Flight Dynamic Problems

[Dangerousdave26]

For my first attempt at creating flight dynamics I thought I would mess around with the FS2004 Pander S.4 Panderjager so far I have taught myself a lot and I am happy with it but...

I collected data from as many sources as I could and for values I could not come up with real world numbers I did the next best thing. I looked at other similar aircraft from that period. I did not copy those values as much as I applied some SWAG to them.

For reference here is the real world data I collected.

<from various sites>
3 Wright Whirlwind R-975-E2 engines, 420 hp each.
Length 12.5 meters, 41.0 feet
span 16.6 meters, 54.4 feet
height 2.9 meters. 7.5 feet
Wing area: 46 sq. meters 150.9 feet
Empty weight: 3,441 Kilo's, 7 586.1 lbs
MTOW 5,736 kilo's, 12 645.7 lbs
Max speed 360 kmh, 223.7 mph, 194.4 kts
cruise speed 300 kmh, 186.4 mph, 162 kts
Landing speed 120 kmh, 74.6 mph, 64.8 kts
Ceiling 5,400 meters 17,716.5 feet
Range 2,950 kilometers, 1,833.0 miles, 1,592.9 nm

R-975E-3: 420 hp (313 kW) @ 2,200 RPM up to 1,400 ft (427 m), 450 hp (336 kW) @ 2,250 RPM for takeoff. Increased supercharging, slightly higher compression ratio.

Propeller diameter: 2.85 m,
Propeller surface: 6.38 m ²
Climb rate 7.6 m/s,
Maximum speed at ground level: 342 km/h, 212 mph, 184.7 kts

<from a German site translated by yahoo>

echnical data: Pander S.IV Panderjager
Use: Long-distance post office airplane
Year of construction: 1933
Crew: 2-3 men
Engine: three air-cooled 9 cylinders radial engines WRIGHT R-975-E2 Whirwind with three-sheet variable-pitch propeller
Take-off power: 420 HP (310 KW)
Continuous duty: 395 HP in 4.000 m (292 KW)
Span: 16.60 m
Length: 12.50 m
largest height: 3.30 m
Propeller diameter: 2.85 m
Propeller surface: 6.38 m ²
Track width: 5.28 m
Wing area: 46.10 m ²
V-type: +5°
Empty mass: 3,210 kg
Takeoff weight normally: 5,150 kg
Takeoff weight maximally: 5,700 kg
Level of fuel in the tank: 1,090 litres
Surface loading: 123,6 kg/m ²
Power loading: 4,52 kg/PS (6.12 kg/kW)
Maximum speed at ground level: 342 km/h
Maximum speed in 4.000m: 364 km/h
Cruising speed in 4.000m: 300km/h
Ceiling: 6,050 m
Climbing achievement: 7.6 m/s
Climbing time on 1.000 m: 2.25 min
Climbing time on 4.000 m: 9.5 min
Range normally: 1,850 km
Range maximally: 3,240 km
Flight duration: 11 h
Armament: none

This information from the German site was the best it gave more detail.

Here is the problem

I can not get the prop speed up to the rated 2250 rpm. The Prop turns at 1,300 (about) RPM. Much like a Wings of Power aircraft this one preforms better with the prop pulled back to 75% than it does the prop full forward. That translates into about 1000 rpm.

The problem this causes is it takes a lot of runway to get off the ground at 100% prop. Almost all of a 9,000' runway. Now if you pull the prop back it takes off almost like I would expect it to if it were running at 2250 RPM.

I have been reading the Aircraft Container manual but it has not provided me any in site.

So even with this problem I have corrected the contact points, center of gravity, brakes, weight issue and if i get the prop fixed I have the speed and range issues figured out.

Something tells me that once I get the prop turning 2200 rpm that will blow all my speed and range numbers.

Anyway so far it has been an enjoyable experience.

Any help is greatly appreciated

Dave

[fliger747]

Typically large piston aircraft engines had a gear ratio somewhere near two, such that the prop turned at about half the engine speed. Are you using AFSD to check out the prop and engine speeds and thrust values? For a very early aircraft prop RPM (not engine) speeds around 1000-1200 RPM would be appropriate.

If you are developing the appropriate engine HP values check the thrust and see if an issue resides in the prop tables.

Good luck! T

[Dangerousdave26]

Thanks Tom

I was looking at FSDat and not AFSD.

I also could not read the gauges in the aircraft and did not take the time to install a panel that would give me a gauge to read.

Stupid me! I knew it had a gear reduction box, I knew the ratio was 1.67 originally in the aircraft.cfg but I failed to realize what that means.

I changed the ratio to 1.80 to bring the prop speed down to 1200 RPM works like a charm.

Thrust is still in the works its good but not perfect.

[sparks]

I wasn't sure what you started with for flight dyanmics on this model, so I went directly to flightsim.com and downloaded it. I found an unfinished flight model that was very difficult to work with. The contact points were particularly messy and FS9 was crashing until I added more points than just the landing gear.

As far as problems with the engine tuning is concerned, the overall aircraft performance numbers have to be reasonable. Prop gear ratio is not that important - it's only important for matching a prop rpm gauge to an engine rpm gauge.

When you have a constant speed prop and engine RPM doesn't reach the rated max, the problem is usually caused by the prop tables. AirWrench tries to match the prop tables to the engines max rated power for climb, max speed at sea level and at critical altitude using the specs for climb rate, and speeds. The optimum climb speed, climb rate, and WEIGHT used to calculate climb rate all factor into the prop table calculations. If the numbers aren't realistic, the resulting prop table coefficients can cause the engine to 'lug' under certain situations and never reach the rated RPM.

I realize this is all pretty general and may be hard to put into practice, so if it helps, here's a link to what I came up with this evening given the specs listed for this aircraft:

http://www.mudpond.org/Panderjager.zip

[aeromed202]

Are you making dynamics from scratch? I only know something about editing cfg's after they are made.

I would just add that as a self-taught cfg fidgeter, with some good results by all accounts, I don't get too hung up on believing that the cfg values were arrived at by meticulous attention to real-world data, unless you are making the aircraft yourself. By that I mean that for any complete aircraft I don't know how the cfg values were obtained as I don't know how to make them. By by trial and error I can see what effect my edits have and from one plane to the next, scales of response will be different. I am working by feeling around in the dark so-to-speak. One makers idea of yaw MOI will be 1400 and a similar model by another maker may be 10000. So I just edit to effect and let the numbers be as they may.

Just my two cents.

[Dangerousdave26]

I wasn't sure what you started with for flight dyanmics on this model, so I went directly to flightsim.com and downloaded it. I found an unfinished flight model that was very difficult to work with. The contact points were particularly messy and FS9 was crashing until I added more points than just the landing gear.

As far as problems with the engine tuning is concerned, the overall aircraft performance numbers have to be reasonable. Prop gear ratio is not that important - it's only important for matching a prop rpm gauge to an engine rpm gauge.

When you have a constant speed prop and engine RPM doesn't reach the rated max, the problem is usually caused by the prop tables. AirWrench tries to match the prop tables to the engines max rated power for climb, max speed at sea level and at critical altitude using the specs for climb rate, and speeds. The optimum climb speed, climb rate, and WEIGHT used to calculate climb rate all factor into the prop table calculations. If the numbers aren't realistic, the resulting prop table coefficients can cause the engine to 'lug' under certain situations and never reach the rated RPM.

I realize this is all pretty general and may be hard to put into practice, so if it helps, here's a link to what I came up with this evening given the specs listed for this aircraft:

http://www.mudpond.org/Panderjager.zip

Great stuff to know

This means I need to add the pay load section to my cfg because this could cause even more concerns.

I am still going to work with my air file / cfg but I would like to know if you have any objections to me using your contact points skid points to be more specific. I have not had time to look into setting those up.

Just my two cents.

And well worth it. :ernae:

One thing that I know is you can not duplicate every thing in FS...

You can only come close... (I think I am quoting Tom there)

And close is defined by the person working on the project.

The question then becomes when do you say close is close enough. :isadizzy:

When you are happy.

[sparks]

Are you making dynamics from scratch?

Yes, I did the air file I linked to from scratch using AirWrench. I wrote AirWrench, and I also develop flight dynamics for commercial products.

FS will do a very reasonable simulation if it's given accurate data in the cfg and air files. The sim is basically a number cruncher that uses equations drawn from aerodynamic engineering texts. It's not black magic - it's based on solid engineering, physics and math.

The physical data contained in the aircraft.cfg is very important. The weight is probably the most important parameter - it's used in motion calculations in every linear direction. (Physics 101: f=ma) The span, area and chord of the main wing are also particularly important because these parameters appear in all the lift and control equations.

MS suggests the following equations to estimate MOI:

MOI = EmptyWeight * (D^2 / K)

Where: Pitch Roll Yaw
D = Length Wingspan 0.5*(Length+Wingspan)
K = 810 1870 770

(sorry about the poorly formatted table - thanks html)

You can't get the most out of FS without building your own air file. You can't make C172 into a C-130 just by changing the aircraft.cfg file.