F-14 low speed prowess vs Other Aircraft

[captain_dalan]

 

Clearly the F-14 hold an advantage in lift to weight ratio, otherwise its' stall speed wouldn't be so much lower and its' ITR so much higher across the board. What SHOULD also then hold true would be that it's STR ought to be better all the way to Mach 0.75, after which point the F-15 takes over.

 

It's not only about lift and drag. Sustaining the turn also involves excess thrust. What the chart is saying (indirectly) is that the GE F110's don't provide enough effective thrust to overcome the induced drag bellow certain air speed. There could be a number of reasons for this (inlet air bleed, engine tuning, inlet geometry....) all of which are purely academic. What ever the reasons, between 150-200KIAS the F-15 just sustains the turn better (at these altitudes).

 

EDIT: GGT beat me to it :P

Edited March 17, 2015 by captain_dalan

[FWind]

Right, let's make sense of it.

First off, the wing loading is irrelevant. Why? Because all by itself it means nothing. Put it this way:

 

What's the wing loading of a glider?

What is its STR?

Why is it lower than that of an A6M? :D

Whos stalls first? Glider or A6M? Why?

 

In case it's still unlear: Thrust (more specifically TWR, but TWR is affected by thrust) is the other component that allows you to maintain an STR.

All engines and inlets are not created equal, and jet engines do not provide constant thrust throughout the airspeed envelope.

 

It looks like the cat's engines lose a lot of thrust below M0.35 compared to the eagle.

 

Arguably that might be a useless part of the envelope.

 

 

 

We noted that I was tired and going back and forth between charts. In the end I chose the wrong one, but the result remains that the two aircraft turn at similar STRs at their respective regimes.

 

 

 

You have charts posted here in this thread.

 

so we better have the F-14B chart in sea level :laugh::laugh::laugh:

[Hummingbird]

It's not only about lift and drag. Sustaining the turn also involves excess thrust. What the chart is saying (indirectly) is that the GE F110's don't provide enough effective thrust to overcome the induced drag bellow certain air speed. There could be a number of reasons for this (inlet air bleed, engine tuning, inlet geometry....) all of which are purely academic. What ever the reasons, between 150-200KIAS the F-15 just sustains the turn better (at these altitudes).

 

EDIT: GGT beat me to it :P

 

I understand the importance of excess thrust, I didn't understand (until now) why a change would suddenly occur in reverse at Mach ~0.35 when the STR was higher all the way from Mach 0.75.

 

Only explanation for this is a loss of thrust, and I've now been given a reason for why/how that could occur.

[GGTharos]

If the inlet is the cause of this loss of thrust, that won't really help.

 

so we better have the F-14B chart in sea level :laugh::laugh::laugh:

[captain_dalan]

I understand the importance of excess thrust, I didn't understand (until now) why a change would suddenly occur in reverse at Mach ~0.35 when the STR was higher all the way from Mach 0.75.

 

Only explanation for this is a loss of thrust, and I've now been given a reason for why/how that could occur.

 

Don't quote me on this one, but i think that in the case of the F-14A at least, this was done on purpose (the inlet air bleed). :book:

[Hummingbird]

If the inlet is the cause of this loss of thrust, that won't really help.

 

Well 5,000 ft naturally WILL help some, by how much is the question though :)

[Hummingbird]

Don't quote me on this one, but i think that in the case of the F-14A at least, this was done on purpose (the inlet air bleed). :book:

 

But the F-15 features a similar device doesn't it?

[captain_dalan]

Well 5,000 ft naturally WILL help some, by how much is the question though :)

 

I can't tell for the B/D, but the A still lags in STR between 130-240 KIAS at SL or 0.2-0.36 TMN. I don't think the B/D would fare that much better. Maybe "shave off" a few knots up and down, but that's it.

 

But the F-15 features a similar device doesn't it?

 

Most jets to. But in the case of the F-14 it may have been utilized in a way that is somewhat more penalizing in the extreme low end of the envelope. I remember having a chat with a guy when i was calculating the original F-14A flight model; i was wondering why the installed thrust of the TF30's was so much less then the theoretical maximum, even when inlet geometry was taken into account. He may have been pulling my leg, but he said that the excessive air bleed was implemented in order to decrease the stall margin. It seams at lower speeds and higher alpha, too much airflow would have had a "choking" effect on the engines.

Edited March 17, 2015 by captain_dalan

[Hummingbird]

I can't tell for the B/D, but the A still lags in STR between 130-240 KIAS at SL or 0.2-0.36 TMN. I don't think the B/D would fare that much better. Maybe "shave off" a few knots up and down, but that's it.

 

What of ITR, that would be about the same between the F-14A and B/D, yes?

 

Is there a way you could do a rough comparison at SL with the B/D ?

 

 

Most jets to. But in the case of the F-14 it may have been utilized in a way that is somewhat more penalizing in the extreme low end of the envelope. I remember having a chat with a guy when i was calculating the original F-14A flight model; i was wondering why the installed thrust of the TF30's was so much less then the theoretical maximum, even when inlet geometry was taken into account. He may have been pulling my leg, but he said that the excessive air bleed was implemented in order to decrease the stall margin. It seams at lower speeds and higher alpha, too much airflow would have had a "choking" effect on the engines.

 

I see, interesting :)

[captain_dalan]

What of ITR, that would be about the same between the F-14A and B/D, yes?

 

Is there a way you could do a rough comparison at SL with the B/D ?

 

 

Any differences in the ITR should be negligible between the variants, although the B's and especially the D's are a bit heavier.

 

Any comparison i could do in that narrow part of the envelope would be a pure guess work. We just have no idea how the engine works in that regime. I have more info on the A if that would help.

[GGTharos]

ITR is much more dependent on the wing loading and high AoA characteristics - so devices like vortex generators etc come into play.

 

Here thrust determines how long you can keep rating at ITR (this is what lets me do a ~15 sec 360 in the F-15 - 35000lbs, clean, SL).

Certainly the F-14 seems to have quite a huge ITR, pretty much second to the flanker which is second to the hornet, IF I recall my numbers correctly.

 

(ITR for flanker in on the order of 32deg/s, Hornet can do in excess of 40 ... again, IIRC).

 

What of ITR, that would be about the same between the F-14A and B/D, yes?

 

Is there a way you could do a rough comparison at SL with the B/D ?

[lunaticfringe]

Except it very clearly does not. Reading charts is still better than quoting a number - I thought we had just gone over this :)

 

Actually, it does, and I thought I'd clarified my "conceptual" misstatement with a linguistic one. May have been erroneously lost in the numerous stops/restarts I go through following these threads.

 

The F-14 *sustains* (literally, as in "has") a available G rate advantage in opposition to the F-15 up to Mach 0.65+. It does not hold a "sustained turning" advantage in that range, it simply has higher G available.

 

Now, the relationship between ITR and STR vs. WL is being stated backwards here: ITR is WL (trending higher) and CL/AoA (can offset low). STR is WL (trending lower), thrust, and shape drag.

 

The F-14 has a high ITR because of its CL/AoA authority, not because of its relatively low WL. And it has STR performance because of the WL, not necessarily (or, despite of) relative to thrust.

 

These factors, as illustrated, can offset one another, or can aid one another.

 

The Flanker is relatively heavy, and has massive lift generation at play, thus its ITR is ridiculous. At the same time, it's thrust isn't necessarily enough to offset the higher WL, meaning it's STR isn't as strong as some expect.

 

The F/A-18 has similar massive lift generation taking place, and a low WL. It's problem isn't thrust, but form drag- illustrated by the fact it has trouble punching the Mach, and recovering energy when in the turning fight.

 

The F-14 and F-15 are similar. The former has higher CL/AoA authority, the latter thrust. WL between the two is generally a wash. Subsequently, the Tomcat is going to have more G earlier in the envelope (byproduct of the wing), whereas the Eagle is going to sustain it better over a wider area (byproduct of the engine).

[GGTharos]

Actually, it does, and I thought I'd clarified my "conceptual" misstatement with a linguistic one. May have been erroneously lost in the numerous stops/restarts I go through following these threads.

 

Obviously lost.

 

The F-14 *sustains* (literally, as in "has") a available G rate advantage in opposition to the F-15 up to Mach 0.65+. It does not hold a "sustained turning" advantage in that range, it simply has higher G available.

Can you tell without doghouse plots for the eagle? I haven't even seen one chart with ITR for the eagle, it's quite vexing :P

[lunaticfringe]

I gave you one in this thread. And G? Chart not necessary. Give me three pieces of data: wing area, AR, and leading edge angle, you've got your CL. Add weight and altitude pressure? Stall. Speed for available G is a function of stall times the square root of said target G number.

 

Larger wing area, larger aspect ratio, lower angle: lower stall. Lower stall: lower speed for available G.

[lunaticfringe]

Oh, my bad- in USAF's infinite wisdom, they didn't add the rate delineation to the 15k doghouse. ;)

[lunaticfringe]

so we better have the F-14B chart in sea level :laugh::laugh::laugh:

 

Doesn't exist.

[GGTharos]

And like a blind bat I missed the chart ;)

 

They have the max ITR at 22deg/s at that altitude, or just slightly less.

 

I've heard higher, but that doesn't mean anything. We can get higher in the sim, but I did that at SL with a much lighter aircraft - high enough to produce an average of 23-24deg (I actually maintained the rate pretty close to 23 a lot of the time IIRC).

 

Probably close enough, I don't know if the max ITR would change based on altitude.

 

Oh, my bad- in USAF's infinite wisdom, they didn't add the rate delineation to the 15k doghouse. ;)

[lunaticfringe]

One thing to remember in all this friendly banter and pissing: charts are nice. They're bite-sized for "small" brains. True aero data is better, and doesn't always conform to the charts, because services like to forget: *all* charts are estimates. A pilot didn't keep topping off the tanker and hit every single intersection on the chart. They grab a bunch, and average based on hard and fast rules. Slow speed values on them are generally suspect because nobody sticks around 0.2/0.3 Mach except during TO. Further, the ability to delineate on a Max CL line when everything is running into it is just about nil.

[captain_dalan]

 

The Flanker is relatively heavy, and has massive lift generation at play, thus its ITR is ridiculous. At the same time, it's thrust isn't necessarily enough to offset the higher WL, meaning it's STR isn't as strong as some expect.

 

True. I was actually quite surprised when i first saw the sustained turning rates for a loaded Flanker. Especially when you take into account the actual size of the load.

 

Certainly the F-14 seems to have quite a huge ITR, pretty much second to the flanker which is second to the hornet, IF I recall my numbers correctly.

 

(ITR for flanker in on the order of 32deg/s, Hornet can do in excess of 40 ... again, IIRC).

 

This is pretty much what i've read/heard most of the time. With the Hornet probably degrading faster then the other two due to external ordinance.

[Hummingbird]

So let me summerize:

 

1. The F-14 & F-15 have about the same WL when taking into account all lifting surfaces (wings, body etc.) with perhaps a small advantage to the F-14

2. The F-14 features a higher CL_max thanks to its higher available AR (swing wing) and high lift devices

3. The F-15 features a higher TW ratio, a difference which increases below Mach ~0.30 due to intake losses on the part of the F-14

 

Result is that the F-14 features a significantly higher ITR across the board (curtesy of its higher LW ratio), and from Mach ~0.30 to Mach 0.75 maintains a noticable advantage in STR due to stable engine performance in this regime, however in the remaining regimes the F-15 holds an STR advantage due to its noticably higher TW ratio.

 

EDIT: Looking at the F-14A chart with the maneuver devices operating the F-14 holds an advantage in STR from Mach 0.3 to 0.75, and not 0.35 to 0.75.

Edited March 17, 2015 by Hummingbird

[Hummingbird]

Regarding performance at SL, looking at the charts an average of 2 deg/sec is gained pr. 5,000 ft in altitude, thus a pretty good guess would be that at SL the F-14 featured an ITR of 23.5 deg/sec @ Mach ~0.45 and 6.5 G.

 

This would translate into a ITR of 24.5 deg/sec @ Mach 0.5 and 7 G at SL.

 

Now as for STR, the F-14 (all versions) features a higher STR than the F-15 between Mach 0.30 to 0.75, however the higher available thrust of the F-14B/D version ought to increase that interval, maybe from Mach 0.2 to 0.9, as well as the difference in max STR by perhaps 0.5 to 0.75 deg/sec. Which in practice means the F-14B/D probably holds a noticable advantage in both ITR and STR at all flying speeds up to Mach 0.9.

 

Remember that we see the exact same happening with the F-15A vs F-15C, where the extra available thrust increased the max STR by 0.75 deg/sec, and same would naturally happen with the F-14A to B/C.

 

Note: This is using the F-14A charts as our baseline and disregarding the F-14B/D charts as they are without the maneuver devices operating, hence the Mach 0.3 - 0.75 STR figure base figure.

Edited March 17, 2015 by Hummingbird

[GGTharos]

You're making assumptions that you shouldn't be making.

 

Drop 5000 feet, and F-15 performance increases as well. This isn't a one-sided thing.

 

Clean up both aircraft and change the engines on the cat and what you get is a box of chocolates, not a turn rate estimate.

[Hummingbird]

You're making assumptions that you shouldn't be making.

 

Drop 5000 feet, and F-15 performance increases as well. This isn't a one-sided thing.

 

Clean up both aircraft and change the engines on the cat and what you get is a box of chocolates, not a turn rate estimate.

 

I'm using F-15 SL figures as well Tharos.

 

A direct comparison can be made at 15,000 ft, with the max STR & ITR being equally affected on both aircraft (again using the available charts to back this up)

[Hummingbird]

Also worthy of note is that the F-14 featured no G limiter, thus it could actually pull 9 G turns without any problems, the ultimate load of the airframe being the same as that of the F-15 as far as I can tell based on the information provided thus far. (11 Gs being pulled at times without issues apparently)

[turkeydriver]

Horizontal tail surface lift and response should benefit the F-14 based on area alone, but the F-15's dogtooth and design might make them more effective than their size would by themselves. Also I've "heard" of a former PC resetting the diagnostic box after reading 14G after an ACM flight-that aircraft would be HARD DOWN for inspection, and while Grumman Ironworks get the jet in the air for 30+ years, the older airframes were so tweaked that the aircraft needed special adjustment just to get the access panels closed. Some blame the airframe stress was due to the carrying of bombs, but only frequent carriage of MK-84 sized bombs with maneuvering and landing would add any real stress. I'd bet money that all that tweaked airframe is from a life of hard ACM.

Edited March 17, 2015 by turkeydriver