19
$\begingroup$

Jet fighters are made to cruise at a high subsonic speed, even at a supersonic speed (supercruise?). It means that they can be flown comfortably at relatively high speeds.

Jet fighters are often used to intercept aircraft, whatever the reason (assistance, escort from national to international airspace, check an unresponsive aircraft,...).

General aviation aircraft can cruise at a slow speed compared to jet fighter. Moreover, if needing assistance, it may fly even slower. I imagine it is possible that a GA aircraft needing assistance cannot fly at a speed above the stall speed of a jet fighter. I know that some countries dispatch choppers to intercept slow aircraft to handle such situation.

I imagine the military doesn't fly their jets at stall speeds when close to another aircraft (a margin is needed).

Is there a speed below which a jet fighter cannot intercept another aircraft? Otherwise, how do they do it? (I imagine you can orbit above another aircraft but it is less convenient to guide it safely to an airport)

$\endgroup$
  • $\begingroup$ Edited title for typo - hope that’s what was intended! $\endgroup$ – Cpt Reynolds Jul 26 '19 at 8:35
  • 8
    $\begingroup$ There was an episode of The Simpsons where Sideshow Bob tried to escape in the Wright Flyer. (Yes, the first airplane). It was so slow that the jet pilots who went after him had to land their planes and run after him on foot. $\endgroup$ – Shawn V. Wilson Jul 27 '19 at 19:55
16
$\begingroup$

There is nothing stopping a faster jet from flying a zigzag pattern behind an intercept target if the goal is to remain in close contact with it, but a large difference in airspeeds would indeed hamper some tasks like observing the cabin for signs of life or unlawful interference. So the answer is yes, a jet fighter cannot perform all the tasks involved in a typical interception of an unresponsive aircraft if the speed delta is too high.

For reference, the takeoff speeds of 4.5 generation fighters like the F-16 are in the 130 kt (240 km/h) region, while the takeoff speed of something like a C172 is around 60 kt (111 km/h). The exact weapon and fuel load of the fighter will of course have an impact, but fighters scrambled for an intercept are typically light and with an almost clean configuration. I have deliberately used takeoff speeds instead of landing speeds to factor in a margin of safety in regards to control authority; both speeds are among the lowest an aircraft can sustain, but during close approaches to a potentially hostile aircraft, some maneuverability is required.

Still, 100+ km/h is a substantial speed gap, and has led to issues in the past:

The Po-2 is also the only biplane credited with a documented jet-kill, as one Lockheed F-94 Starfire was lost while slowing down to 161 km/h (100 mph) – below its stall speed – during an intercept in order to engage the low flying Po-2.

$\endgroup$
  • 2
    $\begingroup$ Presumably Starire -> Starfighter. | The SF didn't need any reason to fall out of the sky - the Polykarpov was just an excuse. |||| Q: "How do you acquire a Starfighter?" A: "Buy a plot of land. and wait." $\endgroup$ – Russell McMahon Jul 27 '19 at 13:28
  • $\begingroup$ In WW2 Po-2s "killed" 109s the same way $\endgroup$ – slebetman Jul 27 '19 at 13:30
  • $\begingroup$ @RussellMcMahon To be honest I was surprised it wasn't a Starfighter. It would have been a fitting cherry on top of its nefarious record. $\endgroup$ – AEhere supports Monica Jul 27 '19 at 14:38
  • $\begingroup$ No, I'd install the U2 wings on the Starfighter, then chase it. $\endgroup$ – Robert DiGiovanni Jul 28 '19 at 23:15
4
$\begingroup$

Military fighter jets are actually not all that fast, when they aren't supersonic. Their cruise speeds are below the cruise speed of airliners. Supersonic flight is mostly meant for interception, while combat speeds are subsonic. The best-range cruise speed for most fighters in the range of 300 knots, due to high drag.

Below their cruise and maneuvering speeds, there is considerable margin to an actual stall. It depends on weight and altitude, so there is no one answer, but it goes down to 100 knots and below.

The most popular GA aircraft, the Cessna 172, has an official cruise speed of 122 knots. This is within the operating limits of most jet fighters, except when overloaded or at altitude above that of GA aircraft. So there is a sufficient amount of overlap between GA and jet speeds, if both want to stay alongside.

However, a GA aircraft can operate much slower, and jet fighters generally won't be able to maintain a steady position close to their stall speeds. While their TWR can be close to 1, the typical fighter aircraft is simply not controllable at a 45-degree angle of attack. Make no mistake, a fighter won't lose a GA plane, but it won't be able to maintain a steady position next to it.

Fighters with thrust vectoring, which are still rare, could sustain mostly side-by-side contact with a GA aircraft. The Pugachev's Cobra maneuver can slow a supermaneuverable plane to near-zero airspeed, although it will have to get back to its stall speed after.

That said, a helicopter is still more practical for assisting a GA airplane, not least because it's much more likely to be able to land near the plane's crash site to help pull the occupants out.

$\endgroup$
  • 4
    $\begingroup$ Wow, the old straight-tail 172 I flew sure didn't cruise at 122 knots. Anyway, would almost be worth busting some Russian airspace just to see someone doing a Cobra maneuver next to me. $\endgroup$ – quiet flyer Jul 26 '19 at 17:40
  • 1
    $\begingroup$ @Therac, I downvoted your answer because I don't agree with the first paragraph. True that the slower the speed the tighter the turn, but no fighter will enter a merge at 200 knots. Speed is life. There is a number called "cornering speed" where the aircraft is fast enough to pull right up to the G limit without entering an accelerated stall, but will stall before overstressing. This is where the best turn performance is achieved. I didn't fly fighters, but in the Prowler this was 300KIAS for a frame of reference. Still, we practiced defensive ACM much faster. $\endgroup$ – Michael Hall Jul 26 '19 at 18:19
  • $\begingroup$ @MichaelHall Ouch, re-read my post - I merged two sentences I shouldn't have. $\endgroup$ – Therac Jul 26 '19 at 18:41
  • $\begingroup$ I still don't like the answer. With flaps down at 120 a fighter can make a radius of turn equivalent to a Cessna at cruise. However, it isn't about the radius, in a fight you want the best turn rate. A better turn rate will generate the angles to achieve an advantage and take a shot. Best rate occurs at cornering speed. I will have to ask an old Hornet friend or do a search on what this typically is for a fighter. I also think cruise is closer to 420 TAS or .7 mach. It was for us... $\endgroup$ – Michael Hall Jul 26 '19 at 20:48
  • 1
    $\begingroup$ @MichaelHall I've read corner speed for an F-16 is around 450 kts. Not sure how typical that number is among fighter aircraft. $\endgroup$ – Todd Wilcox Jul 27 '19 at 7:08
4
$\begingroup$

Depending on aircraft type and weight, flaps down approach speeds for most military fighters is in the range of 120 to 150 KIAS. This is within the speed range of medium to higher performance GA aircraft, but a bit too fast for most fixed pitch prop, fixed landing gear aircraft in common use. Some fighters are quite controllable at high AoA at speeds lower than optimal for approach, but it is unlikely that a fighter pilot would be comfortable flying formation at such a slow speed.

Additionally, there is almost no assistance a military jet could provide to a GA aircraft that would require the two to be in close formation at matching airspeed. The nearest situation necessitating actual close formation flight would be to provide visual verification whether landing gear were fully extended. While close up inspection of an over-center lock is possible to confirm, a general up, down, or stuck halfway assessment is possible from quite a bit further.

Even if it were ideal or possible, unbriefed formation flight between dissimilar aircraft with a GA pilot of unknown proficiency presents its own set of risks. Communication relay to ATC, assigning headings or leading a lost-comm pilot to a hole in the clouds, etc. are legit forms of assistance that could be provided from an interceptor orbiting higher overhead, or leading on at a slightly higher airspeed.

$\endgroup$
  • 1
    $\begingroup$ The most useful assistance a military aircraft could provide to a GA aircraft is guidance if the later lost comms over unfamiliar terrain or cannot be guided over radio due to lack of radar coverage. And that does require the guiding aircraft to fly slow enough that the guided one can keep up with it. $\endgroup$ – Jan Hudec Jul 27 '19 at 19:17
  • $\begingroup$ I fail to see how it answers the question (what is the slowest-speed?) $\endgroup$ – Manu H Jul 27 '19 at 20:49
  • $\begingroup$ @Manu, you are correct, I didn't directly answer the question with a specific airspeed. My point is that you don't have to match speeds to intercept, and you wouldn't likely escort in close formation either. $\endgroup$ – Michael Hall Jul 27 '19 at 21:17
  • $\begingroup$ @MichaelHall You should edit your answer to explicit this point (no lowest speed as you don't need to match bogey's airspeed), otherwise you answer may be flagged as not an answer. $\endgroup$ – Manu H Jul 28 '19 at 7:26
  • $\begingroup$ @Manu H - edited per your suggestion. Please let me know what you think. $\endgroup$ – Michael Hall Jul 29 '19 at 18:31

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.