Date: Thu, 6 Apr 1995 19:25:41 +0300 (EET DST) From: Lahtinen Antti Jussi 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 :