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.

 

[sparks]

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

If you can't tell the difference, it's close enough for all practical purposes.

 

[sparks]

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.

No objections at all. I put it up on my website and posted the URL so it's available for use as-is, to modify and refine, or just examine for your own edification.

 

[Ivan]

Hi Sparks,
I was reading through the "Propellers" document (very educational!) on your site but didn't see a mention of what causes a propeller to "lug". I figure it has to be some relationship between the power coefficient and the graphs in table 512. What is the relationship that would limit the speed of the propeller / engine speed?

Thanks.
- Ivan.

 

[fliger747]

Jerry's Original work book (Excell) approach is a good one. His Airwrench product has continually evolved into a fine tool with many twek adjustments available and some good graphic presentations of the derived aircraft parameters.

Well worth the cost of a decent bottle of wine....

T

 

[Ivan]

Hi Flieger747,

I believe much more in understanding what is going on rather than finding the tool to address the issue. I am sure FDWB and AirWrench are great tools, but I am more interested in understanding basic principles. That is why I am also flipping through quite a few old NACA reports that peripherally address this issue. There are a lot of terms mentioned such as Torque Coefficient that I just don't have a "feel" for yet. I've already put together a couple simple spreadsheets for Advance Ratio and Power Coefficient that let me tweak the numbers and see the changes so that I can get a "feel" for what these numbers are describing.

Thanks.
- Ivan.

 

[sparks]

Hi Sparks,
I was reading through the "Propellers" document (very educational!) on your site but didn't see a mention of what causes a propeller to "lug". I figure it has to be some relationship between the power coefficient and the graphs in table 512. What is the relationship that would limit the speed of the propeller / engine speed?

Thanks.
- Ivan.

The propeller power coeffecient equation is:

Cp = 550 * hp / (rho * rps^3 * diam^5)

The propeller will 'lug' if the value of Cp for a given advance ratio and blade angle is less than what is in the propeller table (511).

 

[Ivan]

Hi Sparks,

I was figuring there would be some kind of internal calculation (not lookup table) that gave the torque required by the propeller at the current angle, rpm and advance ratio (need a formula for that also) which would be compared with the available engine torque at the current rpm. If engine torque was higher, the excess torque would be applied to accelerate the propeller against the inertia value specified in prop record. If the engine torque were lower, the rpm decreases by the same calculation.

I know the Coefficient of Power uses many of the same values that would be used to calculate torque, but how can that alone be sufficient to calculate the rate of increase or decrease in rpm? I am guessing that a torque calculation is hidden between records 511 and 512 in combination with the advance ratio but I just can't see it.

Thanks.
- Ivan.

 

[napamule]

Prop RPM

Ivan,
You do it the way I do it (well, 'simular'). My question is: have you tried changing the prop length and then also adjusting the prop_moi value and trial & error those 2 values? I haven't. Just curious.

You look at cfg and wonder what you have there that you don't have in the air file. No matter, as cfg will over ride the air file. But success depends on 'adopting' FDEs to the MODEL. This is the secret to success when making changes to cfg / air file to get improvemtns.
CB
Napamule

 

[Milton Shupe]

Ivan,

Sometimes tools help improve understanding. At least it helped me to see by tables and graphics, and calcs how the interactions work among the air file tables. The FDWB was immensly helpful in taht regard.

 

[MaddogK]

I've been messing with my prop tables last couple days simply to boost the high altitude performance (ALL other altitude/speeds are 'spot on') but I found the higher I push the power coefficient for the shown beta in AFSD at my max altitude(i.e. beta 21.6), the more the sim wants to use a lower beta value after the edit. It feels like I don't have enough motor at high altitude to get the prop to work properly and I don't want to mess with the engine power as it undoubtedly effect the lower altitude performance. Is there a turbo boost curve I can alter ? Is FDWB still available for D/L ?

 

[MaddogK]

Never mind, found my copy on my 'other' computer, too bad there's no import function as the next couple weeks are gonna be spent on entering data into the spreadsheet.

Heh, also found my MDL disassembler, that might come in handy.

 

[fliger747]

Are you using AFSD to check out all of the parameters? You can see how MP and HP change with altitude and observe prop RPM and pitch angle. It is possible that you are reaching a pitch limit specified in the Cfg file?

The "Critical altitude" entry will affect how high max HP is available with a super/turbo charged engine.

Cheers: T

 

[MaddogK]

Are you using AFSD to check out all of the parameters? You can see how MP and HP change with altitude and observe prop RPM and pitch angle. It is possible that you are reaching a pitch limit specified in the Cfg file?

The "Critical altitude" entry will affect how high max HP is available with a super/turbo charged engine.

Cheers: T

Yes AFSD is my viewer, and it's saying as I push up the efficiency curves @ beta 21.6 and 23 my HP decreases unless I pull prop RPM down to regain the .89 efficiency number, if I leave prop PRM the same as my previous test (lower numbers in sec 512- hi end of beta 21.6 and 23 graph) my airspeed decreases until I eventually stall. I could extend the beta range from 9-23 to 10-60 (another authors FM for the same A/C) but the A/C is on steroids- 50-70 kts too fast at ALL altitudes. Even when I use the slower A/C cfg file with the faster air file the A/C is too WAY to fast so I'm pretty sure these prop curves are the cause.