Good Model

[Tom Goodrick]

After knocking the bad model and deleting it, I went looking for good models of jets. I found two. veneaviones_cii_yv550cp.zip is very good model of the Citation II, currently known as the Bravo. It is easily flown, behaves reliably and looks very good.

fs9sovi.zip is a complex model of the Citation Sovereign that performs nicely after you get it loaded and do a couple of minor things to it. It is by [e-mail]. He put a loy of work into it. For example:

"The passenger cabin is fitted with a number of
features to click on such as functional doors,
tables that extend , a sink with running water
and a flush toilet , clothes closet and a computer screen that functions and can be repositioned."

When I tried loading the aircraft with no modification, FS9 found a file it disliked and would not allow. I could not get past this block. So I did what I normally do anyway. I dumped the panel and copied in one of my standard jet panels. It worked but I lost the fancy VC.

I looked in the aircraft.cfg file and found a few minor things. The MOI's were exactly the same as those of the Learjet 45. Since the empty weight of the Sovereign is almost twice that of the Learjet 45, I corrected the MOI's in proportion to the ratio of empty weights.

I adjusted the CG toward the rear a bit. The model tipped forward sinking its nose gear into the pavement. I checked the gear parameters and found all damping ratios far above unity - bad. Setting the plane back on its feet, it sat better and moved well though the springs are a little more supple - as they should be.

I flew the plane and it seemed okay. After landing I found it was 2400 lb over gross. I decided I needed to get more info on the Sovereign. It was provided in the same download - two PDF files straight from Cessna, one on the design, one on the specs.

For some reason, there were some specs in the PDF that were ignored my the designer. I set the proper BOW and gained about 2400 lbs in payload - the amount the default load was over-gross!

The PDF gave data for three flap settings: 7, 15 and 35. Those were not set in the model. I changed that - no big deal except that we might as well have agreement with the PDF.

I went flying again and paid close attention to speeds and fuel flow. It met the specified climb schedule. At several altitudes it matched speed and fuel flow. It handled well in turns and climbs. There was no hint of divergence at up to 41,000 ft, even when I took it off autopilot and pulsed the stick. I was relieved to see that. At 41,000 ft it showed the capability for 2812 nm absolute range at normal speed of 445 KTAS, Mach 0.77. Next I will test extended range (significantly slower).

All this performance was with full fuel and half the seats filled - carrying only four pax. That is according to specs. You can carry 8 pax on short flights a shorter radius.

It is a good model, given those few small changes. What surprises me is that some main specs from the PDF were not blatantly ignored: MMO is .80, not .90 as Chuck said. VMO is 305 KIAS, not 480 KIAS as Chuck said. But within these proper constraints, the plane flies fine and performs as it should.

I should add that it looks great. Wish I could change the tail number.

[Tom Goodrick]

I finally gave it an N number. While changing back and forth between planes I had the opportunity to simply type in the number. That's rare.

The Sovereign has 9 seats in the most common configuration, one fixed seat opposite the door and facing the door (for a stu) and eight reclining, sliding and rotating seats in a double club. Since max range is only available with 4 pax, companies with a lot of overseas travel might get six seats that spread out more for sleeping.

Here are some interesting tidbits about the performance of this jet from Cessna's PDF furnished to prospective buyers.

Standard takeoff is with 15 deg flaps. (Flap notches are 7, 15, 35). The V2 is 115 KIAS at 30,000 lb (which is max gross for takeoff).

Maximum Landing Weight is 27,100 lb. The Vref at this weight with 35 deg flaps is 110 KIAS. I made V2 and Vref gauges to show the proper values for any weight.

Flap deployment speeds are 250 kias for 7 deg, 200 kias for 15 deg and 175 kias for 35 deg. The 7 deg setting is normally used below 10,000 ft to help stay legal.

Stall speeds are: 35 deg flap 15 deg flap
30,000 lbs________93 kias______101 kias
24,000 lbs________84 kias______90 kias

(Note that these vary with the square root of the weight ratio, the rule that I use in my V speed calculator.)

Cruise Altitude for Various Distances
0-99 nm ________ 6000-14000 ft
100-199 nm_____15000-27000 ft
200-299 nm_____29000-37000 ft
300-499 nm_____37000-43000 ft
500-999 nm_____39000-43000 ft
1000+ _________41000-47000 ft

Time and fuel to climb to altitude for takeoff at msl and 30,000 lb:
15000 ft 4 min 274 lb
25000 ft 7 min 476 lb
35000 ft 12 min 712 lb
41000 ft 18 min 900 lb
45000 ft 47 min 1572 lb (Step climb)
47000 ft 174 min 4289 lb (step climb)

Note that a direct climb can be made only to 41000 ft from a MTOW takeoff. It takes nearly 3 hours to get to 47000 ft. From the cruise performance data it is barely worth it. You gain 14 knots but lose 27 pph. Worth it?

Cruise Speeds and Fuel Flows at Various Altitudes:
Normal Cruise at 29000 lbs
25000 438 ktas 2528 pph
31000 458 ktas 2408 pph
37000 443 ktas 1943 pph
41000 424 ktas 1570 pph

Long Range Cruise at 29000 lbs:
39000 383 ktas 1387 pph
41000 392 ktas 1390 pph
43000 378 ktas 1312 pph

Long Range Cruise at 23000 lbs:
41000 366 kias 1108 pph
43000 357 kias 1045 pph
45000 366 kias 1049 pph
47000 380 kias 1076 pph

From the limited testing I have done, this model performs close to these specs. Somebody was paying attention!

[Tom Goodrick]

So how did I spend yesterday with this exotic model, zooming off to distant lands? No, just tried to fix the little CG/gear problem.

The plane flew fine, taxied fine, took off okay and landed okay. But my CG gauge showed the CG was way up ahead of the wing. That's not good and could lead to stability problems. But I had flown it up to 41,000 ft without seeing any trouble. Past experience, however, has shown that I can get the CG back over the wing and everything will be fine.

So I spent the day diddling around, adjusting the wing_lon_ pos, the CG x pos, the wheel heights, the gear springs and wing chord. I got the CG in the right place - 15% to 25% of the chord back from the leading edge. But then the main gear would float well above the ground in a half compressed configuration and the nose wheel would sink into the ground.

The relation between CG and landing gear behavior in FS9 has always been strange. You load the front of the aircraft and the tail sinks into the ground. Move the CG back and the nose sinks - exactly the opposite from what you'd expect. The Sovereign seems to pivot about a point in the middle of the fuselage without regard for whether the wheels touch the ground or not. The only time I have seen such a thing is when damping is very high and a force is generated based on the speed of the gear toward the ground before it even touches the ground. That is a reasonable effect of the math - reasonable when you know the math. But this is something else.

Maybe I'll just put the CG back on the nose and forget it! It is generally a good idea to get the physics right except when it does no good.

[Tom Goodrick]

Wow. I solved the problem with the gear. A set of parameters for the gear springs was missing! When the gear does not support the aircraft properly on the ground, it is usually a result of confusion about what the parameters do, or a designer will get mixed up and change the wrong parameter. Both are easy to do. It never hurts to look at these parameters and consider their meaning.

Here is what you see in the aircraft.cfg file:


//0 Class
//1 Longitudinal Position (feet)
//2 Lateral Position (feet)
//3 Vertical Position (feet)
//4 Impact Damage Threshold (Feet Per Minute)
//5 Brake Map (0=None, 1=Left, 2=Right)
//6 Wheel Radius (feet)
//7 Steer Angle (degrees)
//8 Static Compression (feet) (0 if rigid)
//9 Max/Static Compression Ratio
//10 Damping Ratio (0=Undamped, 1=Critically Damped)
//11 Extension Time (seconds)
//12 Retraction Time (seconds)
//13 Sound Type
[contact_points]
point.0=1, -9.35, 0.00, -4.05, 2500.1, 0, 0.6000, 36.0, 0.40, 2.5, 0.85, 10.0, 10.0, 0,

The problem I just discovered in the Sovereign was that #9, the max/static compression ratio was missing in all three gear legs.

Note that the numbering was devised by programmers who have warped minds and believe you start counting things with Zero. Do they count their pocket change that way? So after dozens of tries I was reduced to manually counting these parameters from zero. Note that many have decimals and all are separated with commas. It is worthwhile to stretch the window so the lines are not folded. I changed 6,7,8,9 and 10 to get good performance. How the designer ever got good taxi takeoff and landing performance I don't know. I don't think he looked hard for the problem. I noticed a glitch, a little bump, during takeoff when reaching 90 KIAS. This was caused as lift rebalanced the load on the wheels.

Lets look at these parameters:

#3, vertical position. This was adjusted carefully to show contact (no gap between wheel and shadow).

#4, impact damage threshold (ft/min). At first this was set at 1500 which is normal. But after I broke a main gear while taxiing over a rough spot, I set this to 2500 on all wheels. But it may not have been necessary. It was before I discovered the lack of a max/static ratio. This gear is super strong!

#6, wheel radius ft. This is important to get the plane looking right. It was wrong for the main wheels. All wheels had the same radius: .5 feet. Clearly the mains have a much larger radius. I set 1.1 ft.

#7, steer angle for nose wheel. This was 46.5 degrees. That may be close to the actual angle but is too large for FS. With such a large angle we tend to bind the gear by turning the wheel too much. I set 36 degrees. To turn sharply you have to go slowly. 36 degrees is plenty when you are creeping slowly.

#8, static compression ft. "Static" means unloaded at rest on a parking lot. This is the compression of the gear mechanism and has nothing to do with the wheel size. I set 0.4 ft for all three legs. In many cases this differs between the nose and main gears. I chose to keep it simple as a first try. This relates to the amount of stroke that absorbs energy from the vertical motion. A significant stroke makes for a smoother landing as long as there is sufficient damping to avoid a bounce in most cases.

#9, max/static compression ratio. This has no dimensions. It is a ratio of the max compression the gear allows without bottoming and the static compression (at rest). Here it helps to know a little detail of spring/gravity dynamics. If a mass is cushioned by a simple, undamped spiring with a static compression (supporting the weight) of 1 inch and that mass is raised so the spring compression is zero and then the mass is released. the maximum compression of the spring, stopping its downward motion, is exactly twice the static compression or 2 inches in this case. So you want to have a spring that is capable of more than twice the static compression. Thus a good number for this parameters is 2.5. Now this is not perfect because a) there is often an non-zero vertical speed at initial spring compression and b) there is some damping which is an upward force proportional to the vertical speed (that is, the compression rate of the spring). If we were designing gear for a Navy Fighter, I'd use 5.0 here.

#10, damping ratio. Here I see a lot of high numbers like 2, 3 or more. Those are silly. 1 is critically damped which means steady compression with no oscillation - as good as it gets. This value should be between 0.65 to 0.90. The higher values apply to the nose wheels of planes that will flown by professionals who know how to lower a nose wheel gently but firmly.

By the way, the CG is nailed at 25% with the default load.

Now I can get to the business of flying this bird instead of just watching it teeter on its gear. It does need some minor adjustment of drag on final - wheel drag and/or flap drag. I t has too much fun rolling down hill and is hard to reign in for a landing.