ferpe From France, joined Nov 2010, 2666 posts, RR: 58 Posted (1 year 7 months 2 weeks 1 day 23 hours ago) and read 3240 times:
When working to analyse the wings of the modern B and A frames it became appearant that TSFC is not a fixed value for cruise as stated in many schoolbooks. Further the confusion between static sea level TSFC (around 0.3 ) and cruise TSFC (around 0.6 ) is widespread perhaps because the static full power TSFC is almost 50% better then the cruise values,.
I poked around a bit and found this very good graph in "Airplane Aerodynamics and Performance" by Roskam:
This graph shows how thrust and TSFC varies with M and altitude for a high bypass Turbofan. We can see that the static thrust falls a lot and that the TSFC goes to 0.7 at M 0.8 and FL350 and above.
Perhaps some engine wizard could explain a bit more why? What would be the scale of effects from inlet losses?
The most basic reason is that engine efficiency (which TSFC measures) is a strong function of pressure ratio. Lower thrust means lower core speeds means lower pressure ratio means lower TSFC. As you gain altitude density falls, so the mass flow through the engine falls for a particular pressure ratio, so thrust falls. Fuel flow also drops, which helps, but loses aren't as strong a function of density so loses get bigger relative to thrust and overall TSFC falls.
Depends on how good the inlet is. Could be almost nil for a really good inlet at low speed, can be the dominant factor for a marginal inlet at high speed, can be impossible to avoid for very high speeds.
For one thing, thrust is based on momentum which is the velocity difference in the inlet and exhaust velocities. Hence why you lose thrust at speed (up to a point). Also, the mass flow through an engine will change at different operating points, all of which are generally indexed to a design point. This also affects the change in momentum. The mass flow can be computed based on the actual vs. reference based turbine inlet temperature and compressor pressure ratio.
TSFC will generally increase at cruise conditions, but getting up at high altitudes and using less thrust generally means that fuel burn comes out ahead. A first order approximation for thrust lapse is just to multiply the static thrust by the density ratio. In reality you'll get some back at higher mach numbers due to compression effects. (If you get enough of that to not need a compressor, you then have a ramjet) You could also check out this software in tunnel test mode to see how things: http://www.grc.nasa.gov/WWW/k-12/airplane/ngnsim.html
Depends on the plane. For a transport that is generally cruise optimized, probably very little. At cruise the capture area and inlet area will be basically the same but the capture area will be far larger at takeoff. (so much so that the 707 needed blow in doors to get enough mass flow) For a fighter, you'll have huge additive drag losses at lower speeds since the inlets are designed for supersonic operation. And those tend to be longer so you get more friction losses that sometimes necessitate bleeding the boundary layer.
Why do Aerospace Engineering students have to turn things in on time?
- Generally the CF6-80C2 is labeled between 0.56 and 0.605 in these threads, Lisys info point more to 0,6 then 0,56, could be different generations however but i think the 0.56 sounds more like the CF6-80E2 for the 330.
- T500 is more modern then CF6-80C2 and seems to be in the 0.575 range with the T900 0.010 further down.
- The GEnx-1B was listed as 0.5279 at FL370 by Lisys initially but it seems more like 0.530 is the optimum value (above 36000ft and high utilization)
- The table for GE90-115 is the GEnx centered with 0.545 as best value, this seems to be best value for the GE90s i.e. above 36kft.