Hot Weather Cruise

[Tom Goodrick]

In many parts of the US, the temp has been pretty high this summer. I found out how high when I tried to correct a TV weather guy who said the temp at 20,000 ft one morning was only -5C degrees when the normal temp would be -25C. I sent him an email questioning that value and he responded with the temps aloft measured at a station near Birmingham. I checked RW on FS and found -8C at 20,000 ft that morning. That is almost +20C compared to standard. That puts you on the last curve in the performance table for King Airs. So I wondered what effect this would have an aircraft cruise speed using normal cruise power/thrust settings.

I checked a Beech Baron Turbo charged, a Rockwell 690 turbo prop and Cessna Mustang jet. Each was tested at normal cruise power at 20,000 ft.
_____________________________________-25C____-8C
Aircraft________________Power__________ KTAS____KTAS
Beech P Baron 58________76%___________233______222
Rockwell 690____________79%, 77%______268______257___could not set 79%
Cessna Mustang_________65% Throttle____328______321

(Note on jets in FS, throttle position, as noted precisely by my gauge, is best indication of %thrust because thrust is linear with throttle position. I guess I should check this. At least matching pph matches power. I saw 416 pph at -23.9C and 415 pph at -8.1C)

Bottom line is you can lose 10-11 kts cruise spd.

[Tom Goodrick]

Well, I have started looking at performance indicators for jet aircraft. There does seem to be some value in the throttle position. Many jets have markings on the throttles for various phases of flight. I started looking at thrust while flying the Learjet 45. It shows variations and sensitivities that are complex. The fuel flow has always been a very good performance indicator. I suspect that in most cases automatic systems control the power in most jets. The trick is to find a way to control FS jets that produces realistic performance. In the past I have used the fuel flow as an indicator to control climb, cruise and descent. But while doing that I noticed the relationship with throttle position and began using the throttle position for everything except cruise setting because it is important to set the fuel flow accurately in cruise. Of course, other parameters were adjusted during FD development to get the right cruise speed at the right fuel flow at a particular specified altitude based on flight test reports.