FFS technical stuff

Date: Thu, 6 Apr 1995 19:25:41 +0300 (EET DST)
From: Lahtinen Antti Jussi <al76188@cs.tut.fi>
Subject: FFS technical stuff

	Helicopter Rotors

	I tried to re-create various modern helicopters using FFS aircraft
	design rules, and I noticed that FFS designs require much more
	engine power than modern helicopters.
	That is, Huey Cobra uses 0.8 MW engine power to create 11 tons
	of lift, and FFS allows only 4-5 tons of lift per MW. This low
	effiency means that FFS-style Huey clone needs huge engines and
	consumes fuel like hell.
	Huey Cobra weights 9.5 tons, and corresponding FFS designs weights
	24.5 tons.
	However, if the lift/MW values are doubled, modern helicopters can
	be designed.

	----------


	Volume-based aircraft design

	While the present aircraft design rules in FFS are based on the
	weight of the airframe, it is possible to design fixed-wing and
	rotary-wing aircraft by using the design rules from lift-vehicle
	chapter, and design evaluation rules from aircraft chapter.
	The main difference is that you can design armored aircraft with
	lift vehicle rules.

	----------


	Displacement Tons

	When I was re-designing modern vehicles, I noticed that the numbers
	can be deceiving, for 1-ton vehicle is already quite large.
	I checked the volume of various modern vehicles and come up with
	following values:

	Vehicle Name	Approximate Volume in Displacement Tons
	------------	---------------------------------------
	Motor bike	0.2  tons or less
	Family car	0.5-1.0 tons
	Van		1.0-2.0 tons
	APC		2.0 tons (M113)
	Helicopter	1.5 tons (Huey Cobra)
	Jet Fighter	2.0 tons (F-18 Hornet)
	MBT		4.0 tons (T-80)
	Large MBT	6.0 tons (M1 Abrams)

	Compared with these values, 12-ton MBT (Prairie Fire) and 4-ton
	LRV (Pandina) are quite large.


	----------


	Hull Material Volume

	When I calculate the hull material volume (with Exel spreadsheet),
	I use the following formula:

	MV = Material Volume
	DT = Displacement Tons

	MV = (DT*14)-4/3*PI()*(((DT*14)/(4/3*PI()))^(1/3)-0,01)^3

	(Volume of spherical hull, minus the volume of a spherical hull
	which has 1 cm smaller radius. This difference gives the volume
	of 1 cm thich hull armor.)

	This formula gives quite similar values as Hull Size table in FFS,
	page 11, but the results begin to differ in the high and low ends
	of the table.
	
	Rate	Exel	FFS	Rate	Exel	FFS	Rate	Exel	FFS
	1	0.28	0.4	100	6	6	1000	28	28
	2	0.44	0.5	200	6.9	9	2000	44.6	43
	3	0.58	0.7	300	12.6	12	3000	58.4	57
	4	0.70	0.9	400	15.2	15	4000	70.8	70
	5	0.82	1.1	500	17.7	17	5000	82.1	80

	Either this formula is wrong, or there is some errors in the FFS
	table.

-- 
        Antti Lahtinen    :     Justice is Only a Wish of a Weak
        al76188@cs.tut.fi :