Recip Thrust Test 12/1
Thrust Basic formula:
Force= Mass x Acceleration (F= mA)
Mass increases = thrust increases
Acceleration increases= thrust increases
(Opposite is also true)
*Calculate mph to ft/sec would be /60 x /60 (or /360) then x 5280
Gross Thrust
Static thrust, amount of thrust developed by engine when aircraft is standing still on ground
0 forward airspeed, acceleration rate is the highest
Fg= weight/ 32.2 (Ve)
Ve is exhaust velocity or final velocity
No Vi or initial velocity because not moving, it would be 0 since not moving
Net Thrust
Amount of thrust developed by engine when aircraft is in flight
When mass flow is the same for both aircraft, the gross thrust is higher, bc the change in velocity across the engine is greater
Fn= weight/ 32.2 (Ve- Vi)
TSFC
Pounds of flow consumed to produce one lb of thrust for one hour of operation
TSFC= Wf/lbs of thrust
(net or gross)
Wf= total weight of fuel consumed
Ram Air Force
Turbine engines are effective when fly fast bc of ram Air Force
Inlet divergent on subsonic
Increases pressure at inlet compressor
P2 max pressure
Less than mach .1, P2 becomes less ambient
P2 and PA same if Mach .1- .2
Choked nozzle
Reached speed of sound
Limits amount of air that can leave the engine
Air cannot be compressed or get out anymore out
Pressure will increase inside
F= w/g (Ve- Vi) + Ram Air Force + Choked Exhaust Force
Thrust Horsepower (THP)
THP = Fn x MPH/ 375
Thrust into hp
Fn= net thrust
Faster I travel, THP goes up
Thrust producing engine compared to hp producing engine
Propeller Thrust (Turboprop)
Fp= Hp x 375/ mph
Measured in lbs of thrust
Vast majority of thrust is producede by prop
Large volume of air, tiny amount of speed
ESHP
Equivalent shaft horsepower
ESHP= SHP + thrust
SHP= hp delivered to shaft, spins the prop
More common on turboprop
Efficiencies
Propulsive efficiency
External power plant efficiency
How fast the exhaust is leaving
Difference between Ve and Vi, (Ricky’s formula would be P= Vi/ Ve)
There is some kinetic energy that is not experienced in engine (Ve), there is a loss of energy
Faster you travel, Peff would go up
Max prop eff when forward velocity = exit gases
Formula:
Peff= 2/ (1 + Ve/Vi)
Gross Vi is 0 so there would be 0 Peff
100% Peff would be when airspeed matches exhaust speed (not achievable)
Thermal Efficiency
Hp output of engine, how much provides forward thrust
Efficiency numbers not great, but better than recip engines
Recip range= 20-30%
Turbine range= 30-45%, bc exhaust is pretty hot, not cold
Formula:
TE= HP/ HP fuel
(HP: output of engine)
(HP fuel: hp in the fuel)
Thrust produced divided by gal consumed
Expected to have a little higher than radial
Overall Efficiency
Overall eff= P x T
P: propulsive efficiency
T: thermal efficiency
Jets has overall efficiency
Propeller efficiency climbs faster than thermal efficiency descends
Review
Intake area
Pressure: gradually goes up
Velocity: vibrating up and down bc it is slowing down the inlet velocity so it can be at 80 ft/sec
Temperature: goes up slightly
Diffuser
Pressure: peaks in pressure
Velocity: slows down
Temperature: steady
Combustor
Pressure: slight decrease so flame goes out the back, pressure turns into velocity
Velocity: steady, so it stays at 80 ft/sec
Temperature: steep rise in pressure
Turbine Section
Pressure: goes down, zig zag down
Velocity: goes up
Temperature: starting to come back down bc extracting energy
Affect Thrust
Altitude- air pressure, secondary air, less air providing cooling, could over temp the engine
Water injection- increase mass
After burner- add fire, increases velocity, move air going out faster
Methanol injection-
Rain, humidity