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Comment by cjrp | Hacker News Reader

Turbofan (what's on most commercial jet aircraft), not turboprop (jet engine driving a propeller, generally on smaller commercial and some private aircraft).

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ah, neat! looks like they recently switched from 3x turboprop to 4x turbofan

edit: had to correlate sources to find that a bunch of popular aviation blogs got the original design wrong. was actually 3x turbofan -> 4x turbofan

Everyone claiming you are wrong hasn't bothered to google. Literally the first response for "overture turboprop" is a series of articles confirming what you said.

> The Overture supersonic aircraft will be powered by three turboprop engines, which includes two that will be mounted under each wing, while the third engine will be fitted at the end of the fuselage.

https://www.aerospace-technology.com/projects/overture-super...

It really must be a simple typo, it's inaccurate to the point of being nonsensical.

It’s like claiming an upcoming Tesla model will use Ni-Cd batteries. No matter how many blogs claim it, it's clearly wrong.

There is no way you could go supersonic with a turboprop

You can, it's just so loud it becomes health risk to even share a runway with it: https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech

To save others the click: that plane never reached super sonic.

What makes the turboprop itself supersonic is the blades are traveling super sonic. Normal turboprops are limited by keeping their blade tips below super sonic to prevent booms and vibration. It is not special to go super sonic at the blade tips, this experimental plane only served to confirm that.

So it is correct to say no turboprop plane has ever gone super sonic and the idea that boom is turboprop powered is wrong on its face.

That plane didn’t go supersonic. It is impossible for a propeller driven aircraft to go supersonic.

Maybe in a steep dive ? ;-)

Well, it'd be impressively noisy, anyway.

if your next stop is tbe pavement and its a one time use aircraft...

Just the tip...

they're right after all—many blogs got it wrong. maybe boom's PR screwed up

it was never a turbo prop. You'll never get a propeller driven aircraft to supersonic speeds.

And if you try you end up with monstrosities like the XF-84 Thunderscreech

https://en.wikipedia.org/wiki/Republic_XF-84H_Thunderscreech

"Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches (61–76 cm) of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run. Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews. In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H."

Even if your aircraft doesn't come close to the speed of sound, the tips of the propellers will, leading to insane amounts of noise. The Russians, as they are prone to do, didn't care and did it anyways. https://en.wikipedia.org/wiki/Tupolev_Tu-95

A max speed of 575 mph is quite impressive for a prop plane; hasn't ever been recognized by FAI though...

why the downvotes? the original design was trying to use 3x turboprops, and i can see articles on google about them switching to 4x turbofans after the xb-1 work

edit: see parent edit, i was wrong. you guys are pretty good at making someone with misinfo feel bad though

I think the logical explanation is that there was a mistake in the original article. A propeller is not the right choice for supersonic flight, I don't want to say impossible but it wouldn't be far off. They've gone from 3 jet engines to 4, and in both cases they would be turbofans.

The XF-88B Turboprop hit Mach 0.9 in level flight, but it was using an afterburner, which is rather cheating. Supposedly it could go supersonic in dives.

acknowledged in edit

You have to be mixed up; a turboprop has a propeller attached to the jet engine. No-one in their right mind would use a turboprop supersonically. It would make an insane amount of noise.

> a turboprop has a propeller attached to the jet engine.

Since this is the pedant thread I feel obliged to point out that turboprops are propellers attached to gas turbine engines, not jet engines. Jet engines are gas turbine engines that produce thrust using a jet of hot exhaust gas out the back. Gas turbine engines that don't produce thrust using a jet of exhaust gas aren't jet engines; examples are turboprop engines and turboshaft engines (popular in helicopters, some tanks, etc.) Turbofan engines produce at least some of their thrust with an exhaust jet, so it's fair enough to call those jet engines. Probably the truest sort of jet engines are turbojet engines, which are no longer used for commercial aviation and only have some niche applications remaining (for instance cruise missiles.)

Then there are the "jet engines" which aren't gas turbines at all; jetskis use gasoline powered piston engines to produce thrust using a jet of water. And rockets, which don't breath air, could be called jet engines in a sense because they produce thrust using a jet of exhaust gas. But if you go around calling rocket engines "jet engines" you're going to get a lot of people correcting you by pointing out that rocket engines don't breath air. Many rocket engines do contain gas turbines though, using gas turbines to power propellant pumps, e.g. turbopumps...

And if we really want to get into the weeds, some piston powered aircraft get a small amount of thrust from their exhaust too. And some exploit the "Meredith effect", wherein air over the radiators gets heated and produces a small amount of thrust. These effects may contribute a few percentage points of the total thrust of the plane, and in truth, some turboprop configurations do this too. But >90% of the thrust is coming from the propeller, not the exhaust.

That is true, I was restricting my vocabulary to avoid adding additional concepts.

Define “right mind”

The Republic XF-84H "Thunderscreech" was an American experimental turboprop aircraft derived from the F-84F Thunderstreak. Powered by a turbine engine that was mated to a supersonic propeller, the XF-84H had the potential of setting the unofficial air speed record for propeller-driven aircraft, but was unable to overcome aerodynamic deficiencies and engine reliability problems, resulting in the program's cancellation.

[. . .]

Unlike standard propellers that turn at subsonic speeds, the outer 24–30 inches (61–76 cm) of the blades on the XF-84H's propeller traveled faster than the speed of sound even at idle thrust, producing a continuous visible sonic boom that radiated laterally from the propellers for hundreds of yards. The shock wave was actually powerful enough to knock a man down; an unfortunate crew chief who was inside a nearby C-47 was severely incapacitated during a 30-minute ground run. Coupled with the already considerable noise from the subsonic aspect of the propeller and the T40's dual turbine sections, the aircraft was notorious for inducing severe nausea and headaches among ground crews. In one report, a Republic engineer suffered a seizure after close range exposure to the shock waves emanating from a powered-up XF-84H.

The XF-84H design top speed was Mach .9 and probably made it to .7 in testing.

https://en.m.wikipedia.org/wiki/Republic_XF-84H_Thunderscree...

Replying since I can’t edit my other post. I didn’t downvote you, I just let you know that you were very likely wrong and that may have been the source of downvotes. I presumed it was a statement made in good faith and was going to be corrected in short order and it has.

It’s the original source of the mistake that should feel bad as they’re supposed to be experts in the field.

The original design was meant to use 3x turbojets, not turboprops.

What does "turbo" mean in this context? Are these things somehow using exhaust to spin a turbine to force air into the intake faster for boosted performance? Or is it just jive, spin, marketing, to sound cool?

The XB-1 used 3 turbojet engines.

i didn't say anything about xb-1's propulsion

I acknowledge that; I was trying to bridge from the things you did say to facts about the Boom aircraft development.

Share the links?

Not sure what you were thinking of, but they were never planning on a turboprop since that's not really possible.

Since we talk about a start up selling power points to VCs, why not use tirboprops? Same as having typos in Nigerian oil prince inheritence mails to weed out the targets, sorry investors, that might think too much?

I find it incredibly hard to believe that turboprop was ever on the table.

r/noStupidQuestions ;

Can these be 'stacked' - Can you have a turboFan in line with a turboProp such that the output of the wash of the Fan feeds into the Prop, but with a portion of the wash spinning to thrust on the outer ring of output.=, via a design in the cowlings which is hyper directed thrust vents (think the grid of straws used to funnel water into a cohesive column, which can be directed)

Imagine a small diameter turboprop behind a much larger turbofan

Technically yes but it would make performance worse. Props are efficient because of their large size, which allows them to push a lot of air, if you shrink it then you give up that advantage. If you had extra power left over to turn a prop, you'd want to use it to turn a bigger fan, or just leave that power in the exhaust.

As far as has ever been discovered - no, this won't work. You can think of a fan as just a prop enclosed in a housing (jet engine). A prop loses effectiveness at the speed of sound because the air passing the prop gets a high pressure shock wave that effectively makes the prop not prop-shaped, so it can't move air. Picture it as using a hammer on glass instead of pushing on glass with your hand, one works way better than the other for generating thrust. The only way (so far) to have an effective prop is to have the prop tips move slower than the speed of sound. Engine designers worked around this by slowing the air down around the prop. To do this, they moved it inside a tube. They slowed the air down so that the prop (fan) can travel more slowly, and then they heat the air behind the prop to gain excess pressure and thrust. And this is exactly what a mach 1+ jet engine does. The opening at the front of the engine forces air in, and the design of the inlet slows the air down so it is subsonic, along with a corresponding increase in pressure. Some fans (props) which are now effective because they spin at subsonic speeds compress it more, so more air can enter the engine. Then they burn fuel to heat up the air, increasing the pressure. After that they have a few more fans that run in reverse to drive the fans at the front of the engine, and finally exhaust this hotter, bigger, more high pressure air out the back of the engine to produce thrust.

So hopefully you see how your question is an interesting one, and one that has already been sorta done. Turbofans and turboprops are really quite similar, but at mach speeds only the turbofans have the right environment to be able to work efficiently. Your idea would have the prop in a supersonic air stream, which would make it effectively useless.

>a high pressure shock wave that effectively makes the prop not prop-shaped, so it can't move air.

Leading micro eddys can solve this.

If the induction is a straight stream, it will fail - you need to direct micro eddys

If you do this with mechanical means (deflection cowlings) you will hit a limit.

The ideal design is in the funneling of eddys as they traverse in a super spiral between the front eddy and as it spirals to the thrust vector.

however, pre-ionizaton, and then magnetic ion direction can swirl the eddy to the desired output. However, AIR is not the thrust component at this time, its ionized energy which is being "thrust" (thrust is typically thought of as a 'push' - but this is actually a 'pull'

Identify a spot, pull yourself to it. As opp

What about the Tu-95? It's famously loud due to (says Wikipedia) supersonic propeller tips. Do you know why they're still effective?

I am convinced that if you were to dimple the propeller (tips) such as a golf ball is pitted, you would reduce this effect.

Further, if you wer to dimple/convex in an alternating pattern the leading edge of any aero ... efficiencies would increase.

Micro-dimples are better.

Understand the eddys, as Da vinci would say....

The Tu-95’s props are paired for counter-rotational torque balance. The second prop does not add additional thrust.

The supersonic speed was the question, not the contrarotation.

Well, samstave's original question was one of adding power with an inline stack. As I understood it, that isn't the purpose for he Tu-95's pair of props since they share a power source. The below explanation [0] has some interesting analysis based on Russian language documentation about how torque is divided between the prop pairs. Additionally the paper linked from the Wikipedia contra-rotating prop page "Analysis of a contra-rotating propeller driven transport aircraft" [1] has a great section on fuel savings, which probably has contributed to the Tu-95's success.

Thus, the front prop gets almost 20% more torque than the rear prop.

0. https://aviation.stackexchange.com/questions/74787/why-dont-...

1. https://www.rcgroups.com/forums/showatt.php?attachmentid=281...

Thank you all TIL so much.

I believe they're sacrificing efficiency for performance.

Edit: removed battery voltage analogy.

You might be interested in reading this stack overflow answer about differences between turbo prop, jet, and fan. https://aviation.stackexchange.com/questions/71301/what-is-t...

Another aspect to consider is changing engine characteristics for different conditions. An example is changing the structures guiding air into the engine.

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

This is why we CAN have nice things.

If everyone were to take their response as if they were educating a youngling, to grow with an understanding, the world would be a better place.

Why would you do that though, instead of just running a multi-stage turbofan such as they used on the F-100 engine (F15 and F16).

"can" vs "can come up with an implementation that provides any performance benefits in any set of real world circumstances"

You could. But there's no way the efficiency and complexity penalty having props feeding fans or fans feeding props comes out ahead of "pick one and make it bigger"

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