Can you scale the thrust to weight ratio of a turboprop?

[nwopilot]

Ok this may be a very dumb question to anyone with an engineering background, but I have genuine curiosity.

Lets assume money and runway length are not a factor.

Is it possible to make an exact replica of a PC12, but for it to be 10 times the size. I want the engine to be the exact same engine, just each part of it would be 10 times the size. I want the fuselage to be 10 times the size. I want everything to be 10 times the size (I guess the cockpit and pax seat sizes can be adjusted).

Is this even possible from an engineering standpoint? Will the aircraft be able to maintain the same thrust to weight ratio if everything is exactly 10 times bigger? What about 50 times bigger?

Would a turbojet be any different?

[Boreas]

Off the top of my head, the gigantic propeller up font would be a problem... It wont work due to centrifugal stresses and shockwave formation.

And the engine wouldn't be scalable. Fluid dynamics would require re-engineering of the core and likely the auxiliary components. Also, a fuselage 10x the size would presumable have several "decks" of passenger seating that would need to be integrated into the current pressure vessel - an engineering nightmare. The list goes on.

Bottom line, if you're gonna make something even twice as big, much less an order of magnitude bigger, its best to start from scratch.

[digits_]

Ok this may be a very dumb question to anyone with an engineering background, but I have genuine curiosity.

Lets assume money and runway length are not a factor.

Is it possible to make an exact replica of a PC12, but for it to be 10 times the size. I want the engine to be the exact same engine, just each part of it would be 10 times the size. I want the fuselage to be 10 times the size. I want everything to be 10 times the size (I guess the cockpit and pax seat sizes can be adjusted).

Is this even possible from an engineering standpoint? Will the aircraft be able to maintain the same thrust to weight ratio if everything is exactly 10 times bigger? What about 50 times bigger?

Would a turbojet be any different?

If you make a fuselage radius 10 times bigger and the length as well, your weight would be roughly 100 times the original weight.

Even more if you increase the thickness of the material to give it extra strength.

[Meatservo]

If you're talking about just putting a PC-12 into an imaginary "growing machine", I imagine for metallurgical and fluid-dynamic reasons you'd have trouble getting the fuel control unit to function properly, in particular the pneumatic components with their tiny orifices and bellows and "calibrated leaks" and so on. It would still presumably be using 1:1 scale air and fuel, so things like fuel nozzles would be ridiculously oversize: if the HP fuel pump could still move the fuel at that scale turning at its designed speed without cavitating, the ten-times-bigger orifices in the nozzles would be like firehoses.

Also I feel the plane would be extremely heavy. A ten-times-bigger PC12 would have a wingspan of 160 meters, which is bigger than the biggest plane now in existence, that weird "Stratolaunch" thing at 117 meters. Nevertheless, I doubt planes that size use metal that is ten times thicker than the metal on the PC-12. Maybe they do, I don't know, but it occurs to me that if you want a plane that is ten times bigger, you can't just build the exact same structure with metal that is ten times thicker. Metal ductility and frangibility don't scale linearly and you'd probably end up with some pretty weird stresses in the scaled-up version. Actually it would probably end up being massively over-built. A PC-12 that has been scaled up ten times isn't going to be ten times heavier. It's be more like a thousand times heavier. A Boeing 737 is more than ten times heavier than a PC-12 and it's definitely not ten times bigger. Your giant PC-12 would probably be too heavy to fly.

There's all sorts of things I can think of. Who knows what the battery would do at that size. With the cathode and anode plates having 100 times the area the amp/hour rating of the battery would be massive, but the voltage would probably still be 24 volts due to the chemistry. But the plane's wiring, being ten times thicker, would draw a lot more power than the little version. A LOT more. Could the battery withstand being discharged at that rate? I don't actually know. Would giant relays still function properly at only 24 volts? I don't know that either. Would giant capacitors still have the same capacitance? Therefore would giant radios still recieve on the correct frequencies? Don't know.

For insight, you can look on "Youtube" and find radio-controlled models that are ten times smaller than their fullsize counterparts. Even turbine-driven ones... I'm not kidding. Again, if you scaled one of those up you'd end up with something a lot beefier and heavier than a real PC-12... if you shrank a PC-12 ten times, the aluminium would be like tin foil and the engine parts would be too small to work properly with full-sized air.

Interesting question for sure. But your giant plane probably wouldn't even start, and even if it could taxi out of the craters it would push into the asphalt, it would be way too heavy to fly.

[AuxBatOn]

When you scale an air vessel up, some aerodynamic issues that were masked by a smaller scale will be uncovered at larger scale. Remember the Super Hornet was “just a 25% bigger Hornet” (it was actually considered a minor modification originally, sort of an expansion of an approved type design. They had significant handling qualities issues that took a lot of time and effort to find the causes and fix them.

[Chris M]

Compare, for example, a 737-600 to a 777-200. The 777 is twice-ish the length of the 737 (209' vs 102') but the 777 is between 3.8 and 5.3 times heavier, and has 3.5 to 5.8 times the thrust.

For a basic solid the weight of an object increases proportionally to any dimensional increase, cubed. Eg, an imaginary 1x1x1' cube that weighs 1 lb, if doubled in size, will be 2x2x2' = 8 cubic feet and 8 lbs. Complex structures aren't quite the same - there are efficiencies gained as you increase the size and therefor the weight of the aircraft don't increase at the same pace, but it is still a non-linear relationship. Make something X times the size and you get a lot more than X times the weight, and in the airplane world that means a lot more than X times the power required.

I ran into a similar issue while designing a set of floats for my RC Cessna 182. If I model the floats as a direct scale down of the real thing I'll end up with way more flotation than a real plane would have. The displaced volume of the floats is a cubed relationship to the scale factor, and because the medium being displaced, water, is of uniform density, the flotation capability is also a cubed relationship. However, the weight of the model isn't so well related to its scale factor and ends up weighing quite a bit less than its "scale weight". The plane is gratuitously overpowered so I may end up ballasting it down somewhat to get closer to the weight it should be.

[Tailwind W10]

nwopilot, from a layman's perspective it seems reasonable, but as has already been mentioned, it's a lot more complex than just scaling.

digits_ you've mentioned a squaring relationship for weight, but as weight corresponds to volume (3 dimensions) rather than area, you'd be closer to reality to say the new plane would be 1,000x heavier.

Back to the original question, scaling up a PC-12 x10 would result in an airplane 473' long, 534' wingspan and 140' high. That's approximately twice the dimensions of an A380! 4x the area, 8x the weight!!! Needless to say the engineering challenges would be daunting. Did you really want to do it with a single engine?

When designing an airplane, the starting point is the mission the airplane must perform. Dimensions, power requirements, speeds, loadings, materials, all things are governed by that mission. In your thought experiment, what would the mission of this airplane be?

Gerry

[digits_]

digits_ you've mentioned a squaring relationship for weight, but as weight corresponds to volume (3 dimensions) rather than area, you'd be closer to reality to say the new plane would be 1,000x heavier.

True, but the fuselage of a plane is mainly air (hollow), so it won't be 1000 times heavier, it's not solid. Between 100 and 1000 times would be a good guess.

Chris his example mentions the comparison of a boeing twice the length and it works out to a weight factor increase of 3.8 to 5.3, so closer to a squaring relationship.

[telex]

Seriously, how much pop and chips do you need to move?

[iflyforpie]

Aerodynamically the Reynolds number comes into play.

A Reynolds number is a dimensionless number which in aviation is primarily used to properly scale wind tunnel test results to the life sized aircraft. It’s based on flight speed times wing chord divided by the kinematic viscosity of the air. In order for a model or small aircraft to have the same flight characteristics as a larger one, the Reynolds number must be identical.

[Tailwind W10]

digits_ you've mentioned a squaring relationship for weight, but as weight corresponds to volume (3 dimensions) rather than area, you'd be closer to reality to say the new plane would be 1,000x heavier.

True, but the fuselage of a plane is mainly air (hollow), so it won't be 1000 times heavier, it's not solid. Between 100 and 1000 times would be a good guess.

Chris his example mentions the comparison of a boeing twice the length and it works out to a weight factor increase of 3.8 to 5.3, so closer to a squaring relationship.

Point very well taken. All material thicknesses will need to be increased to take the loads of the larger and heavier structure. If those thicknesses end up being 10 times larger than the original, then the weight increase by the cube relationship would hold. Bottom line one would need to engineer the airframe to find out for sure.

Gerry

[nwopilot]

This is why I love av canada. Unique perspectives on design — generating thoughts that had never crossed my mind.

Seriously, how much pop and chips do you need to move?

This comment had me in tears

In your thought experiment, what would the mission of this airplane be?

Gerry

I really has no planned mission for the aircraft ( I guess it would look kinda cool in my backyard ). Well...I was looking through some online stuff on single engines, and I was kinda suprised to find out that the largest (heaviest) SE turboprop was the Air Tractor. I can see the obvious reasons why no one wants a large single engine aircraft, but it got me thinking about the complex challenges that might arise. If we had unlimited resources, what is the largest (heaviest) SE turboprop that we could develop?

[goingnowherefast]

The tiny AT-802 is only 16,000lbs.

There were some pretty big single engine piston aircraft in WW2.
P-47 17,500lbs
Avenger 17,900lbs
Guardian 22,600lbs
Skyraider 25,000lbs
AM Mauler 25,737lbs

Douglas Skypirate was never fully developed but it was 34 760lbs

There were also quite a few military aircraft in the 50s that had high powered turboprop engines, but not many of them left the testing phase.
Douglass Skyshark 23,000lbs
Westland Wyvern 24,550lbs
Republic XF-84H 27,000lbs

I'd imagine the only limitation would be the power requirements and resulting propeller size. At some point you need more power than you can build a propeller to handle. Longer blades mean you move the engine higher up, but then you have huge landing gear, and it starts to turn into an ungodly thing. Counter-rotating propellers and adding more blades help the issue, but there is still a limit. I suppose you could have one engine driving multiple propellers mounted on the wings, but then what's the point? The gear boxes and drive shafts would be so complex, it's ridiculous.


There are also some pretty large single engine jet aircraft that don't suffer from the "too big to fit" propeller problem, but this is a thread on turboprops.

[Tailwind W10]

... what is the largest (heaviest) SE turboprop that we could develop?

Here's a couple of examples other than those listed already.
Back when turboprops were first being developed they needed test beds
http://aerofiles.com/boe-b17turbo.jpg
https://i.pinimg.com/originals/44/89/27 ... 9c9848.jpg
Note the pistons are feathered.

This is evendently a Wright XT-35 engine of about 5,000 hp

The TU-95 Bear bomber had the Kuznetsov NK-12 engine of about 15,000 hp. Looks like it was mainly used on multi engine airplanes, the only example I can find where it was used singly was in the A-90 Orlyonok, which isn't a true 'airplane', but a vehicle that spends all it's time in ground effect.
https://upload.wikimedia.org/wikipedia/ ... onok_4.JPG

Gerry

[photofly]

XF-84H:

Lin Hendrix, one of the Republic test pilots assigned to the program, flew the aircraft once and refused to ever fly it again, claiming "it never flew over 450 knots (830 km/h) indicated, since at that speed, it developed an unhappy practice of 'snaking', apparently losing longitudinal stability".[14] Hendrix also told the formidable Republic project engineer, "You aren't big enough and there aren't enough of you to get me in that thing again".[13] The other test flights were fraught with engine failures, and persistent hydraulic, nose gear, and vibration problems.[2] Test pilot Hank Beaird took the XF-84H up 11 times, with 10 of these flights ending in forced landings.[15]

:-O