FFS Helicopter Design

Date: Tue, 13 Jun 1995 10:03:17 +0300 (EET DST)
From: Lahtinen Antti Jussi <al76188@cs.tut.fi>
Subject: FFS Helicopter Design

	FFS Helicopter Design

	A while ago I wrote about the difficulties of re-crating modern
	helicopters with FFS design rules. Because the rules would not
	work correctly, I did some literature research about real-life
	helicopter design, and found the mathematical model of power
	consumption in hovering and flight.

	Complete helicopter model is very complex, and while it gives
	quite accurate power consumption values for various airspeed,
	weight, and drag combinations, it is too massive to be used
	with FFS design rules. However, the model can be used to
	pinpoint some errors in FFS.


	Design Strategy

	Helicopters can be designed using either aircraft of lift
	vehicle rules. Both rules give quite similar end results, but
	lift vehicles can be designed with armor.
	Note that page 22: Lift Vehicle Chassis Configuration table and
	page 30 Airframe table have similar values (only examined from
	different perspectives) and all Airframes can be used as Lift
	Vehicle Chassis.

	Airframes not listed in Lift Vehicle Chassis table
	Type		Volume	SA	Price	Min	Max	Eff
	Autogyro	0.005	70	1.02	40	200	0.65
	Wing-in-Ground	0.05	70	1.02	75	400	0.90

	(These airframes are not suitable for helicopters, but I use
	these values when I design PARWIG vehicles.)


	Rotors

	FFS, page 32: Rotor Assemblies lift values are too low. I could
	model only MTR and NOTAR rotors, and lift values for other
	rotor types can only be estimated.

	Type	Lift (ton/MW)
	MTR	8
	NOTAR	8.4
	TMR	10 ?
	CMR	10 ?
	X-Wing	10 ?

	I could not find enough data to check the actual weights of
	different rotor assemblies, so I use the values from the Rotor
	Assemblies table, but substituting:

	Wt: Weight in tons, per _MW output_ of power plant.
	Volume in m3 is equal to the weight of rotor. (This is the 
	actual volume of the rotor assembly. Note that parked 
	helicopter may still take large amount of hangar space, because 
	the rotor blades can be quite long.)

	Power Plants

	Power plant can be either Improved Internal Combustion Engine
	(piston engine) or Gas Turbine. Almost all modern helicopters
	use gas turbines ("turboshaft"), and piston engines are mainly
	used in radio controlled drones.
	One possible helicopter power plant is rotor tip-drive, that 
	means tip-mounted pulsejets (AZHRAE) or ramjets. Pulsejets are 
	used mainly in target-drones, and ramjets are used in TJ-5 and
	NHI Kolibrie helicopters. I am still unable to find enough 
	design data for rotor tip-drive engines.

	FFS, page 63: Chemical Power Plants table shows quite poor
	values for gas turbines. The following values are taken from
	Pratt & Whitney PT6 -series turboprops and turboshafts, and the
	values are for the whole engine installation (including gas
	generator, free power turbine, reduction gearbox and drive
	shaft).

	TL	MW	Mass	kl/hour	Name
	6	1.9	0.5	0.39	PT6A-6
	7	3.4	0.7	0.345	PT6A-42
	8	4.9	0.9	0.3	PT6A-68
	9	6.4	1.1	0.255	Estimated future model
	10	7.9	1.3	0.21	Estimated future model

	Note: values are for 1 cubic meter of engine. Original engine
	volumes (Min Vol ?) were about 0.22 m3. TL 8+ values are
	estimates based on PT6 technical evolution curves.

	Gas turbine is more suitable helicopter power plant than MHD
	generator, because gas turbine produces mechanical energy and
	MHD generator produces electrical energy.
	Magnetohydrodynamic generation of power is based on the direct
	conversion of fuel energy into electrical energy by flowing a
	heated, electrical conductive fluid or gas through a magnetic
	field. To achieve this, fuel is often doped with impurities, so
	that exhaust gas is electrically conductive, and this exhaust
	gas is piped through magnetic field. The impurities in fuel
	often make the exhaust gas highly toxic.
	Note: the MHD _turbine_ described in text in page 63 is
	completely unknown to me. As far as I know, the modern MHD
	power generators are not based on turbine technology.

	Sources:
	Jukka Raunio; Helikopteriteoria. Published by Finnish Border 
	Guard Department, 1989.
	R.W. Prouty; Helicopter Aerodynamics. PJS Publications Inc. 
	USA, 1985.
	Journal of Engineering for Gas Turbines and Power; The PT6 
	Engine: 30 Years of Gas Turbine Technology Evolution. 
	Transactions of the ASME, APRIL 1994, Vol. 116.

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