"Very" High Speed Performace Question (Sorry)

[wangus]

Hope I've posted this in correct forum...

I've just been reading about the F-14, and more specifically its top speed was given in MPH. In a modern airline one cruises at, grab a figure, Mach 0.79, to avoid supersonic airflow over the wings? The engines aren't operating at maximum thrust, but perhaps N1 93%. More power is available, but an increase in speed would exceed MachCrit?
If I'm correct, military supersonic aircraft are immune to supersonic airflow issues over the wing. Is the aircraft top speed simply limited by thrust running out at high speeds??? Presumably the airframe would happily go faster?? Also the IAS at the quoted speeds of 1400 MPH at circa FL600 (ceiling height) would presumably be approaching low speed stall speeds (IAS)?
Also, above the tropopause, mach 1 is about 573 knots. 38.94xsqare root absolute T. Mach 2 would then be 1320 MPH (converted), but the aircraft is described as capable of mach 1 point something, but 1400 is over Mach 2.....:ugh:
can somebody see where I'm confusing myself? I would be most grateful.

[Genghis the Engineer]

Are you sure that they are quoting IAS and not TAS? Many aviation writers, and especially aircraft company marketing departments don't understand the difference so usually quote whichever sounds most impressive - which is usually TAS.

G

[Rengineer]

Maybe a little bit, though it's all a while back that I studied that. For one thing, even fast jets typically have a maximum mach number. If you look at a fast jet's mach-altitude diagram, maximum mach is really defined by the power of the engines, the flutter limit, and the temperature limit. Any of these may limit the top mach number achievable. In addition, for an individual aircraft type, you may want to limit the region where the compression shock exists, and this can introduce additional limits on mach number.

Hope this helps at least a little bit :}

[RetiredF4]

Max Mach of the Tomcat is 2.34!
See the numbers far down on the page Wiki F-14 (http://en.wikipedia.org/wiki/Grumman_F-14_Tomcat)

The speed limit in combat jets is simplified defined by the engine performance and the inlet-temperature of the airflow to the compressor (duct-temp-high).

In Test-flights we did max speed-runs (i flew F-4 Phantom II) till the needle wouldn´t go further, my max speed was 2.23 Mach at FL 390 with the Recce Version RF-4E.

franzl

[TLB]

Most fighters will have a max KIAS (knots indicated air speed) limit AND a maximum mach number for the reasons mentioned above

[wiggy]

RetiredF4
In Test-flights we did max speed-runs (i flew F-4 Phantom II) till the needle wouldn´t go further, my max speed was 2.23 Mach at FL 390 with the Recce Version RF-4E.


:{:{:{ only because I flew the "M"......

[27mm]

Don't be too disheartened, Wiggy. I recall cranking up an F4M to 1.6M at FL360 (in standard RAFG fit, 2 tanks, gunpod) over Belgium during Hi-Flier/SS intercepts - my WSO was very ticked off, as it made the final turn radius quite a bit larger than at the usual 1.3M....

[BOAC]

Wangus - I'm not sure you have actually had an answer to your question Also the IAS at the quoted speeds of 1400 MPH at circa FL600 (ceiling height) would presumably be approaching low speed stall speeds (IAS)? - at FL600, the IAS at a TAS of 1400 MPH is a fair bit away from the stall IAS. I don't have the conversion tables to hand, but I'm sure someone will have.

It sounds like a fairly basic book there if it quotes a top speed in MPH.

[fantom]

We did 1.7M in a gate off Holland during an exercise and I assured the nav they would not catch us.

There were two GAF 104s and a Frightning already formating on us...

Buggah.

[BOAC]

....and formation at that speed is t r i c k y !

[fantom]

Yeah, but I was leader...

[wangus]

Thanks for your inputs. A further thought / question which I would appreciate your comments / input on. (I hope my ramblings appear coherent!!)

I've just dug out the book again (cheap child's fighter guide), and looked at the figures quoted. 1545 MPH max speed, ceiling of 60,000.
1545 MPH converts to 1342 Knots. Now, using my whizwheel, I'm getting a rough calculation (not taking compressibility into account) that an IAS of 422 knots (inner scale) gives TAS of 1342 knots (outer scale) when at 60,000 and MINUS 56.5*C. Does that mean from a pilot's perspective, if you were taking off to perform an intercept, you could accelerate to 422 knots during your initial climb, and then maintain an IAS of 422 knots for the entire flight as you climbed to FL600. Your TAS and Mach # would steadily be rising rapidly obviously, ultimately to figures above??
Above tropopause, Mach 1 is roughly 574 knots (?). 1342 knots gives theoretical mach 2.34 as Retired F4 kindly pointed out. That I comprehend. I think.... I suppose the one thing I'm still not clear on, is what is the high speed flight crew primary concern regading speed. Are they monitoring MACH#, IAS, or both equally? Can you zip along at Mach 2.34 and have a limiting IAS structurally? Surely not? (Frozen ATPL training didn't address this!)
Thanks again for comments so far, and I look forward to your further comments.

[John Farley]

wangus

From a book in my library:

There are more exciting things to read about than the theory and practice of what speed should be flown on the climb. However one question I have been asked more than once is why Concorde had a climbing speed that started as a constant indicated airspeed (IAS) but later became a constant indicated mach number (IMN). This changeover is not unique to Concorde. It is something that affects all aircraft capable of flying high as well as those with a supersonic capability in level flight. As ever the explanation of this phenomenon comes in two sizes - short and long.

The short explanation is that, as you climb at a constant IAS, regardless of your aircraft type, your mach number is actually steadily increasing thanks to the reducing air temperature and density. If your aircraft has sufficient power to climb high enough, the mach number will eventually reach that at which a pretty steep drag rise starts to happen, even if your climbing speed is only 80 kt IAS. This mach-related drag rise would probably start in the range of say 0.60 to 0.90 IMN depending on the shape of the aircraft and, in particular, the thickness chord ratio of the wing. Naturally such a drag increase will quickly put a stop to any climb rate. It is therefore necessary to hold an IMN a little below the one where the steep drag rise happens in order to keep the best climb going. Eventually of course full power will be needed to hold height at that speed. Once at this point, you have reached the absolute ceiling of the aircraft. To summarise, all aircraft climb initially at constant IAS and increasing IMN and then change over to a constant IMN and a reducing IAS.

The longer explanation has to do with how climbing speeds are chosen and the theory that lies behind them. Here some differences start to emerge both between the theoretical optimum and what it is sensible to expect pilots to fly and also between piston-powered and jet aircraft. Any reasonable text book on the subject is likely to cover the topic of climbing in a few equations but I have always found it more useful to understand the words behind the equations.

There are three things going on in any climb at a constant IAS. First the engine is having to overcome the drag that would be present at the same speed in level flight, second it has to provide the energy necessary to ‘winch’ the weight up vertically at a speed equal to the rate of climb and finally it has to accelerate the aircraft because at constant IAS the true airspeed (TAS) is steadily increasing as height is increased. I do accept that the latter term is small enough to be ignored in low performance aircraft but it is not insignificant with jets that have high rates of climb.

There follows several pages of detail but you probably are bored by now........

Once the climb is done and you are in the high speed level (or diving) business then ALL aircraft have an IAS limit for structural reasons as you clearly understand. However not all aircraft have a Mach limit. If a type has one it is probably related to an undesirable trim change or other control issue. Only those types which can sustain a very high TAS are likely to have a structural temperature problem which may be reflected to the pilot as a sustained Mach limit or an actual structural temperature indication limit.

[wangus]

Thanks. That all makes sense, and NO it doesn't bore me, or else I wouldn't have asked.
So to use my example then, an F-14 Tomcat at FL600, Mach 2.34, TAS of 1342 knots, and IAS of 422 knots, the IAS is clearly not approaching unsafe. The most likely scenario here is the engines are maxed out / suffering effectively from hypoxia?
I though the shockwave issues only occurred as you pass through Mach 1, and you effectively punch your way through. I didn't realize they continued and repeated up to Mach2, Mach3. The shockwaves on the wing cause a nose down effect approaching M1, then again near M2??

[RetiredF4]

Again a simplified answer from my side (being no engineer, but a retired fighter jock): The airflow through the engines has to stay subsonic. Therefore the engine inlets have variable ramps, which control the airflow to the engines and reduce the speed of it to subsonic. That again heats up the air and that was the limiting speed factor in the RF-4E. "duct Temp high warning".

It might be different in other jets, but in most of them the pure structural limit is not a factor.

franzl

[wangus]

Eureka. Thanks retired F4. Something in my brain has just clicked and that makes sense regarding limitations. So if a hypothetical leap in engine technology occurred, there is no reason these existing airframes couldn't in fact fly faster.

[RetiredF4]

With higher speeds other issues like temperature of the whole airframe arise.

If you are interested in more details, i recommend the book from Paul F Crickmore "Lockheed SR-71 Blackbird" published with Osprey Air Combat. The order Number is ISBN 0-85045-653-3.

I dont know if it is still available somewhere, but it is very entertaining.
franzl

[Double Zero]

I make a point of asking Test Pilots which aircraft they would like to fly, given the choice.

A very experienced ex-RAF Test Pilot said " F-16, you can get 850kts IAS in that - with the Lightning one was limited to 650 kts IAS or the intake would overheat ".

I would have thought things like the F-15 or Russian jobs might do even better, but apart from the F-15 this was before the advent of the others.

[glad rag]

http://www.f-16.net/f-16_versions_article20.html

[zulu_01]

the CAS limits are for the sea level performance primarily due to flutter/drag/temp/structural limits.
the M.no limits are applicable at higher altitude and again due to Flutter/temp/engine intake design etc.

[WeekendFlyer]

Just a few points for consideration:

1. Once above about Mach 0.3 you must take compressibility into account when calculating TAS from IAS or CAS. The effect of it (scale altitude correction), i.e. the difference between CAS and EAS, becomes significant quite quickly.

2. Pressure errors (which affect the relationship between IAS and CAS) can do some crazy things in the transonic region (above about Mach 0.85) and can also be a bit odd when supersonic. For solving this problem a modern air data computer really helps.

3. Another consideration is the aircraft Specific Excess Power (SEP). This changes with height and speed and is a combination of engine performance and airframe drag factors. The best climb performance is obtained by flying speeds that optimise SEP and can involve climbing at a set speed, then descending to gain speed, then climbing again. An efficient climb is all about energy management.

4. The tropopause has a huge effect on climb and cruise performance because once through it you no longer benefit from reducing temperature and air density as you climb (temperature above the tropopause is constant).

5. You are probably wasting your time trying to back-calculate speeds from non-technical publications. When speeds are expressed in MPH there is no way of knowing if they converted from a Mach No of TAS, and whether the conversion used the sea-level Mach No or the Mach No at altitude.

6. All subsonic high performance jet aircraft I know of tend to use a constant IAS climb until a specific height, at which point a constant Mach No climb is conducted. Supersonic is a totally different ball game because getting through the transonic region requires an awful lot of thrust so this acceleration is normally done in level flight or a descent, before climbing again at constant Mach.

Hope this all helps!

WF

[goldfish85]

The reason civil transport limit themselves to 0.75 or thereabouts is to avoid the drag rise as the airflow goes supersonic over the top of the wing. Years ago, when fuel was cheap, we would cruise at 0.85 and you could see the shockwave on top of the wing.


Goldfish