Is my ship design wrong?

[Torraqe]

Hello!

Short, fast post:

My center of mass is at the center of the spaceplane, my center of lift is inside the center of mass. My thrust vector is behind the center of mass, and aligned to it. Yet, when I try to control the craft it starts spinning wildly, and I have to abort, it can't even take off.

I tried to put it in the launcher, changed its wings, and now I used not 4 but 8 of those little black control surfaces what are absolutely useless (4 at the back of the wing, 4 at the front of the wing, hidden inside it). The are aligned perfectly and move synchronously.

Are these designs completely wrong? What should I do?

I got completely stuck in the career, I can't make enough money because each lunch requires as much money or more money than the reward. I can't jump forward technologically too. How to I reach 18000 m to measure the temperature with these parts?

Any answer to these questions would be very helpful.

Thanks.

[Superfluous J]

Push your COL back a bit. Your COM needs to be in FRONT of the COL, not in the same location.

Think of an arrow or a dart. There's a reason the heavy metal tip is at the front.

Also, the elevons gets confused near the COM. If they're all behind it (which they should be if your wings are far enough back for the above to be true) then they'll work correctly. Any that go in front of the COM will reverse themselves.

[swjr-swis]

Edit: @bewing, @Streetwind, and @Red Iron Crown point out a few important things I missed in this post, make sure to read/use those too:

• credit bewing - the Wheasly engine loses a lot of thrust before getting even close to the target altitude of 18 km. Consider alternative or additional engines.
• credit Streetwind - the rear gear is so far back that it will make pitching up to lift off very difficult. Far from CoM - long leverage arm - takes a lot of force. Place them close behind the CoM.
• credit Red Iron Crown - RIC explained more clearly the interaction between control surfaces and the CoM.

 

23 hours ago, Torraqe said:

My center of mass is at the center of the spaceplane, my center of lift is inside the center of mass. My thrust vector is behind the center of mass, and aligned to it. Yet, when I try to control the craft it starts spinning wildly, and I have to abort, it can't even take off.

1) The control surfaces are in front of the CoM. The editor won't tell you this, but that will make them reverse how they work once the plane is launched: when you try to pitch up, they will actually be pitching you down. You can correct this by pulling the 'Authority' slider to the left and make it negative.

Edit: I didn't word the above very well. A more accurate way of explaining the CoM/control surface reversing problem added by @Red Iron Crown:

17 minutes ago, Red Iron Crown said:

Sort of. Control surfaces ahead of CoM work fine (see every canard ever), the trouble arises when the wing is attached ahead of the CoM while the surfaces are behind it, that's when they reverse in pitch and yaw. Reversing them with the authority slider works as long as the surfaces aren't being used for roll, otherwise it makes them reverse for roll while being correct for pitch.

2) They are also very close to the CoM, which means they will be almost useless for pitching because they have no arm for leverage. Two of the smallest control surfaces somewhere on the tail fin will be much more effective in pitching the plane than these.

3) Once you correct those things, you likely won't need the double wings for the plane to lift and fly, or the extra fins on the wing tips. Will save you parts, money, drag, fuel, and give you a higher top speed. (*)

 

23 hours ago, Torraqe said:

I tried to put it in the launcher, changed its wings, and now I used not 4 but 8 of those little black control surfaces what are absolutely useless (4 at the back of the wing, 4 at the front of the wing, hidden inside it). The are aligned perfectly and move synchronously.

Are these designs completely wrong? What should I do?

I got completely stuck in the career, I can't make enough money because each lunch requires as much money or more money than the reward. I can't jump forward technologically too. How to I reach 18000 m to measure the temperature with these parts?

2) The control surfaces, especially those on the back of the wing, are still too close to the CoM to be of much help with pitching. A few smaller ones on the tail will be more effective.

3) Here too, once you correct the pitching, you have way more wing surface than this plane needs to fly. A pair of the slanted wings (the ones on the end of the wing) should really be all that's needed. (*)

 

Let us know how you're faring.

 

(*: unless of course you are after a specific look for the plane, then there really are no rules . But right now it seems you are more interested in the practical matter of just making it lift off and fly rather than making it look a specific way.)

Edited July 11, 2016 by swjr-swis
added referal to helpful comments from others

[bewing]

On an airplane, there is an axis of: pitch, roll, and yaw. Yaw is controlled by the tail, and you have that covered. Roll is handled by the two horizontal control surfaces out at the tips of the wings -- when one tips up, the opposite side tips down, and the plane rolls. So you have that covered.

But pitch control is tricky. You control pitch by having control surfaces at the very front of the plane, or at the very back end. All your control surfaces are in the middle. They have no leverage to make the nose of the plane go up or down. So your plane will certainly crash, and that is why.

Once you have that basic problem solved, one little helpful tip is that you generally want to have your CoL ball be a little bit behind your CoM ball. Even with the pitch control problem fixed, you will probably find your plane is a little too responsive to the controls. You will just tap S, and the front end of your plane will shoot up. Tap W and you will instantly go into a dive. So move one of your wings back. Probably all the way to the back of your plane.

Beyond that, a wheesley engine can easily get you to 10km altitude. Your design can probably fly at 12km. With extreme care, you can make a perfectly designed wheesley-based jet reach 16,698 meters for an instant (I know because I managed to do it once as a lark). But to reach 18km, you cannot do it this way. You use jet engines to get the plane close to your target. Then you have a little rocket on board and you light that to get you from 10km altitude up to 18km or 20km to take your measurements. Switching from jet mode to rocket mode and back again two or three times in a single flight is tricky, but doable. But it is very easy to just do it once. If you would like an example airplane that flies nicely and has rocket boost to take those high altitude measurements, just say so and I will post one.

If you are on normal difficulty, and you can't make money on your contracts -- then you are spending way too much on each launch. So you are doing something a bit wrong. Are you trying to make your money by doing space contracts? Or by flying airplanes and testing parts? Or some of both?

As far as science goes -- there are many tricks for getting science. Do you know that even at the very beginning of the game that there are 19 places around KSC where you can get science points? There are about 500 science points available within walking/swimming distance of KSC, for almost no money. If you would like instructions, tips, or hints, just say so. Of course, you have already unlocked the Wheesley, the thermometer, and the AV-R8, so you are making some progress.

Edited July 10, 2016 by bewing

[Torraqe]

Wow thanks a lot guys, need a little time to read them all.

You are awesome!

[Streetwind]

4 hours ago, swjr-swis said:

1) The control surfaces are in front of the CoM. The editor won't tell you this, but that will make them reverse how they work once the plane is launched: when you try to pitch up, they will actually be pitching you down. You can correct this by pulling the 'Authority' slider to the left and make it negative.

2) They are also very close to the CoM, which means they will be almost useless for pitching because they have no arm for leverage. Two of the smallest control surfaces somewhere on the tail fin will be much more effective in pitching the plane than these.

3) Once you correct those things, you likely won't need the double wings for the plane to lift and fly, or the extra fins on the wing tips. Will save you parts, money, drag, fuel, and give you a higher top speed. (*)

In addition to this excellent advice:

Your plane fails to take off because your aft landing gear is too far in the back. Think of your aft landing gear like a fulcrum around which the plane pivots in order to lift the nose up. If that fulcrum is too far back, the force provided by the control surfaces must lift the weight of the entire plane in order to make it nose up. Since control surfaces are not wings, they are not made to generate enough lift to carry the entire plane. ...Especially if they are positioned with next to no lever arm, as in this example

Basically, you managed to put the elevators around where you'd want the wheels, and the wheels around where you'd want the elevators. This is the main reason that many aircraft have their elevators on elongated tails, well behind the main body, while the landing gear is installed under or near the main wing.

Edited July 10, 2016 by Streetwind

[bewing]

One other quick tip -- if you compare those early swept wings to the type B wing connectors you will find the following: the type B connectors weigh less, cost less, and give you more lift. So the swept wings are a bad deal.

[Warzouz]

I think @Streetwind explained it, but your rear landing wheels is far too rear. That's why you can't take off.

But as you COL is really near you COM, your plain will be very unstable. And as fuel is consumed, the COM may move and if it goes behind COL, your plane will flip or flat spin.

COM must always be in front of COL (like a dart). The farther it is, the more stable the plane is, but the harder it is to steer. You have to try different configurations and verify COL/COM when loaded and unloaded. The flipping plane which was flying very nicely after take-off is a classic.

Keep in mind that even rules are the same for planes and rockets. Rockets are usually less subject to unstability because they usually don't diverge from prograde much. (rockets don't "fly" much). So building plains is harder than rockets.

[Red Iron Crown]

22 hours ago, swjr-swis said:

1) The control surfaces are in front of the CoM. The editor won't tell you this, but that will make them reverse how they work once the plane is launched: when you try to pitch up, they will actually be pitching you down. You can correct this by pulling the 'Authority' slider to the left and make it negative.

Sort of. Control surfaces ahead of CoM work fine (see every canard ever), the trouble arises when the wing is attached ahead of the CoM while the surfaces are behind it, that's when they reverse in pitch and yaw. Reversing them with the authority slider works as long as the surfaces aren't being used for roll, otherwise it makes them reverse for roll while being correct for pitch.

[swjr-swis]

17 minutes ago, Red Iron Crown said:

Sort of. Control surfaces ahead of CoM work fine (see every canard ever), the trouble arises when the wing is attached ahead of the CoM while the surfaces are behind it, that's when they reverse in pitch and yaw. Reversing them with the authority slider works as long as the surfaces aren't being used for roll, otherwise it makes them reverse for roll while being correct for pitch.

Thanks, RIC. I did a bad job of wording this, even though I am aware of it working that way. You expressed it more accurately.

[swjr-swis]

22 hours ago, bewing said:

Beyond that, a wheesley engine can easily get you to 10km altitude. Your design can probably fly at 12km. With extreme care, you can make a perfectly designed wheesley-based jet reach 16,698 meters for an instant (I know because I managed to do it once as a lark). But to reach 18km, you cannot do it this way. You use jet engines to get the plane close to your target. Then you have a little rocket on board and you light that to get you from 10km altitude up to 18km or 20km to take your measurements.

Emphasis added, because this is a very good point. The Wheasly seemed to be quite a bit better since its last updates, so I went and did some more tests: it starts losing a whole lot of thrust very quickly at a disappointingly low altitude. Even the humble Junos maintain more thrust once passing 8-10km.

Replacing the two Wheasly nacelles by a set of 7 Junos (center and 6 in radial symmetry around it) not only managed to dip the plane over 18km without needing rocket engines, it also turned the plane supersonic, sustaining up to 610m/s @ 13km. This is not just due to the thrust - apparently a set of 7x 0.625m drag 'stacks' produces considerably less drag than a single 1.25m stack.

The Wheasly uses the least fuel of all jets, and has the thrust reverser... otherwise it appears to still be very underwhelming, especially if it even 'out-drags' a spamming of Junos.

 

20 hours ago, Streetwind said:

Your plane fails to take off because your aft landing gear is too far in the back.

Good catch, I failed to notice the position of that gear.

Advanced tip: if the main wings are given just a tiny bit of angle of attack (pitched up a few degrees compared to the body of the plane), the plane can lift off without needing to pitch at all - and it helps keeps the drag down in flight, giving it better performance.