agnitfom2000

Actually meant ground idle thrust.  Found some plots for the J-79 that at sea level showed about 490 lb.  At 3 engines around 1,500 lb.; aircraft has a typical take-off weight of 80,000 lb. so at about 1.9%.

Unfortunately, the aircraft I'm trying to model (the XB-68, essentially a 3 engine version of the B-58) is a historical paper study so it has very minimal information.  The saving grace is that it does have a fairly common engine, the J-79.  I'll try searching the flight manuals of some aircraft with this engine installed to try and find the flight idle thrust.  I just got hold of the F-4J NATOPS so I will start there.

 

From the experience with the G550, this behavior on the aircraft I'm modeling is at least plausible.  My industry background is materials, structures, making high temperature components out of exotic alloys, and program management.  I have much less experience on overall aircraft performance, so I really appreciate the assist from both of you.

One of the things I see in FSX is that jet aircraft tend to accelerate on the runway even with the throttle at idle.  The example I tested went from 0 to 54 knots on 13,000 feet of runway.  Even more extreme than my old car with the idle screw set to the max! Everything in the model set-up was as realistic as possible.  Drag scalars and the jet engine thrust scalar were all set to 1.

 

My question for any jet pilots in this forum is whether this behavior is realistic, or is the jet engine performance model in FSX producing too much thrust at idle?  If it is realistic, I'll leave things alone.  If not, I'll do some experimenting to try and minimize the effect.

 

Thanks

David Allan

Clicking on the following link will make it easier to find: https://a2asimulations.com/product-category/fsx/

Are you referring to the badly twisted sky bridge or something else? I've also seen AI airliners at rest bouncing around on their landing gear spewing fire.

Thanks for the kind comments! I really appreciate the support. As far as publishing the S-92, the floats were the last problem. Except for writing up the documentation and creating the package, everything is ready. I'm hoping to get it out within the next month.

 

David

 

PS: I'll try out the contact points too.

I never thought about using lights to illustrate the different types of contact points. That's a great idea!

 

David

My condolences. I'm very sorry to hear that. I have really enjoyed his work.

 

David

I was out of town for a couple of weeks, but after returning I was able to make the floats work properly. See the picture:

I ended up using Model Converter X to transform the model downwards about 1 1/2 feet. The animations also transformed properly. I did not adjust the float points or scrape points in the Aircraft.cfg. I did adjust the CG and landing gear contact points vertically. Here are the final results. I realize max feet/second speed is higher than realistic.

 

[contact_points]

0.......1......2........3.....4......5...6.....7......8.....9....10....11..12...13...14..15.....16..

;gear

point.0=1, 12.7, 0.00, -4.50, 9500, 0, 0.5187, 75.0, 0.500, 1.2, 0.6, 5.0, 5.0, 0, 120.0, 200.0

point.1=1, -7.7, -5.20, -4.50, 9500, 1, 0.5187, 0.0, 0.500, 1.2, 0.6, 5.3, 5.3, 2, 120.0, 200.0

point.2=1, -7.7, 5.20, -4.50, 9500, 2, 0.5187, 0.0, 0.500, 1.2, 0.6, 5.3, 5.3, 3, 120.0, 200.0

;bottom

point.3=2, 18.50, -4.70, 2.70, 8000, 0, 0.0000, 0.0, 0.000, 0.0, 1.0, 0.0, 0.0, 4, 0.0, 0.0

point.4=2, 18.50, 4.70, 2.70, 8000, 0, 0.0000, 0.0, 0.000, 0.0, 1.0, 0.0, 0.0, 4, 0.0, 0.0

point.5=2, -9.600, -4.70, 2.70, 8000, 0, 0.0000, 0.0, 0.000, 0.0, 1.0, 0.0, 0.0, 4, 0.0, 0.0

point.6=2, -9.600, 4.70, 2.70, 8000, 0, 0.0000, 0.0, 0.000, 0.0, 1.0, 0.0, 0.0, 4, 0.0, 0.0

point.7=2, -2.000, -7.30, 2.70, 8000, 0, 0.0000, 0.0, 0.000, 0.0, 1.0, 0.0, 0.0, 4, 0.0, 0.0

point.8=2, -2.000, 7.30, 2.70, 8000, 0, 0.0000, 0.0, 0.000, 0.0, 1.0, 0.0, 0.0, 4, 0.0, 0.0

;float

point.9= 4, 8.50, -9.50, -0.50, 8000, 0, 0.0000, 0.0, 0.100, 1.5, 0.6, 0.0, 0.0, 4, 0.0, 0.0

point.10=4, 8.50, 9.50, -0.50, 8000, 0, 0.0000, 0.0, 0.100, 1.5, 0.6, 0.0, 0.0, 4, 0.0, 0.0

point.11=4, -5.60, -9.50, -0.50, 8000, 0, 0.0000, 0.0, 0.100, 1.5, 0.6, 0.0, 0.0, 4, 0.0, 0.0

point.12=4, -5.60, 9.50, -0.50, 8000, 0, 0.0000, 0.0, 0.100, 1.5, 0.6, 0.0, 0.0, 4, 0.0, 0.0

 

It worked OK for both FSX and P3Dv4.

 

Thanks to everyone who helped out with this problem!

I've had some partial success. It will float for any set of points where the Z value is less than -0.5 (-1, -2 etc. works). -1 did work, which was higher than the landing gear points. See picture below:

 

 

Strangely enough, I had the exact same response when I took the model file and put it in the EH-101 directory and used those aircraft.cfg and .air files. I could only lower the aircraft so far before I got the crash again. I'm beginning to think the problem is the compiled model.

 

Beean, I had put in ridiculously high values for the impact threshold to try and eliminate it as a cause. Looking at it again, I'm honestly not sure why it is different. Probably a silly typo. :confused:

Thanks everyone for the responses.

 

By the amphibious EH-101 I was referring to Carl Vokes' FSX Making The EH101 Helicopter Float. He provides contact points for the standard EH-101 provided with FSX. If you search flightsim.com, filter on FSX Helicopters, and search for the word float, his item comes up first in the search.

 

I have tried out both suggestions, and even tried combining them. Unfortunately it still crashes at the same height between the bottom of the gear and the bottom of the fuselage. I've tried both opening up the simulation with the helicopter in a water landing area, and landing the helicopter on the water with the gear retracted. Even the really gentle landing from a 10 foot hover didn't work. It looked like it was working for a few seconds but then crashed.

 

I'm thinking I have something else going on beyond the contact points, but I am not sure what.

Thanks for the response. I did actually try that. Unfortunately I don't know the rules behind the contact points and how the different types relate to each other. I can make it work by putting it into the aircraft configuration file for the amphibious EH-101, but the model is way out of position. I could translate the model and recompile, but I would have to rework all the animations. Next time I'll make all the contact points work first before doing the animations.

I'm almost ready to post a set of FSX/P3Dv4 S-92 aircraft that I have built over the last year. Everything works well except for the flotation. The aircraft crashes about 1 foot above the water. It works perfectly fine on land.

 

Attached below is the list of contact points and an illustration of the points. The yellow points are lights and not relevant to the discussion. The three lowest dark red points correspond to the lowered position of the landing gear. The dark red points above correspond to the float points or the scrape points. The aircraft crashes into the water at a height halfway between the bottom of the aircraft and the landing gear contact points. The crash position does not correspond to any of the points.

 

 

[contact_points]

0......1....2......3.....4....5....6.....7......8.....9...10...11...12...13...14......15

;gear

point.0=1, 12.7, 0.000, -1.20, 9100, 0, 0.5187,75.0, 0.500, 1.2, 0.6, 5.0, 5.0, 7.0, 120.0, 200.0

point.1=1, -7.7, -5.2, -1.10, 9500, 1, 0.5187, 0.0, 0.500, 1.2, 0.6, 5.3, 5.3, 7.0, 120.0, 200.0

point.2=1, -7.7, 5.2, -1.10, 9500, 2, 0.5187, 0.0, 0.500, 1.2, 0.6, 5.3, 5.3, 7.0, 120.0, 200.0

;bottom

point.3=2, 18.50, -4.70, 1.70, 80000, 0, 0.0000, 0.0, 0.000000, 0.0, 0.000000, 0.0, 0.0, 4.0, 0.0, 0.0

point.4=2, 18.50, 4.70, 1.70, 80000, 0, 0.0000, 0.0, 0.000000, 0.0, 0.000000, 0.0, 0.0, 4.0, 0.0, 0.0

point.5=2, -9.600, -4.70, 1.70, 80000, 0, 0.0000, 0.0, 0.000000, 0.0, 0.000000, 0.0, 0.0, 4.0, 0.0, 0.0

point.6=2, -9.600, 4.70, 1.70, 80000, 0, 0.0000, 0.0, 0.000000, 0.0, 0.000000, 0.0, 0.0, 4.0, 0.0, 0.0

point.7=2, -2.000, -7.30, 1.70, 80000, 0, 0.0000, 0.0, 0.000000, 0.0, 0.000000, 0.0, 0.0, 4.0, 0.0, 0.0

point.8=2, -2.000, 7.30, 1.70, 160000, 0, 0.0000, 0.0, 0.000000, 0.0, 0.000000, 0.0, 0.0, 4.0, 0.0, 0.0

;float

point.9=4, 8.50, -9.5, 2.5, 18800, 0, 0, 0, 0.0, 0.0, 0.0, 0, 0, 0, 0, 0

point.10=4, 8.50, 9.5, 2.5, 18800, 0, 0, 0, 0.0, 0.0, 0.0, 0, 0, 0, 0, 0

point.11=4, -5.60, -9.5, 2.5, 18800, 0, 0, 0, 0.0, 0.0, 0.0, 0, 0, 0, 0, 0

point.12=4, -5.60, 9.5, 2.5, 18800, 0, 0, 0, 0.0, 0.0, 0.0, 0, 0, 0, 0, 0

 

Note the ridiculously high impact damage thresholds.

 

The CG position is given below:

 

reference_datum_position= 0.000, 0.000, -0.350

empty_weight_CG_position= 0.000, 0.000, 7.000

 

I did have engine and fuel centroid points that corresponded with the approximate crash position. I moved those to the proper position, but it did not have any effect.

 

Inflation or non-inflation of the float bags does not make a difference (not that I expected it to).

 

I have compared the EH-101 and S-92A models in Model Developer and I have not been able to find any significant or relevant differences.

 

Other things I have tried that have not had an effect:

1) Removing all crash boxes from the model compile

2) Substituting the EH-101 AIR file for the S-92A AIR file

3) Increasing the impact damage thresholds to really high numbers.

 

The only thing I have gotten to work was to put the S-92A model file into the EH-101 file. That change actually worked. However, the aircraft was way out of position. I tried to adjust the aircraft position in the aircraft.cfg file but ran back into the crash problem. I could transform the aircraft prior to compiling but I'd rather not, since I would then have to rework 25+ animation sequences.

 

I have run out of other ideas on things to try. Any further ideas?

 

Thanks.

 

David Allan

agnitfom2000

 

PS: I thought about positing this in the aircraft development forum, but that forum doesn't seem to get anything near the readership.