Altitude flight dynamics.

[nickpike 0]

Hi guys,Problem. I'm happy with the performance of my aircraft overall, but above 35 000 feet, and particularly at 40 000 feet, the nose rises quite steeply, cruising with an AOA of about 8 degrees probably because the speed is dropping off. How can I keep the nose down at these altitudes.Your help would be appreciated,nick BTW, Thanks for the help so far Milton.

[John_Cillis 2,406]

I guess no one can help until you say which aircraft you're seeing the problem in. Many aircraft can't climb past FL350, and many of those that can need to have some fuel burned off to go from one flight level to the next--step climbing it's called.-John

[Guest Douglas K]

Stick to the basics. The four forces acting on an aircraft in flight are:1. Lift2. Gravity (Weight)3. Thrust4. DragAdjust as necessary to suit the conditions at the altitudes involved.If you need help with the specifics of making those adjustments, we can probably work something out. But as John said more information about the aircraft in question is needed. It's quite possible that the aircraft you're modeling SHOULD fly with a +8 degree pitch attitude at high altitude, but without knowing the aircraft type it's a little difficult to say. Douglas

[nickpike 0]

Thanks for the input guys. The aircraft is my own version of the F-117A Nighthawk. I was thinking that an increase in thrust would sort this out but would knock out the realism for other aspects like takeoff and lower altitude flying. I was wondering if a fix might be more subtle in some of the aircraft characteristics tables in the .air file for example. I've attached a picture of said bird.Thanks for your help,nick

[Guest Douglas K]

Good looking model.The maximum level speed for the F-117 is estimated by some to be slightly in excess of Mach 1 at high altitude, the operating speed is usually listed as .92 Mach. Performance info is understandably scarce, but we can assume that this aircraft would be a good performer at FL350 to FL400, and wouldn

[nickpike 0]

From what I read on the net, this bird is subsonic with a max level speed of 0.82M and cruising at 0.60M at 30 000 ft. She's not that powerful as the runway length is quoted at 11 000 ft. Ceiling 45 000 ft. Weights are as you stated. My wing area is 912.7.The tooltip value is TOOLTIPTEXT_ATTITUDE_INDICATOR_BANK_PITCHThe test values I got were (conducted with the values set in AP and AP on)Altitude***Speed****INCIDENCE ALPHA****Tooltip****Throttle20 000**** 0.8 *********** -1 ********** 1 ******** 57%40 000**** 0.8 *********** 1 ********** -1 ******** 62%40 000**** 0.7 *********** 2 ********** -2 ******** 60%40 000**** 0.6 *********** 3 ********** -3 ******** 67%40 000**** 0.5 *********** 6 ********** -6 ******** 79%Setting to mach 0.4, the nose just continues to rise and she stalls out before reaching 0.4 and the throttles are at 100%.Max speed tests.5 000 **** 0.95 ********** -1 ********** 1 ******** 100%40 000**** 0.90 ********** 1 ********** -1 ******** 100%Many thanks for your attention,nick

[John_Cillis 2,406]

Mach .4 is just too low for that altitude. Think in terms of airspeed too. Mach .8 at FL350 is around 280KTS indicated. Half that, 140KTS, would be very close to stall for a jet like the F117. You may not even notice the problem if you've set the sim to report true vs. indicated airspeed.-JohnEdit:If you find the engine isn't sustaining enough power in higher altitudes, you can try reducing the intake area in 50 pct. increments until the performance curves start to line up.-John

[nickpike 0]

Many thanks. I've found a fairly comprehensive doc on the web that's full of a lot of info, and it states that the absolute ceiling is actually 39 000 ft, and bearing in mind your comment, maybe I don't really have a problem. I don't have performance curves to work to. How would I know if the intake area was optimised.Thanks again,nick

[John_Cillis 2,406]

"How would I know if the intake area was optimised."You really wouldn't know. There's a rule of thumb some FDE designers use, but many ignore the parameter. I use it for quick and dirty adjustments, where performance seems anemic in the flight levels. But it's not the best way to tweak an FDE since there's other tables that control more finely the engine's performance at a given mach number and N1 setting.-John

[Guest Douglas K]

>>>> Many thanks. I've found a fairly comprehensive doc on the webthat's full of a lot of info, and it states that the absoluteceiling is actually 39 000 ft, and bearing in mind yourcomment, maybe I don't really have a problem.<<<

[nickpike 0]

Hey, many thanks for the info. I'll take a look at AAM as I do tend to play with the tables. For example, I adjust table values to allow high thrust with the brakes on and no rolling. Always used aired to date. The inlet area info has been useful. Many thanks,nick

[Guest Ron Freimuth]

>>Changing the inlet area will cause a change in intake ram drag>and thrust; drag is directly proportional to the value, thrust>is inversely proportional. So higher values entered there will>increase the drag and decrease the thrust etc. Table 1507>allows for a more refined adjustment of the same thing, ram>drag and net thrust. >>>You might want to visit:>> http://www.avhistory.org/scripts/MegaBBS/f...w.asp?forumid=5>DouglasAlso:http://perso.wanadoo.fr/hsors/FS_Soft/fsdocs.html Jan 2005 versions of my aired.ini and a commented aircraft.cfg file are there. I see I didn't make it very clear that reducing 'Intake Area' reduces Ram Drag at high speeds, thus is an easy way to adjust thrust in high cruise FL's. Ron

[Guest IanK]

Hello,Can you give me a link to the F-117 performance doc as FL390 seems an odd one. They certainly make a noise when flying overhead our airport.Ian