Jet Engines Losing propulsion requirement while running up to full power. VTOL Aircraft testing.

[LineOfRahl]

Hey guys, I seem to have found a bug of some kind. I decided to add engines for VTOL capabilities on a fighter jet, and CANNOT seem to get all 4 lift engines to work. I have 4 "Wheesley" turbines, with nacelles for the static suction. 

2 of them work as they should providing 100% prop requirement. 

2 of them do not work the same way, and I cannot find any reason why. I have reloaded the craft, deleted and replaced the engines/nacelles. Moved them up down and around, etc. I cannot figure out why they don't work.

Weird thing is, they all start off at 100% and then 2 of them start to drop as the thrust climbs.

Also does the forum not allow image URL from google images? It won't let me insert any images??????

I'll upload photos as soon as I figure out why It won't let me....

https://photos.google.com/photo/AF1QipMOmuuZJmYpHV_L06Lvdp8L9gtan0Vjmh_ZEE5q

[Rakete]

Try imgur.com to host photos.

 

[LineOfRahl]

I figured it out lol I was trying to use the "Insert image from URL" button instead of just pasting the link into the body.

These are the various stages of what is happening. Not shown in images, but possibly relevant, all 4 nacelles show the same airflow/effective velocity.

https://photos.google.com/photo/AF1QipMqTugjjxlcY2511p0S27GRoC3vlY0osIKwe45J

https://photos.google.com/photo/AF1QipNl7TU5nSwgp3_grVn82CUHgcsqmsYXGqdIXyRI

https://photos.google.com/photo/AF1QipOC8J2GIym6JfSpQoRor0ES2J9VcholaEd7FXkl

https://photos.google.com/photo/AF1QipMOmuuZJmYpHV_L06Lvdp8L9gtan0Vjmh_ZEE5q

[LineOfRahl]

Well I found the solution. 4 nacelles is not enough to provide air to the engines. Since it "drains evenly" it chooses some engines to give 100% to and the others get what remains.

I bumped it up to 5 engines and 7 nacelles and it is working fine. A little clipping later and you can barely tell there's 7 nacelles on there lol

https://photos.google.com/photo/AF1QipNq_gZADO-GahuGjSF2O9CoVr9WDFZ3CzzbVZRQ

https://photos.google.com/photo/AF1QipNv-M65cEoQWgCcT0jpoahQ0WsLfLN0_qgzR6DH

I could have saved a lot of time if the values given for the intake air amount (5.0 "units") and the air requirements of the engine ("Intake air" 29.60/sec) made any sense whatsoever...

If I plug those numbers into a calculator 5*29.60 means the engines need 148/sec intake air, while the nacelles provide 7*5 "units" for a total of 35.

Based on this I can determine that 148/sec air is <= 35 "units" of air so 1 "unit" from the intake is roughly = 4.23/sec "intake air" for the engine.

This is why consistency with terminology is so very helpful lol

-Regards

[quazarz]

I only just started playing with air-breathing engines yesterday to build an SSTO and was building everything on the assumption that "units" just means units/second. The reality seems so much more confusing. Is it because air intake is dependent on other variables like velocity/atmosphere? Can you extend the altitude that the engines are effective by adding more intakes?

[LineOfRahl]

Yes it is dependent on velocity and pressure. When you're building an SSTO the number of intakes doesn't matter as long as they provide enough to get the plane going on the runway. I have experimented with this and you can feed 6 Rapier engines with only 2 Shock Cone intakes. The forward motion provides a very powerful ram-air effect. Once you get it over the 400m/s hump, the effective drag actually starts to drop, while the air intake continues to increase, and away to orbit. Just be sure you have enough oxidizer, once the engines cut out you need to accelerate quite a bit to achieve stable orbit.

Unfortunately no. You cannot increase the effective altitude that way. The effective altitude is dictated by the engine only. SSTO's are very difficult, and not very useful, without Rapier engines.  They have the highest ceiling, and they benefit the most from ram-air. I ended up launching all my spaceplanes with rockets until I unlocked the rapier engines lol

Something like this works well. https://photos.google.com/photo/AF1QipPFRH8TTz4WcdN5cfWRuI7tVdeK5QFaFLbfZ9_d

Make sure everything is inside of a bay to keep drag to a minimum, and keep it simple to keep the mass down. 

Have fun experimenting!

[quazarz]

Cool, thanks! I did manage to get one working yesterday using a mk2 fuselage, 3 whiplash and 4 nuclear engines. I was trying to use rapiers but I didn't like having to balance the amount of oxidizer, always seemed to run out too early to get anywhere really useful. With the nukes, it's a long climb but I can get up to orbit with 2k dV left in the tanks... if it doesn't flip and slam against the atmo on the way up. That's the biggest thing I'm struggling with now, is getting that to stop happening. I took it to Eve for my first ever fly-by after refueling it at the Mun, super inefficient trip but 4k dV made it happen.

Edited January 4, 2021 by quazarz

[LineOfRahl]

Awesome! Eve is one of my favorite bodies lol even if it is the most difficult to take off from. 

 If it's flipping like that is it possible the engines thrust is not centered with the mass? Or you may need a couple more reaction wheels to maintain control. When you're in the upper atmosphere, things can get pretty unstable if the thrust is even a little off center with respect to the mass. The air up there provides very little aerodynamic stability, and a lot of drag if the AOA suddenly changes. 

Is it getting rear-heavy when the fuel starts to run closer to the 50% mark? Nuke engines are very heavy and I found that keeping some fuel in a forward tank while climbing can be very helpful. You can just "turn off" the forward most tanks to keep the mass where it should be, and re-activate them once you're clear of the atmosphere. If you're getting to orbit with 2k dV you probably don't need all the fuel tanks active. 

 

[quazarz]

1 minute ago, LineOfRahl said:

Is it getting rear-heavy when the fuel starts to run closer to the 50% mark?

This might be what is happening. It's very nearly mirrored across the horizontal axis so I don't think it's a thrust misalignment, there might be some milimeter level differences between top and bottom. I am not sure exactly how I am supposed to read/utilize the "center of lift" indicator but I tried to place it squarely on the center of mass. I will experiment later with moving the fuel tanks towards the front since they are all pretty concentrated towards the rear. Also have way too much air intake based on what you are telling me.

[jimmymcgoochie]

@quazarz I think the problem is that your plane is too tail heavy- all your fuel tanks are near the back along with the jets and those heavy NERVs which will make the plane unstable. The weight needs to be shifted forwards, whether that’s by redistributing the fuel tanks and setting the fuel priority to drain the tanks in a more balanced way, by moving the wings further back to shift the centre of mass in front of the centre of lift (and moving the engine nacelles forwards to return them to the position they’re in now) or swapping the docking port and payload bay around to shift the heavier port to the front, assuming the payload bay is empty or at least weighs less than the docking port.

You could also try angling your wings slightly, a few degrees higher at the leading edge will make them generate more lift and can help takeoff speeds as well as balancing your centre of lift. Just remember that  ore lift at low speeds becomes more drag at high speeds, so there’s a balance to strike between taking off quickly and making it to orbit efficiently.

[Vanamonde]

Looks like it was a gameplay issue? Sounds like it though I can't view the pics. At any rate, thread moved to the Gameplay sub. 

[LineOfRahl]

If you're that close with the thrust/mass centers, I doubt that's the problem.

19 hours ago, jimmymcgoochie said:

@quazarz I think the problem is that your plane is too tail heavy- all your fuel tanks are near the back along with the jets and those heavy NERVs which will make the plane unstable. The weight needs to be shifted forwards, whether that’s by redistributing the fuel tanks and setting the fuel priority to drain the tanks in a more balanced way, by moving the wings further back to shift the centre of mass in front of the centre of lift (and moving the engine nacelles forwards to return them to the position they’re in now) or swapping the docking port and payload bay around to shift the heavier port to the front, assuming the payload bay is empty or at least weighs less than the docking port.

You could also try angling your wings slightly, a few degrees higher at the leading edge will make them generate more lift and can help takeoff speeds as well as balancing your centre of lift. Just remember that  ore lift at low speeds becomes more drag at high speeds, so there’s a balance to strike between taking off quickly and making it to orbit efficiently.

I think you're right, it might be tail-heavy. ^^^

Re-arranging it so that small MK2 tank is directly behind the cockpit would give you that ballast capability.

Could you possibly  scoot the NERV engines/tanks forward a bit? it doesn't look like they're close enough to the fuselage or wings to have thrust interference issues. (Though they still can if the game so chooses lol) Moving those engines even just a couple meters can drastically change the whole weight balance, and in such a way that even when the fuel runs low it maintains a good balance. 

15 hours ago, Vanamonde said:

Looks like it was a gameplay issue? Sounds like it though I can't view the pics. At any rate, thread moved to the Gameplay sub. 

Thanks man! Definitely a gameplay issue lol