to speculate about stall speed?
 

ScaleBirdsPaul

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I’ve been following them on Instagram. Seems like they’ve got a real nice setup and are making serious progress. I hope they succeed. My only thoughts on the project are that in a recent post they stood by the partially assembled airframe, and it was smaller than I expected. Not surprising given their cruise and efficiency goals. Also, the construction technique is interesting, it’s being fabricated in more smaller panels/pieces than I would expect from a fully composite airplane. I’m not a composite guy so I’m not passing judgement, just an observation.
 

wsimpso1

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I'm not sure how speed scales with power in aircraft.
...
I think drag varies with the square of velocity, so to go twice as fast takes 4 times the power.
Nope. Twice as fast takes eight times the power at constant drag. Derivation follows:

Drag varies with square of speed. Work is force through a distance. Take work done and divide by the time it took to do it, and you have power which is also force times speed. Sooo, power with same Cd and frontal area is speed cubed.

Billski
 
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