Why is my aircraft flipping.

[JP_Magoo]

I'm having trouble building a jet. for some reason it flips when I apply the elevator. The center of mass and lift are in the same place, and the elevons controlling pitch are set to almost zero(engine gimble is also off).

The reason I know it's not the elevator intensity is because when I hold the elevator it starts to turn very slowly, but then speeds up until it ends up flipping, and subsequently stalling. I have built lots of aircraft, and i have had this problem a few times. I have a hunch it has to do with mass, and the wing's surface area. Although i can't be sure. 

If anyone could help, who's a bit more skilled at aircraft design, that would be great.

[4x4cheesecake]

19 minutes ago, JP_Magoo said:

The center of mass and lift are in the same place,

In the same place? That sounds pretty flippy Try to move the CoL behind the CoM (so the blue lift vector does no longer touch the CoM bubble) and the jet should become more stable.

Also, in which direction does the CoM shift with empty tanks?

[JP_Magoo]

Thanks for your help, it worked. I don't know which direction the CoM would go, but i didn't test it for more than a minute or so. So I wouldn't know.

 

[Gargamel]

39 minutes ago, JP_Magoo said:

Thanks for your help, it worked. I don't know which direction the CoM would go, but i didn't test it for more than a minute or so. So I wouldn't know.

 

In the SPH, just manually empty all the tanks.  You will see the COM visibly shift.  You can then make adjustments in your design to account for full tanks v empty. 

[Deddly]

Another useful tip is you grab the craft by the root part and rotate it to see how the CoL moves relative to the CoM. Sometimes you can see that just a tiny change in attitude will make the craft completely unstable. Other times, it's no problem having the CoL directly centred on the CoM.

[adsii1970]

In addition to the great information above, let me also share some historical aviation. Maybe it will help, maybe it won't.

You can also play around with wing angles. There are times that simply putting the leading edge of a wing 5 degrees from horizontal can be enough to generate a lot of lift. A good example of this is the old DC-3. The wings were not horizontal, but actually were angled along two axes - it has a slight angle on the leading edge AND along the length of the wing. I've made a near close replica in KSP and can tell you this simple trick not only will give a craft stability, but capable of carrying large cargo loads.

Here's a video of a fully restored DC-3. Watch how it nearly jumps off the runway when it gets up to speed. This is what good aircraft designs do. If you look at the B-25, the B-17, and other aircraft made at the same time, the focus was on lift + thrust = flight. Within more modern aircraft drawing, the formula is reversed: thrust + lift (for example, the F-4 Phantom). The true test is to design your craft, fly it to an altitude of about 5,000 meters, then cut your engines. If you've relied too much on thrust, your craft will begin to lose altitude like a rock and become nearly uncontrollable. If you've designed the craft to maximize lift, your plane will still lose altitude, but will take on the characteristics of a glider.

 

Sorry about the first image that didn't take. The C-47 and the DC-3 are the same air frame. The DC-3 is the civilian version, sold to passenger and freight air carriers worldwide. The C-47 is the military air frame. Hope the image helps.

Edited August 20, 2018 by adsii1970
Fixed the front stetck which was missing.

[JP_Magoo]

7 hours ago, adsii1970 said:

You can also play around with wing angles. There are times that simply putting the leading edge of a wing 5 degrees from horizontal can be enough to generate a lot of lift.

I do that sometimes, if the CoM and CoL are off by to much. But i try to find other options before resorting to it, because it can make your craft less aerodynamic.

Edited August 20, 2018 by JP_Magoo

[adsii1970]

1 minute ago, JP_Magoo said:

I do that sometimes, if the CoM and CoL are off by to much. But i try to find other options before resorting to it, because it can make your craft less aerodynamic.

Not in KSP. This has been one of the major complaints about the soup-o-sphere since .18. Yes, it has gotten a lot better, but the way the game handles drag and lift isn't exactly real world. Besides, it is a real world solution used on real world aircraft - which in my opinion, makes it fair game. If you look at the image of the C-47/DC-3, and then the drawings at the bottom of the post, that's what the designers of that aircraft did - the wings are tilted at the root by about 5 degrees and there's a secondary tilt at the tip. And this design does work in KSP as well.

But that's the good thing about KSP - it basically is a free-form game. We don't have to agree to have a lot of fun!

[JP_Magoo]

I think i was thinking of another problem with doing that. I'm pretty sure i added canards to an aircraft and offset them to gain enough lift. The only problem was the aircraft (in time warp) would start rolling to the left. For some reason the SAS could not adjust for it. But it only happened in time warp, so it's not really an issue. I guess I've just had bad experiences with it, so i unconsciously never choose it.

[Not Sure]

This happens often with shuttle designs. Having your col behind your com isn’t enough. They both need to be relatively in the center of the craft, not too far back and not too far forward (forward is uncommon). The reason is because the col shifts forwards when pitching up due to body lift among other things and so when you pitch up, it just wants to keep pitching up and up until it flips.

[Mahnarch]

On 8/20/2018 at 11:59 AM, adsii1970 said:

In addition to the great information above, let me also share some historical aviation. Maybe it will help, maybe it won't.

You can also play around with wing angles. There are times that simply putting the leading edge of a wing 5 degrees from horizontal can be enough to generate a lot of lift. A good example of this is the old DC-3. The wings were not horizontal, but actually were angled along two axes - it has a slight angle on the leading edge AND along the length of the wing. I've made a near close replica in KSP and can tell you this simple trick not only will give a craft stability, but capable of carrying large cargo loads.

Here's a video of a fully restored DC-3. Watch how it nearly jumps off the runway when it gets up to speed. This is what good aircraft designs do. If you look at the B-25, the B-17, and other aircraft made at the same time, the focus was on lift + thrust = flight. Within more modern aircraft drawing, the formula is reversed: thrust + lift (for example, the F-4 Phantom). The true test is to design your craft, fly it to an altitude of about 5,000 meters, then cut your engines. If you've relied too much on thrust, your craft will begin to lose altitude like a rock and become nearly uncontrollable. If you've designed the craft to maximize lift, your plane will still lose altitude, but will take on the characteristics of a glider.

 

Sorry about the first image that didn't take. The C-47 and the DC-3 are the same air frame. The DC-3 is the civilian version, sold to passenger and freight air carriers worldwide. The C-47 is the military air frame. Hope the image helps.

The DC-3 is the main reason I went to flight school.

15 years later and I still haven't flown one. 

[adsii1970]

5 minutes ago, Mahnarch said:

The DC-3 is the main reason I went to flight school.

15 years later and I still haven't flown one. 

Neither have I. When I was a kid, I wanted to get one and restore it. And all for the sake of flying it to air shows. They are beautiful planes and for a while, workhorse for both the U.S. military and civilian aviation. About a year ago, I stumbled on a website which listed all the current airlines which still had the DC-3 on their active flight roster. Granted, most of them are in South America or Africa, but still, a beautiful aircraft.

There's even an airline in the Canadian Northwest... http://buffaloairways.com/index.php?page=douglas-dc-3

https://www.cnn.com/travel/article/aviation-douglas-dc-3/index.html

http://www.airliners.net/forum/viewtopic.php?t=289959

Oh, and here's this:

Yup, a Kerbalized C-47 (this is the cargo/military version of the craft)...

[Mahnarch]

I was in flight school in 2001-2002, so my last flight sim was from that era.

I'd always practice my lessons in the Skyhawk and -somehow - always managed to end up in the DC-3 by the evening.