Comment by alwa

Comment by alwa 4 days ago

8 replies

View on Hacker News

(1) The turbofan category of jet engine seems to inspire a lot of very pretty animated technical diagrams—here’s one set from a German manufacturer [0]. Now if only we could convince Bartozs Ciechanowski to take on such a subject… [1]

(2) I know glider pilots who fly without any fuel at all, once aloft… sounds not unlike the 150-200km glide range that @MaxikCZ mentions at idle from cruising altitude.

[0] https://aeroreport.de/en/good-to-know/how-does-a-turbofan-en...

[1] e.g. https://ciechanow.ski/airfoil/

devilbunny 4 days ago

Aircraft that are designed as gliders are much lighter and thus have much longer glide range than aircraft that aren't. They're so lightweight that they can climb on thermals. A 737 is not going to be able to do that, but a regular glider can't fly at 400 knots.

Reply View 3 replies
  • MaxikCZ 4 days ago

    > thus have much longer glide range

    Im gonna be a little pedantic, but the weight has surprisingly small effect on glide range, actually none of the weight affect the range directly, its all from secondary effects.

    The glide is given mainly by drag and lift (so body and wing geometry), correlated to certain speed. The weight isnt in the equation at all. What weight does, is increases the speed in which the aircraft achieves this maximum glide ratio, and in higher speed you have higher drag, which finally reduces the range.

    Thats why many modern gliders have water tanks in wings, to increase the weight of the glider, moving planes speed of best glide ratio higher, allowing for more efficiency at higher speeds. Its worth it if the atmospheric condition provide strong lifts. Pilot can then dump the water in flight to reduce the wing load, allowing them to land with less speed, or just keep in the air longer as thermals get weaker in the afternoon/evening

    (source, I used to be a glider pilot)

    Reply View 2 replies
    • justsid 4 days ago

      It should also be noted that gliders have crazy aspect ratios. Airliner wings are designed for completely different flight envelopes than gliders, it’s all a game of what you optimize for and what trade offs you are willing and/or required to make.

      But of course that doesn’t mean that airliners can’t glide well, the Gimly Glider and Air Transat flight come to mind. But gliders can definitely beat an airliner in terms of performance.

      Reply View 0 replies
    • devilbunny 4 days ago

      You are, of course, correct, and thanks for clarifying.

      Reply View 0 replies
throw_a_grenade 4 days ago

Re: (2): There's a difference between sailplanes and gliders. Sailplanes are gliders that can “soar”, i.e. gain altitude just from the air that is moving up for some reason. Your friends have licence that says „Sailplane Pilot Licence”, not „Glider”.

The distinction is less pronounced nowadays, because there is no mondern aircraft designed as gliders-but-not-sailplanes, but historically there were planes that fit this niche, mostly military transport of WW1 and WW2 vintage.

Passenger jets (with engines turned off) are relatively decent gliders, but incapable of soaring. So no, you can't get more that about 20:1 glide ratio no matter how good is the weather (for sailplanes).

Reply View 0 replies
johndubchak 4 days ago

Regarding the turbofan and [0], above...if you're communicating to a non-engineer (me), how does the design get to the point of such complexity? I would love to learn the design story behind such an incredibly complex piece of machinery.

I am being serious, if you cannot tell.

Reply View 1 reply
  • Gibbon1 3 days ago

    For the same thrust it's more efficient to accelerate a large mass of air a small amount than t accelerate a small mass of air a large amount. The fan is what gives you that.

    I rough guessed the cost of fuel over a 737's life as $150 million. Where the engines cost something around $30 million. That pushes the engineering economics towards maximizing the engines efficiency.

    I'm suspicious that bypass ratio's for turbofans are close to maxed out. The diameter of the fan gets unwieldy. That was the design issue that the 737 Max was trying to get around. With bad results. Possible the future is hybrid designs with two engines and 4 or more electrically driven fans.

    Reply View 0 replies