Going to air shows I noticed that when fighter jets did slow fight, it was just as loud as high speed passes. Why is this?
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3$\begingroup$ Ignoring everything else, a slow pass is ... well slow, it takes longer. Is 100 decibels for 0.1 seconds actually "louder" than 90 for 2 seconds, in terms of perception? $\endgroup$– epsCommented Oct 9, 2022 at 0:27
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4$\begingroup$ In order to understand why engine noise and speed are at best loosely correlated, consider these two extremes: airplane is diving straight nose down with engines off: zero engine noise, high speed. And the airplane is hovering without moving at all: lots of engine noise, because there is no lift and the engine needs to use a lot of power just to fight gravity, but the speed is zero. $\endgroup$– Jörg W MittagCommented Oct 9, 2022 at 10:53
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1$\begingroup$ Fighters have low bypass engines compared to airliners. That means the exhaust comes out much faster than on airliners making a lot more noise. It is also why fighters have a much higher pitch to the engine noise. $\endgroup$– Ross MillikanCommented Oct 11, 2022 at 1:54
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3 Answers
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When fighters are going slow they still need to generate enough lift to maintain level flight. This is done by increasing the angle of attack, and coincidently 100% of the time the induced drag also increases. (but don't confuse this with a cause/effect relationship!)
As induced drag increases, more thrust is needed to overcome it. This is called operating on the "backside of the power curve". At some point almost full power is needed just to overcome the drag and maintain level flight, which is why you hear the noise.
There are lots of questions and answers dealing with drag curves that will help you understand this on a deeper level if you are interested.
answered Oct 8, 2022 at 15:23
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28$\begingroup$ Don't think of slow flight as the same as driving slowly in your car. Think of it as driving your car up a very steep mountain - you will slow down, yes, but you will also need to rev your engine up on low gear. $\endgroup$ Commented Oct 9, 2022 at 3:02
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6$\begingroup$ @quietflyer, motivation for that phraseology is to keep the aero PHDs who insist there’s no cause and effect relationship off my back. ;) $\endgroup$ Commented Oct 9, 2022 at 14:57
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3$\begingroup$ @quietflyer hence why the analogy was the slope of the mountain and not the speed of the car! $\endgroup$ Commented Oct 10, 2022 at 12:30
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4$\begingroup$ @quietflyer I'm not sure what is so difficult about "think of slow flight as climbing a hill, not as driving slowly" for a layperson. Yes, you aren't going up. So what? It's an analogy. $\endgroup$ Commented Oct 10, 2022 at 12:34
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3$\begingroup$ Ok guys, please take the automotive metaphor over to chat - this is not productive or particularly insightful... $\endgroup$ Commented Oct 10, 2022 at 15:25
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This regime has a name on the "Power Curve", it's called the "Region of Reverse Command". It's called that because as the aircraft goes below a certain speed, it requires an increasing amount of power the slower you go to retain the same amount of lift.
Being in this area of the curve can be quite dangerous, and getting outside the backside of the curve is often referred to as "being behind the curve", and is actually something I think about somewhat frequently.
Story time:
I was out flying one day with my SO and two of our friends in a 172, it had a STOL kit and 160HP conversion, it also wasn't a particularly high density altitude day (this may have saved our lives). They really wanted to check out their house from above like most first-time plane rides. We cruised up there and they called their kids out of the house to come wave from the cul-de-sac.
In order to loiter I set the plane up for slow flight like I'd practiced countless times (I actually really enjoy chugging around like this), tossed in some flaps (don't remember now how much exactly), and throttled back and pitched to bring 'er down into the laid back position that a plane takes at that point in the curve. As the speed slowed I laid into the throttle to keep us a handful of knots above stall and orbited the house for a few minutes while watching the altimeter like a hawk and they had a good ole time trying to yell out the windows.
Why would the plane's mods have saved our lives? Well, while I was above the required "safe" altitude for the area, one thing that I hadn't taken into account is how much the stall speed changes at the wing loading with 4 adults in the thing. If I hadn't had the extra headroom afforded by those mods, I may have gotten into a point in the power curve where powering out of the reverse control regime before hitting the ground wouldn't be possible. This realization is why it's indelibly burned into my mind.
This is one area of flight where remembering "pitch controls airspeed, and throttle controls altitude" can be a literal lifesaver.
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1$\begingroup$ Am I right in assuming a plane with a positive thrust-to-weight ratio simply has that "maximum power available" remain above the "power required" all the way down to zero airspeed, so that the risk of "not enough time to power out of reversed command regime" is not an issue? $\endgroup$– toweCommented Oct 11, 2022 at 9:50
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$\begingroup$ This would actually be an interesting question to pose on the site. Thinking about it I could consider a few different possible answers which would differ if the aircraft had vectored thrust or not. because the amount of thrust against gravity at some alpha will vary inversely to the lift generated by the wings, I do think the curve would look quite different, but I do not think that it would completely remove the area below the reverse command region except maybe if there was vectored thrust or if it had a VERY high TtW ratio. $\endgroup$ Commented Nov 1, 2022 at 1:31
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At very low speeds, with a high AOA (angle of attack), they are using a lot of the thrust to just hold it up in the air, rather than lift over the wings.
Also, the crowd likes it...:)
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$\begingroup$ This is the elephant in the room. Those passes could be a lot quieter, but the pilots are trained to make a lot of noise when passing over the crowds, then quieten down as they leave the show area $\endgroup$– RichardCommented Oct 9, 2022 at 7:51
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1$\begingroup$ @Richard, for a given thrust setting needed to fly at a certain airspeed, how would the pilot increase or decrease the noise? $\endgroup$ Commented Oct 9, 2022 at 14:59
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2$\begingroup$ @MichaelHall i guess pilot has option to increase drag and use different thrust setting for certain airspeed too. Let’s say full flap with huge drag and higher thrust setting to make more noise. $\endgroup$ Commented Oct 9, 2022 at 16:33
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1$\begingroup$ Well, you are free to presume that the goal is to make noise, but it is actually a slow flight demonstration and noise is simply a byproduct. Pilots are just not "trained to make a lot of noise"... $\endgroup$ Commented Oct 9, 2022 at 18:16
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1$\begingroup$ The last airshow that I saw, ended with supersonic-capable jets in various formations and choreography. Of course, they're forbidden from actually going supersonic, but to make the noise, they also create a lot of thrust. The combination of afterburner and speedbrakes together, seems a bit odd until you realize what their goal really is. It was also interesting to see a silent approach, and then a very short "fade-in" as one crossed in front of where I was. Not a boom, but a very short fade from silence to very loud. $\endgroup$– AaronDCommented Oct 9, 2022 at 20:36