Discussion: F-14, F-15 Hybrid Idea

[KJ_Lesnick]

a thousand pounds a minute , 27000 kgs an hour , take a sfc ratio of 2 which was the norm for J-79 then you will be talking about an engine of 13600 kg thrust .I don't have a likely candidate in my mind .So let's  presume 1.65 which was the A/B value for '404 in late 80's , you will have 16500 kg of thrust ; that will be an F-18 .Should be quite OK I guess.

Okay... I'm confused, you said further down on this post that a SFC of 0.8 produces 67 kg per minute (4,020 kgs an hour).  How did 2.7 which is just around 3 times that high equal 27,000 kgs an hour? 

as I posted the 2.7 value can be a misprint , but I have read that turbofans burn more in afterburner ; higher the bypass ratio , higher the sfc though the relation is not probably linear ; I mean two times the bypass would not give two times the sfc .I don't know , just expect it to be so.

I thought they'd be more efficient on AB as you'd have a multi-stage compressor (which produces a pretty good pressure ratio -- probably equal that to the pressure of the core's exhaust as it reaches the afterburner, and considering the core's exhaust is 65% air and the fan is 100% air... although, come to think about it, the core flow is moving a lot faster probably even then as both streams are subsonic and the speed of sound is determined by air temperature and the exhaust is a lot hotter than the fan...)

and afterburners are gas guzzlers ...

5000 kgs at a nominal sfc of 0.8 gives a fuel flow of 67 kgs per minute , same engine would be doing 8000 with 1.75 and the fuel is spent at a rate of 233 kg/dk... 1.6 times the thrust , 3.5 times the fuel .Afterburners are inefficient .

As I said, how is it that 0.8 produces a fuel consumption of 67 kgs a minute (4,020 an hour), and 2.7 produces 27,000 kgs an hour...

an anectode on people who evaluate ; in full A/B a Su-7 will spend all of its fuel in 8 minutes . It is proof that Russians are barely off the Stone Age. Actually the F-16 does the same 20 years later .How is this to be evaluated ?

The Su-7 has more than one afterburner setting?  I thought all jets until like the J-79 had afterburner as simply ON or OFF?


KJ Lesnick

[KJ_Lesnick]

So what's the formula for SFC?  Amount of thrust per, amount of fuel burned, per amount of time right?

Kendra Lesnick

[KJ_Lesnick]

sfc value times engine thrust gives fuel expenditure in an hour . J-79 sfc was about .85 at military and 1.97 at afterburner if I am not mistaken so it depends on which setting you are calculating .

Understood

[KJ_Lesnick]

Okay, back to the primary issue, the F-14/F-15 Hybrid Concept.

How would a wing like the F-15U (way larger than the F-15A/C's), but the thickness (not t/c ratio, just physical thickness) being either equal or slightly greater than the F-15 (proportionally thinner than the F-15U in either case) perform compared to the F-15's wing at high-speed, and low-speeds? 


Kendra Lesnick

[KJ_Lesnick]

r16,

Normally, I actually would actually be inclined to go with your idea.  However, from what I remember the F-15U had significant speed penalties compared to the F-15A/C (more wing-area, more drag, plus with 40% more fuel capacity, more weight), and actually I'm trying to get a plane with the same maximum-speeds as the F-15A/C.  So the best solution I could think of was to add wing-area which sounds like it would increase lift (and fuel capacity too technically since the extra wing-area could carry more gas), but keep the same thickness or only add a little bit more. 

Do you have any idea if you would gain or lose performance if you increased the wing area but kept the thickness the same?  Because when I look at it, I can see conflicting information

1.) More area equals more lift and lighter wing-loading
2.) The wing will be proportionally thinner as the physical thickness is the same resulting in a lower t/c ratio and less lift per given square foot of wing. 

I'm not sure which factor would win out.  Instinctively I think the increase in wing area would make up for the thinner surface, but instincts aren't always right you know. 


Kendra Lesnick

[KJ_Lesnick]

To R16,
I've posted a question on the Secret-Projects forum inquiring as to how much of a wing-area difference there is between the F-15U and the F-15A/C.  If I can get that data, I know a few people I could ask how an F-15 would fly if it's wing-area was increased X-percent, and retained the same thickness. 


Kendra Lensick

[KJ_Lesnick]

Was the F-15's design in anyway influenced by the F-14?  (As the F-15 appears to be designed as a plane that can beat virtually anybody in a dog-fight)

Kendra Lesnick
BTW:  If anybody has any data regarding General Dynamics F/X design, please reply

[KJ_Lesnick]

So the F-15 was not influenced by the F-14 design then?

Kendra

[KJ_Lesnick]

Well, I have heard cases in the past where two simultaneous programs influenced each other...

[overscan]

The answer is no.

McDonnell-Douglas came a good second to Grumman in VFX (F-14) with a design that was very different indeed from their FX (F-15) submission. The F-15 configuration was complete before the VFX winner was known. McDD would hardly have been copying features from a rival VFX design.

[KJ_Lesnick]

Out of curiousity, why did the General Dynamics FX program use swing-wings (From what I remember the FX was supposed to weigh 40,000 lbs at combat weight -- I doubt you'd be able to achieve that with swing-wings...)? 


KJ_Lesnick
 

[KJ_Lesnick]

r16,

Okay, I understand what you're saying, but what I'm wondering is... if a 40,000 lb VG design could have been cooked up, then why was the F-14 so heavy (I mean the F-4, which was for the Navy was a 40,000 pounder, and the F-15A was like 38,100 without any weapons and 40,100 with the four sparrows on it, and was an Air-Force so there's obviously not too big a weight difference if the strength is built in right?)?


KJ

[Archibald]

My own little suggestion : try to consider the french Mirage G (a less-known prototype which flew between 1967 and 1970)

Two-seat, TF-30, 18 tons.

[KJ_Lesnick]

ı would doubt F-14 could be done as a 18 ton aircraft as it is about 19 empty . It is massive but it is all useful weight ,at least generally . You would have to give up the Phoenix for starters I think .

All useful weight?  How so?  You mean just to carry six phoenix's, it jacks up the structural weight that much?  That doesn't make much sense to me as the F-4 could carry something like 16,000 or so pounds of bombs!


Archibald,

My own little suggestion : try to consider the french Mirage G (a less-known prototype which flew between 1967 and 1970)

Two-seat, TF-30, 18 tons.

Now that's a nice-looking design?  Why didn't the French develop it, were there any problems with it?  (By the way -- Is a TF-306 that TF-30 design the french were trying to re-work into a turbo-ramjet?)


Kendra Lesnick

[KJ_Lesnick]

r16,

well , you could make it to carry 16 AIM-54s if you wanted to , but the idea is to "use it in a useful way" . Remembering the F-15 which is not exactly small  would carry only 2 , at least in a naval setting , tends to support the idea the Tomcat is just about right size .

I'm not exactly sure that I understand your statement about being able to "usefully carry them" -- I thought being able to simply carry them under the plane without causing any harm to the aircraft was good enough.

I thought the USN never seriously thought of using an F-15 for Naval use?  Regardless, I'm surprised it could only carry so few of them.     

As an aside I have seen the suggestion that the 6 Phoenix missions are not allowed as carrier decks are not cleared to absorb the landing shock that would result .

What is the maximum amount of AIM-54's an F-14 can bring-back to the carrier?

To land x amounts of bombs on carrier , the poor USN had to buy entirely new Hornets since the legacy ones lacked the lift ,or the wing area .

I didn't know that.  Although to be honest, I think it would have been better if they went with F-14's as they could carry large amounts of bombs and were actually better in the bombing role than the F-18 was.

And sizing something up sizes everything up .The F-16 is less than 9 tons empty while the F-2 is 12 . And what is F-2 apart from a larger winged F-16 ?

Actually, I thought the F-2 was the same thing as an F-16 and was unaware of it's wing-size difference, but it is odd that it would have such a large wing-area difference. 

if USN sees a Phantom with 16000 pounds of bombs on it approaching to land on a carrier , I am so sure that they would open fire with everything ... Even it was 1962 and the plane had Stars and Bars the size of the carrier itself .

Well, that's probably true!


KJ