Jeff Hill's latest claim

[Bell]

Reheat, I know there is a simple answer for this so please humor me.

How can one part of an aircraft go supersonic without the "whole aircraft" doing so?

Interesting thread guys...

Not exactly the same but somehow related... we encouter objects that move at different speeds at the same time on a daily basis (obviously the scientists on here will proof me wrong )

 

[Corsair 115]

No, I don't believe so. It would severely buffet and might "tuck" severely prior to going through the Mach for the whole aircraft. That's an educated guess BTW.

In regards to a passenger jet approaching Mach 1 in a dive, would compressibility enter the picture at all? Or has that been designed out of aircraft since the issue was first encountered in some WWII fighter types?

 

[rwguinn]

In regards to a passenger jet approaching Mach 1 in a dive, would compressibility enter the picture at all? Or has that been designed out of aircraft since the issue was first encountered in some WWII fighter types?

???
Compressibility IS the phenomenon.
You have to use natural laws, not design them out...

 

[uk_dave]

Reheat, I know there is a simple answer for this so please humor me.

How can one part of an aircraft go supersonic without the "whole aircraft" doing so?

Interesting thread guys...

I remember seeing a documentary about Concorde and how the air intake into the engine had to be slowed to below supersonic speed for them to function. Not sure if that counts as part of the plane though.

 

[Cheap Shot]

REHEAT does altitude make a difference on speed, due to density is that correct. I remember a long time ago at least 25 or 30 years ago, someone had a privately owned Starfighter 104, and I beleive he flew it at just over 900 MPH at 100 AGL over a dry lake bed, can't remeber his name. But the same aircraft at a higher altitude would be over 1400 mph, is that due to density, temprature, or anything else. I remember watching it, and there was some concern over ground effect when the aircraft made its run, it would be nice if there was a video of it. The 104 looked nice it was all painted up.

 

[Reheat]

In regards to a passenger jet approaching Mach 1 in a dive, would compressibility enter the picture at all? Or has that been designed out of aircraft since the issue was first encountered in some WWII fighter types?

Compressibility is always present as rwguinn just said. Designs such as Mach trimmers on the horizontal stabilizer and wing sweep are just two of the designs to lessens the adverse affects and raises the Critical Mach over the wing surface allowing the aircraft to go faster without the adverse effects such Mach Buffet and Mach Tuck so prominent during the earlier years of aircraft development.

ETA - These designs also significantly improved fuel consumption due to the lesser and later onset of drag which is due to a higher Critical Mach over the wing.

 

[Corsair 115]

Compressibility IS the phenomenon.
You have to use natural laws, not design them out...

Yeah, I probably didn't phrase that very well. I was thinking along the lines of technical solutions lessening it as a concern.

Reheat answered nicely in the post above what I was trying to get at.

 

[Reheat]

REHEAT does altitude make a difference on speed, due to density is that correct. I remember a long time ago at least 25 or 30 years ago, someone had a privately owned Starfighter 104, and I beleive he flew it at just over 900 MPH at 100 AGL over a dry lake bed, can't remeber his name. But the same aircraft at a higher altitude would be over 1400 mph, is that due to density, temprature, or anything else. I remember watching it, and there was some concern over ground effect when the aircraft made its run, it would be nice if there was a video of it. The 104 looked nice it was all painted up.

I'm watering my newly reseeded lawn so answers will be short and disjointed.

IAS = Indicated Airspeed
CAS = Calibrated Airspeed = IAS corrected for installation and position error.
EAS = Equivalent Airspeed = CAS corrected for compressibility
TAS = True Airspeed = EAS corrected for temperature and pressure (air density).

900 MPH IAS at low level (SEA Level) in a standard atmosphere would be nearly equal to 900 TAS.

900 MPH IAS at altitude would be a CONSIDERABLY faster TAS perhaps as high as 1400 MPH, but I'd have to calculate it to be exact. Sounds reasonable at first glance.

As you previously noted you use Mach # at altitude for reference because it is simpler to determine and adjust.

Ground effect on a F-104 at low altitude/high speed would be negligible. Hardly noticeable at all.

 

[Reheat]

In regards to a passenger jet approaching Mach 1 in a dive, would compressibility enter the picture at all? Or has that been designed out of aircraft since the issue was first encountered in some WWII fighter types?

The WWII fighters encounter Critical Mach in the realm of about .70. The early jet fighters were in the realm of .78 - .80. Then we reached the era of the Century Series Fighters which were all supersonic capable because of wing sweep and other design features.

Older jet transports such as the B-707 and DC-8 Critical Mach was in the range of about .86-.88 or so. Todays transports are in the range of .95 or higher.

When R. Mackey sees this thread I'm sure he can add valuable information that will be more accurate that my memory recall.

For those who are curious about supersonic flight. In fighters (at least) since the 60's the ONLY way to determine supersonic versus subsonic flight was/is to look at the Mach Indicator. There are no other indications at all. Well, the early ones did need a bit of power as supersonic drag increased.

I understand that the Concorde was more fuel efficient at Mach 1.7-2.0 that it was at .95. Interesting phenomena.....

 

[R.Mackey]

Reheat, I know there is a simple answer for this so please humor me.

How can one part of an aircraft go supersonic without the "whole aircraft" doing so?

Different shape and/or angle of attack. Perhaps R. Mackey can explain it better than I can.

Bear in mind that the Mach Indicator is sensing at the pitot tube not on the wings and other places on the aircraft.

Basically, there may be points on the wings or other parts that achieve Mach Crit prior to a Mach 1 indication in the cockpit.

Right. The key factor is, again, compressibility. Also, it's important to remember that the speed of sound is a function of air temperature -- at altitude, where air is colder, Mach 1 is considerably slower than it is at sea level. The speed of sound is approximately linear with respect to temperature for small perturbations around the critical speed.

What happens in an aircraft slightly above Mach 1 is that the air, heated by skin friction, will see a significant rise in what Mach 1 means as it passes to the rear of the aircraft. Therefore, it is possible for the nose to be slightly supersonic, while the tail can be slightly subsonic, even though both are moving at the same speed.

This is also why speed indicators, e.g. pitostatic probes, are generally mounted at the front of the aircraft, extending away from skin effects and local buffeting.

The instability near Mach 1 also describes one of the gremlins that hampered early supersonic flight. Because the air passes in and out of supersonic conditions right at this speed, as governed by interference with the airframe, there is no way for a stable shock to form. This creates severe vibrations, detaching the airflow and leading to stalls and even more vibrations, and soon the aircraft has lost all control authority. The development history of the Bell X-1 is almost entirely concerned with discovery and control of these phenomena.

 

[Reheat]

What happens in an aircraft slightly above Mach 1 is that the air, heated by skin friction, will see a significant rise in what Mach 1 means as it passes to the rear of the aircraft. Therefore, it is possible for the nose to be slightly supersonic, while the tail can be slightly subsonic, even though both are moving at the same speed.

Perhaps a better way to think of this is NOT in terms of speed as all parts are, of course, moving at the same speed, but think instead of differing airflow rates over different surfaces. Aircraft surfaces are designed to increase the airflow across the top of the surfaces. This is most true of the wing, in order to generate lift. That differing airflow rate over different surfaces in addition to R. Mackey's explanation should clarify this issue for those who think I'm crazy.

 

[R.Mackey]

Regarding critical Mach on airfoils, Mach buffeting, etc., a quick primer is on pp. 15-6 through 15-9 of the FAA Airplane Handbook.

These effects aren't the same as one part "traveling" faster than another -- instead, the airfoil of an ordinary aircraft generates lift by accelerating the fluid. If that fluid is already near Mach 1, then the airfoil won't behave in quite the same way anymore, leading to locally supersonic conditions and stalling for different reasons.

Most supersonic aircraft have extremely thin wings. The F-104 Starfighter is perhaps the ultimate example of this.

ETA: Yup, read Reheat's mind, I did.

 

[Reheat]

I remember a long time ago at least 25 or 30 years ago, someone had a privately owned Starfighter 104, and I beleive he flew it at just over 900 MPH at 100 AGL over a dry lake bed, can't remeber his name. But the same aircraft at a higher altitude would be over 1400 mph, is that due to density, temprature, or anything else. I remember watching it, and there was some concern over ground effect when the aircraft made its run, it would be nice if there was a video of it. The 104 looked nice it was all painted up.

It was Darryl Greenamyer. Here's the only reference that I've found. IIRC, the aircraft was a beautiful bright red. No photo yet, but I'm still looking.

After setting his piston-engine speed record in 1969, Greenamyer assembled the world's first privately owned F-104 Starfighter from scrounged parts. The project took him 12 years. When complete, the plane was 2,000 pounds lighter than a stock F-104, carried 300 extra gallons of fuel, and sported a red velvet instrument panel. On October 24, 1977, in Tonopah, Nevada, he broke the world absolute low-altitude speed record, previously set by a U.S. Navy F-4 Phantom, streaking twice through a 3-km trap at an average speed of 998 mph less than 80 feet over the desert. His record remains unbroken.

http://www.popsci.com/popsci/printe...d1155b4a1db84010vgnvcm1000004eecbccdrcrd.html

Here we go.....

 

[Reheat]

Stundie Material

This has to be stundie material.....

From LCF

I will answer a few concerns you have..

in order to get a large jet to fly at lower altitudes you must KILL LIFT.
the geometry of the wing is as such to create LIFT.. but under say
300 feet or so... you have to kill lift to penetrate the thicker air.
that is that thing that comes down off the back side of the wing.

@500+ mph you will need to be way up there to avoid vibration as
the air rushes under the wing... the lower you are the lower the air
speed to avoid these vibrations which can rip the wings off the plane.
(this is my understanding of speed & air - areonautics) I'm not an engineer
this guy is and he is good. and I found no errors in his logic and he
has no agenda... end of story this guy says NO PLANE - you had
better understand NO PLANE> and totally consistant with the data.

 

[Dave Rogers]

I'm no aeronautical engineer (apart from the occasional Airfix kit*), but isn't "that thing that comes down off the back side of the wing" the flap, which actually increases lift at low speeds?

Dave

*OK, a couple of hundred, but who's counting?

 

[apathoid]

I'm no aeronautical engineer (apart from the occasional Airfix kit*), but isn't "that thing that comes down off the back side of the wing" the flap, which actually increases lift at low speeds?

Dave


*OK, a couple of hundred, but who's counting?

Might he be referring to the spoilers? Which deploy on touchdown to kill lift - pinning the plane to the runway... And perhaps I'm wrong, but flaps and slats actually just increase surface area and camber, allowing the plane to fly slower. I think that, if anything, they increase drag, which is evident by the need to increase thrust after flap extension....

Alot of non-aviation folk seem to seem the the flaps slow the airplane down. They do, but that's not their intended purpose. The spoilers are what you'd expect to see if the plane is too high or too fast......

eta: I suppose it's possible for flaps to increase both lift and drag....

 

[CurtC]

To find out more about this topic and Jeff's inability to understand, please see this thread over at LCF:

http://z10.invisionfree.com/Loose_Change_Forum/index.php?showtopic=14781

 

[Reheat]

I'm no aeronautical engineer (apart from the occasional Airfix kit*), but isn't "that thing that comes down off the back side of the wing" the flap, which actually increases lift at low speeds?

The entire thought of "killing lift" is so stupid that to explain it removes the hilarity of the statement.

If one were to "kill lift" (by adding drag devices) the aircraft would eventually crash!

Maybe it's flaps he's talking about or the lift vector itself (I really don't know) as the entire though of "killing lift" in order to fly at low altitude is hilarious.

Flaps or similar "high lift devices" do not increase lift, they merely shift the center of pressure on the wing while altering the lift vector thereby reducing stall speed which allows an aircraft to fly slower.

Don't worry too much about it. I guess it's necessary to understand the dynamics of flight to find it as hilarious as I do.

 

[CurtC]

Alot of non-aviation folk seem to seem the the flaps slow the airplane down. They do, but that's not their intended purpose. The spoilers are what you'd expect to see if the plane is too high or too fast......

But flaps do play a key role in slowing the plane down. They increase lift, which is useful for keeping the plane aloft at low speeds, but they definitely increase drag as well. It's not that easy to slow a 100,000 pound machine down from 550 mph to less than 200 mph, while at the same time going "downhill" from 30,000 feet, while not having anything to push against but air. They use the flaps and landing gear. If you notice while flying, the spoilers on the wings are not used very much while in the air. They are some, but more as control surfaces.

 

[apathoid]

But flaps do play a key role in slowing the plane down. They increase lift, which is useful for keeping the plane aloft at low speeds, but they definitely increase drag as well. It's not that easy to slow a 100,000 pound machine down from 550 mph to less than 200 mph, while at the same time going "downhill" from 30,000 feet, while not having anything to push against but air. They use the flaps and landing gear. If you notice while flying, the spoilers on the wings are not used very much while in the air. They are some, but more as control surfaces.

You're pretty much right, but I'd add that you only see flaps extended when the airplane is already below 230 kts or so. In fact, airliners have flap "speed limits" and IIRC, the limit for 1 degree of flap is 250 kts or so..The landing gear doesn't go down til 1500 feet or so, when the aiplane is about 180 kts or less. To get down from 35,000, the pilots simply reduce power and sort of "glide" down. Alot of times, especially on heavier airplanes, you'll see spoilers go up just before 10,000' is reached, to try to slow to meet the 250 kts speed limit...

I only made the flap/spoiler distinction to point out that the flap's primary purpose isn't slowing the plane down, as the LC poster is trying to imply..