[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[TeeGee]

Does anybody know how to improve the forward velocity derivatives as well as the downward velocity dervates? All of my designs have these issues, but I can usually manage them during flight and reentry, it's just for sake of perfection.

I would like to design a craft that is stable across all speeds.

[diegzumillo]

I would like to design a craft that is stable across all speeds.

I would like to know if this is even possible.

[luckyhendrix]

I have had the same problem with Xw with some craft , yet it did not seems to make the aircraft uncontrollable.

It tried to look up what affects Xw but with not luck.

Note that the situation you're simulating : mach5 with sea level air density (1.2) , is almost impossible to reach.

Generally if you reach mach 5 you are already at the edge of the atmosphere.If your plane is stable when air density is around 0.1 and at Mach 5, you should be fine.

I think it is impossible to have a plane stable for "all speed", that is, all altitude-speed-fuel levels combinations.

But it should be possible to design a plane that stays stable under all normal flight conditions it has to go trough to complete its mission.

Edited June 14, 2014 by luckyhendrix

[iamzac]

Thank you for a great plugin that allows building of more realistic planes!

I had some problems with FAR which I mostly solved but I still don't understand why they happen.

In short:

My planes have a tendency to pitch up and flip (leading to disintegration) in some conditions even though the COL is behind COM and the AOA is small.

Long description:

First I built this blackbird replica: http://i.imgur.com/gZ5G6G5.png which worked fine with no problems.

Then I wanted to build a spaceplane version so I added oxidizer, replaced the engines with rapiers and other small modifications.

According to SPH the COM moved back a little but was still in front of COL.

I had no problems taking off and flying at subsonic speed but every time it reached Mach 1 it exploded.

It didn't matter the height, the thrust, the angle.

To better see the problem I accelerated very slowly at higher altitudes to see the problem happen more slowly and what I saw was that when reaching Mach 1 it suddenly displayed large scale stall and there was a sudden pitch up which was the one causing the high dynamic pressure to disintegrate the plane.

I didn't understand why this was happening because the COL was already behind the COM, the angle of attack was small, I wasn't even touching the controls and at transonic and supersonic speeds the COL should move to the back and the nose should have a tendency to pitch down not up... but anyway the logical action was to try to move COL more back so I did and now my spaceplane version looks like this: http://i.imgur.com/BltpfiX.png and can go past Mach 1 without exploding.

I no longer have pictures with the exploding version but if needed I can modify it and take some.

Question 1: But I still don't understand: why was this happening in the first place?

After solving this problem I encountered another problem which I have with all my spaceplanes and FAR, including this model for example: http://i.imgur.com/2yvoOgN.png

At heights above 20.000 meters and speeds above Mach 4 my spaceplanes have the tendency to suddenly pitch up, usually when I try to pitch down and then flip and of course explode.

The solution I have found to this problem is to try to enter 20.000 almost perfectly level and then let the pitch increase and not correct it more than once and just go to rocket mode and space as quickly as possible in order not to flip, but this means that I loose deltav because I don't gather all the horizontal speed that I could get from air breathing mode.

Question 2: So why does this happen and how can I solve this?

The slow constant pitching up happens in both stock aerodynamics and with FAR always so I am not asking about that (although I am not sure why that happens either) the problem is the sudden pitch up even though AOA is small, and the COL is way behing COM and I don't pitch too fast.

The COL is so behind COM that I have some trouble when landing because then I no longer have problems with the plane pitching up, at low speeds and altitudes it behaves normally, which means that it will keep trying to go nose down so I have to keep pitching up so landing is a bit ugly.

I tried moving fuel to the front continuously while moving up to move the COM even further in front but it still happens.

If I enable all the FAR stability helping tools it's even worse: it's almost guaranteed that when I will try to pitch down it will suddenly flip up.

I designed an experimental spaceplane with the engines in front so that the COM will be close to the front of the plane and the center of thrust be also ahead thinking that as an arrow is stable because the COM is in front and when my planes flip the have a tendency to go with their rear forward maybe this will help but it didn't, still the same flip.

I don't have screenshots but I can take some and upload if necessary (it looks ugly).

This eight engine stock cargo design : http://i.imgur.com/6pza3pG.png which also has the COM in front seems to work fine but I didn't test it enough, I only used it once and I might have not stayed enough gathering horizontal speed to check if this problems was still happening.

But this shouldn't be the problem because high speed planes like SR-71 and XB-70 usually have the COM close to the rear.

This problem also happens during reentry so when I return I have to point the nose prograde until I reach bellow 15.000 m and bellow Mach 4 and only then I can begin to safely and slowly pitch up, but I would like to reenter space shuttle style, pitched up but like I said that is impossible even if I move all my fuel in front.

I was also thinking that maybe the fact that Blackbird has forward chines that cat like canards and XB-70 also has canards maybe I should add some canards too but not only it would ruin the look but like I showed already other spaceplanes I built have canards and still have this problems and the spaceshuttle doesn't have canards and can reenter at high pitch angle.

Sorry for such a lengthy post...

[DivisionByZero]

In addition to checking the air density...

The pitching moments can often be corrected by changing the relative separation of the CoM and CoL. The easy way to check this is to click "empty" if you know the CoM moves a certain way when the tanks are dry. If you look closely at the red number, it may not turn green, but if you look at its magnitude you will get a sense of the CoM movement helped or not and could be used to correct the design.

The other thing to check is if the instability is a problem or not. You can do this with the simulation window.

I made some notes on this stuff in the FAR wiki. Maybe they will help? https://github.com/ferram4/Ferram-Aerospace-Research/wiki/An-Example-SSTO-Design-Process

Good luck!

[Hodo]

I had some problems with FAR which I mostly solved but I still don't understand why they happen.

...

This is what I can guess at. Your CoM and CoL is shifting as you fly. The fuel is draining from the front tanks and causing the CoM to shift towards the rear of the aircraft, moving it behind your CoL. Which is causing your instability problem.

Fix for this is simple. You can download TAC Fuel Balancer, and get the RCS build aid which will help with the design of your craft showing the empty and loaded.

[BudgetHedgehog]

Even better - PWB Fuel Balancer. Can balance fuel (surprise!) around a specific CoM that you set in the editor.

[iamzac]

@DivisionByZero:

Thanks a lot, your guide was very helpful, it turns out it was a dynamic stability issue which I could solve very quickly with the help of the simulation.

I still don't understand exactly what was happening but now it's fine, and after I finish reading the wiki maybe I will have an idea about what was happening.

This will be very very useful in future designs.

I was going to recommend for this to be linked from the first page but I see that it already is a link to the wiki but I didn't saw it before...

Do you know what phi (Φ) is in lateral simulation? I think that it might be some lateral (angular?) speed but I am not sure.

[Alshain]

Is there a guide to this for people who aren't pilots or aircraft designers? I have everything in the green and I can't get off the runway. I get up to 60m/s and it veers out of control.

[Hodo]

Is there a guide to this for people who aren't pilots or aircraft designers? I have everything in the green and I can't get off the runway. I get up to 60m/s and it veers out of control.

Do you have a picture of your craft?

My best guess is that your landing gear are out of alignment, to much camber. If that is the case then it is a common problem is KSP not FAR.

And 60m/s isn't that fast, depending on the size of your wings and the amount of lift you are generating with those wings will affect your take off speed. The other thing that could be sending your craft out of control is the wing flex, if you are using a lot of wing panels to create your wings they will flex thus creating uneven lift, which will send your craft out of control. Use struts to reinforce your wings to keep them from flexing to much.

[Alshain]

Do you have a picture of your craft?

My best guess is that your landing gear are out of alignment, to much camber. If that is the case then it is a common problem is KSP not FAR.

And 60m/s isn't that fast, depending on the size of your wings and the amount of lift you are generating with those wings will affect your take off speed. The other thing that could be sending your craft out of control is the wing flex, if you are using a lot of wing panels to create your wings they will flex thus creating uneven lift, which will send your craft out of control. Use struts to reinforce your wings to keep them from flexing to much.

No I got it, it was too much control on the tweakables. It goes straight now, but then off the end of the runway and into the water. Guess I need to work on lift. Thanks though.

[Hodo]

No I got it, it was too much control on the tweakables. It goes straight now, but then off the end of the runway and into the water. Guess I need to work on lift. Thanks though.

Best bit of advice I can give with the control surfaces is to look at actual aircraft and see how they are set. There is a reason why majority of all the aircraft in the air use the same setup.

[Alshain]

Yeah, I just turned the max control from 15 to 10 and it fixed the problem. Just added a little more lift and it flew for a little while, before I pitched up to hard and it fell apart.

[Hodo]

Yeah, I just turned the max control from 15 to 10 and it fixed the problem. Just added a little more lift and it flew for a little while, before I pitched up to hard and it fell apart.

http://img.photobucket.com/albums/v445/Alshain/screenshot3.png

Ok the pitching up hard after a while is because your fuel is draining from the front and shifting the CoM behind the CoL. Easy fix for this is to get either TAC Fuel Balancer, or the PWB Fuel Balancer. Another handy tool is RCS Build aid.

[Alshain]

Ok the pitching up hard after a while is because your fuel is draining from the front and shifting the CoM behind the CoL. Easy fix for this is to get either TAC Fuel Balancer, or the PWB Fuel Balancer. Another handy tool is RCS Build aid.

I think it was my fault, I just pulled up too hard. On my second attempt... orbit @ 90km. I have TAC but didn't use it. Now I realize I have no idea how to land.

[Hodo]

Landing is easy, it is crashing while under control.

Landing just make sure your decent rate isn't to high, generally less than 10m/s, and keep your pitch high enough that you can keep that glide slope, but not to steep that your tail strikes the ground on landing.

[diegzumillo]

Is there a guide to this for people who aren't pilots or aircraft designers? I have everything in the green and I can't get off the runway. I get up to 60m/s and it veers out of control.

I found that wiki/github on the OP to be very helpful. Did you read it? it's really easy to miss amidst other links!

[Alshain]

My bigger concern is landing somewhere... landable.

I found that wiki/github on the OP to be very helpful. Did you read it? it's really easy to miss amidst other links!

I did but it's quite incomplete. I still have no idea what the red, green and blue lines mean.

Edited June 15, 2014 by Alshain

[Hodo]

In simplest terms.

The lines are the lift rating, drag rating, and lift vs drag ratio. If I remember them right.

[Alshain]

Well I landed in the desert but a little too fast and it hit a bump and blew up. I was on the ground though @ about 85m/s. Oh well, try try again, but not right now.

[Hodo]

Well I landed in the desert but a little too fast and it hit a bump and blew up. I was on the ground though @ about 85m/s. Oh well, try try again, but not right now.

I found landing speed is around 10m/s less than my take off speed. Which is about what I shoot for. You can lower this further with the use of flaps.

[AndreyATGB]

Or use B9 airbrakes, which are totally not OP at all right now.

[Hodo]

Or use B9 airbrakes, which are totally not OP at all right now.

Not talking about stopping, but increasing your lift while slowing down... thus flaps. I can land my Mach2+ fighter with flaps at around 170mph, which is almost 80mph slower then the take off speed due to flap settings.

[DaMichel]

Back to KSP again. At least i finished the heavy version of the plane i showed previously. This one can deliver 18t in LKO on only two of the small B9 engines. It has a TWR < 0.7 and struggles to take off with the test payload. Once in the air it is quite easy to fly into space, it just takes a long time. Excluding some failures in "Simulations" for learning how to fly it, i made it back to KSC in one piece

Notable design changes compared to the small version include a larger wing area to support the additional weight and a larger vertical tail since it was a bit too twitchy. Oh and i angled the large wing part a little bit upward to help generate lift for take off.

P.S. I have a new version which has functioning air brakes, too (extending sideways from the vertical stabilizer, plus a pair of spoilers on the main wings) but didn't take pictures of it.

[DivisionByZero]

I've found learning how to use the flaps an spoilers very rewarding, even if it's a bit non-standard for delta wings. They still help slow the craft down during landing.