Tuesday, April 14, 2015

VSKYLABS XF-85 'Goblin'


VSKYLABS McDonnell XF-85 'Goblin'

Including a short discussion about: Is it a plausible Dogfighter?



General information:
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 constrains, which required the aircraft to fit into the bomb bay of a B-36 bomber (being a mother ship), resulted in an exotic result.

It's 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.


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 it's flight model controls, Center of Gravity and artificial stability characteristics was a real challenge; This aircraft is relatively heavy for it's size, up to 0.66 Thrust to Weight ratio, features tiny horizontal stabilizers with a 'V' configuration Elevators (which are less effective). Is this flying machine was supposed to serve as an escort fighter which could dogfight interceptors, and make it's kill by using it's gun's?...

In a fighter pilot perspective, being just small (in order to be a 'parasite' on a mother-plane), is not always the best tactics 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 it's safety issues regarding the docking phase of it's flight, it could have survive an actual dogfight with a P-51 Mustang, a Republic P-47 Thunderbolt, or other 50' matched Jet fighter...

It's a 50' era single Jet tiny and bulky aircraft, carried by a large Convair B-36 'Peacemaker', which is a strategic bomber, deeply above enemy territory. Such 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 bomber should be effective, or else, the high numbers will win. If the project of the XF-85 would have carried on, does the XF-85 would have meet it's expectations as a bomber protector? I'm not sure, and we have not discussed yet about it's very low fuel capacity and it's 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 the XF-85, and how we have to fly it correctly and perform aerobatic/dogfight maneuvers, despite it's 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 you best friend for now :)

VSKYLABS XF-85 model is slightly different from the real prototype by having a 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 it's handling, Pitch trim work is continues, 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 XF-85 is setup for 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'll use 100% engine power, the Afterburner will kick in, and you'll get another 3000 lbs of thrust. Now you can 'Rock 'n' Roll' with the aircraft, compensating it's under maneuverability with extra power.

If you want to fly dogfight style maneuvers like a real 50' era jet fighter pilot in such 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 in to a catch - in order to accelerate back to the maneuverability area in it's flight envelope (best continuous turn rate), you'll have stop turning, and you can't effort that time and degrees during a dogfight (unless you are going for a shot, and even though, be prepared for surprises, so, save energy!). Vertical maneuvers are not affordable or possible unless you've planned it ahead. It might be more practical to execute vertical maneuvers at low altitudes, and not below 350 Knots in the starting point.

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

Hope you'll like it!

JetManHuss.



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