The IAI Kfir Corner Velocity: A Flight-Envelope Analysis

[Aircraft Performance] issued 25th April 2015

The VSKYLABS IAI 'Kfir' TC-2 is a "Turn Performance" flight model, designed to explore and match the turn-rate performance of the actual ‘Kfir’ aircraft in X-Plane 9/10.

It is an X-Plane ‘Kfir’ maneuverability demonstrator.


General References:
The current flight model of the VSKYLABS IAI 'Kfir' TC-2 gives a general perspective of its real turn performance, based on its Turn Performance Diagrams at 5,000, 15,000, and 25,000 feet. The ‘Corner Velocity’ was the main goal of achievement (actual performance diagrams of the ‘Kfir’ are attached below, for your use).

Corner Velocity:
In simple words, 'Corner Velocity', which is also called 'corner speed', is the minimum speed at which maximum G can be obtained (at full engine power). Fly faster than it, and you can instantaneously pull to a maximum, and even over-G, turn (and lose energy in the process). Fly slower, and pulling all the way will get your turn rate to drop rapidly, so the bad guy will gain angles as you'll lose energy.

Flying at the speed of 'Corner Velocity' is a good tactic in a dogfight. You are maneuvering at the best sustained turn rate of your aircraft, without losing energy, flying the air-combat geometry, and at the same time, you have the potential to 'go for the kill', or make defensive maneuvers by pulling harder into the maximum-G or highest turn-rate zone (which is usually not a sustained situation because of drag and thrust-to-weight ratio issues).

For example, the corner velocity of the F-4E at 15,000 feet and maximum thrust is just above Mach 0.8, and it can hold up to 7 G’s sustained turn (depending on its configuration) without losing energy. Its turn rate in this condition would be ~14 degrees per second. The corner velocity of the 'Kfir', at the same altitude, is around Mach 0.7, but its turn rate would be ~16 degrees per second. So, at 15,000 feet, the 'Kfir’s' turn rate would be slightly higher, and its corner speed would be slightly lower than the F-4E. I will discuss dogfight tactics in another post :)

X-Plane flight performance note:
The VSKYLABS IAI ‘Kfir’ TC-2 flight model was tuned to match its turn performance, but to do that in X-Plane 9/10, engine thrust had to be changed to higher values, and there are some changes (in the "backstage") to the wing area and thickness, which are not visible and do not affect its general delta-wing handling characteristics. These changes slightly affected the accuracy of some non-drag-consuming zones in its flight envelope, but, all in all, and based on my experience, it’s a good ‘Kfir’ maneuverability demonstrator. (Supersonic flight characteristics are not fully tested in this version, yet…).

Here are performance charts for the 'Kfir C-7', which is quite similar (the C-7 was a more powerful version):

The McDonnell XF-85 Goblin: A Plausible Dogfighter?

 [Test-Pilot Report] issued 14th April 2015

*Test-Flight report and insights, conducted in X-Plane 9/10:

The McDonnell XF-85 Goblin was designed to be a single-seat "parasite" escort fighter that could be carried by a large bomber. Development of two prototypes was ordered in March 1947. Design constraints, which required the aircraft to fit into the bomb bay of a B-36 bomber (being a mother ship), resulted in an exotic result.

Its tiny, short fuselage was fitted with folding swept wings, of 21 ft 1.5 in (6.44 m) span. It was powered by a 3,000 lb (1,400 kgf) Westinghouse J34-WE-7 turbojet. There were no landing gears except for emergency skids. The fighter was intended to return to the parent aircraft and dock with a trapeze, by means of a retracting hook.

*Image attribution:

By Author listed as "U.S. Air Force photo" - National Museum of the US Air Force (Direct Link) - Image listed as "080227-F-1234S-050.jpg", Public Domain, https://commons.wikimedia.org/w/index.php?curid=16214546

Was the “Parasite” concept plausible?:

Although I was expecting stability and handling issues in the making of this small, short, and heavy aircraft, I found out that configuring its flight model controls, Center of Gravity, and artificial stability characteristics was a real challenge. This aircraft is relatively heavy for its size, up to a 0.66 thrust-to-weight ratio, features tiny horizontal stabilizers with a "V" configuration, and elevators (which are less effective). Is this flying machine supposed to serve as an escort fighter which could dogfight interceptors and make its kill by using its guns?…

From a fighter pilot perspective, being just small (in order to be a "parasite" on a mother-plane) is not always the best tactic to win, or even survive, a dogfight. There were test flights to check the concept of being a "parasite" on a bomber, in and out, but I’m not sure that if this project would have survived its safety issues regarding the docking phase of its flight, it could have survived an actual dogfight with a P-51 Mustang, a Republic P-47 Thunderbolt, or another 50’s-matched jet fighter…

It’s a 50’s era single-jet, tiny and bulky aircraft, carried by a large Convair B-36 Peacemaker, which is a strategic bomber, deep above enemy territory. Such a strategic mission, in those days, was subjected to massive numbers of hostile interceptors, which defended their land (therefore being able to take off in large numbers). Protection of such a bomber should be effective, or else the high numbers will win. If the project of the XF-85 would have carried on, would the XF-85 have met its expectations as a bomber protector? I’m not sure, and we have not discussed yet its very low fuel capacity and its early jet-engine fuel-consumption characteristics.

Flying the XF-85 in X-Plane can give us a little perspective on how difficult it might be handling the XF-85, and how we have to fly it correctly and perform aerobatic/dogfight maneuvers, despite its unwanted “habits”.

Flying tips for X-Plane flight simulator:

First of all, it’s a "handling and performance" project of mine, so there isn’t a fancy 3D cockpit. The HUD is your best friend for now :)

The VSKYLABS XF-85 model is slightly different from the real prototype by having optional fictional landing gears, an afterburner, and some modified artificial-stability features. The purpose of these is to raise the fun factor of operating this aircraft in the simulator.

Flying the tiny/bulky XF-85 feels almost like flying a heavy and dirty-configuration fighter jet. You have relatively low authority regarding its handling, pitch trim work is continuous, and when maneuvering, you have to constantly manage your potential (airspeed and altitude). It’s not an agile fighter as you might expect it to be (this particular version of the XF-85 is set up for a good balance between "control" and "feel", but it’s not the final or "perfect" setup, as I’ll keep running my updates).

The thrust-to-weight ratio of the flight model is the same as in the prototype. If you want to practice flying with this ratio, use 98%-99% of engine power. If you use 100% engine power, the afterburner will kick in, and you’ll get another 3,000 lbs of thrust. Now you can "Rock ’n’ Roll" with the aircraft, compensating its under-maneuverability with extra power.

If you want to fly dogfight-style maneuvers like a real 50’s-era jet fighter pilot in such an aircraft, get your speed up to at least 300 knots, and keep your turns at 4-5 G’s. If you want to have a tight turn, use full power (99% or 100%), and turn while the nose of the aircraft is pointing below the horizon, using gravitation as an additional power source. Getting yourself below 300 knots in a dogfight will get you into a catch - in order to accelerate back to the maneuverability area in its flight envelope (best continuous turn rate), you’ll have to stop turning, and you can’t afford that time and degrees during a dogfight (unless you are going for a shot, and even then, be prepared for surprises - so save energy!). Vertical maneuvers are not affordable or possible unless you’ve planned them ahead. It might be more practical to execute vertical maneuvers at low altitudes, and not below 350 knots at the starting point.

This is definitely not my last update for the XF-85, so keep tracking my updates.

Hope you’ll like it!

JMH