How to obtain negative lift?

[Apaseall]

I would like to press my rover into the ground when using it on planets with an atmosphere.

I can and do build planes. Space planes too. So I can add lift to help my rover fly.

What I want to do is press it downwards, not lift it up. The body of my rover is a space plane part. That comes with its own lift. As the rover speed increases it may be reducing its apparent weight and so be reducing friction which would normally be used to be able to steer the rover.

One work around is to use action groups to lock steering if like me you have more than 2 wheels. Also reduce the amount each wheel is able to turn. The first needs me to remember to lock more and more pairs of wheels as speed increases during play. The other can be set at rover build time.

My reason for asking is that my current understanding is that lift is defined for a part such that it lifts the craft up, and that merely flipping the part over will not change the direction of the lift.

Am I wrong? If not, any way I can get what I would like?

Thanks.

[Lisias]

Use negative angles of attack.

Lift in KSP is greatly simplified - the shape of surface is meaningless, so there's no difference on flipping the wing upside-down as we would do on reallife.

What I do is tilt down the wing's front border. This creates "negative lift".

[mystifeid]

4 hours ago, Apaseall said:

I would like to press my rover into the ground when using it on planets with an atmosphere.

Are you sure this is necessary? Try testing your design at KSC. Here is a Mk2 fuselage at 387m/s with aerodynamic forces shown and ... no lift.

[Gargamel]

12 minutes ago, mystifeid said:

Are you sure this is necessary?

Repeating....   Aside from ice caps and runways, there aren't many places in the solar system flat enough with an atmosphere to get a rover going fast enough to create lift anyways.   There are plenty of places to create lift and take off, but the speed required to create a downforce would destroy most rover designs, due to the bump and undulations of the terrain. 

[Gordon Fecyk]

11 hours ago, Apaseall said:

What I want to do is press it downwards, not lift it up.

Sounds like you want a spoiler, like what F1 cars have in real life.

Most elevon parts support an extended and retracted mode in Stock, so you can use them as rudimentary flaps. You can also toggle inverted mode to turn them into spoilers. This can be handy when landing a stock aircraft as well to plant the craft on the ground at touchdown.

Alternately, just add some static wing parts and rotate them slightly downward.

[Apaseall]

16 hours ago, Gargamel said:

Repeating....   Aside from ice caps and runways, there aren't many places in the solar system flat enough with an atmosphere to get a rover going fast enough to create lift anyways.   There are plenty of places to create lift and take off, but the speed required to create a downforce would destroy most rover designs, due to the bump and undulations of the terrain. 

The issue is not actual take off, but the reduction in friction. This is needed for steering the rover, not going fast in a straight line.

[mystifeid]

21 minutes ago, Apaseall said:

The issue is not actual take off, but the reduction in friction. This is needed for steering the rover, not going fast in a straight line.

So can you switch on the aerodynamic force overlay (F12) and post a screenshot of the lift being generated by your rover?

[Apaseall]

Lol. Now that I have looked at it, the little blue arrows are pointing downwards. Oops. I should have looked rather than guessing. Right off to find another way to improve my steering. Thanks.

[mystifeid]

5 minutes ago, Apaseall said:

Lol. Now that I have looked at it, the little blue arrows are pointing downwards. Oops. I should have looked rather than guessing. Right off to find another way to improve my steering. Thanks.

It's actually generally a good thing to reduce the friction on steering wheels and increase friction on non-steering wheels.

[Apaseall]

I know what I am wishing for is not actually realistic. I want it for personal playability as I am a bit fed up of spinning round when I try to make heading changes at speed lol.

[mystifeid]

26 minutes ago, Apaseall said:

I am a bit fed up of spinning round when I try to make heading changes at speed lol.

That's why reducing the friction on the (hopefully front) steering wheels is a good idea. You want them to be able to slide rather than dig in. Other than that make the rover very low and very wide. And even with the best designed rover, the snap barrel roll is still a possibility on low g moons/planets. Have a good lightweight rcs system and get used to flipping it on quickly in an emergency. You can also use Mechjeb's stability control if you get really fed up.  Inertia is a b*tch though and really, the best way to improve turning at speed is to keep redesigning lower and wider.

Edited August 27, 2018 by mystifeid

[TanDeeJay]

You dont have reaction wheels on your rover by any chance that are set to normal or pilot only? If your steering control keys are feeding into the reaction wheels,  it might be the reaction wheels trying to yaw your rover that is spining it.

Also, caps lock key changes the sensitivity of the controls. 

[Gargamel]

17 hours ago, Apaseall said:

The issue is not actual take off, but the reduction in friction. This is needed for steering the rover, not going fast in a straight line.

Wait.... so you want downforce for steering, but you want it by not going fast? 

[Lisias]

17 hours ago, mystifeid said:

It's actually generally a good thing to reduce the friction on steering wheels and increase friction on non-steering wheels.

As a "survivor" of an aquaplaning (managed to regain control of the car before damage was done - my seat, however, didn't made it…. ), I strongly disagree.  

Edited August 28, 2018 by Lisias
typo. again.

[mystifeid]

3 hours ago, Lisias said:

As a "survivor" of an aquaplaning (managed to regain control of the car before damage was done - my seat, however, didn't made it…. ), I strongly disagree.

You'll notice that I included "generally" because I was thinking I could maybe build a rover for which that statement would not be applicable.

But had you in fact increased friction on non-steering wheels and decreased it on steering wheels? If you had, are you blaming your loss of control on that setup? And are you basing your opinion on just one incident?

Currently I am circumnavigating Eve. This is my last circumnavigation of all the planets/moons in the stock Kerbol system. I am the survivor/victim of countless incidents and over time I have become very good at regaining control in different types of rovers without damage.

Loss of control is often caused by one or both of two things - terrain and aggressive course correction. Not much you can do about the terrain except anticipate the effect it will have on the rover and/or change the angle/speed at which you meet a feature. There are also bugs in the terrain. I have seen rips in the space-time continuum on Minmus and Ike, driven into bulldust on Gilly and on Eve I keep encountering invisible patches of quicksand which slow and slew the rover. This last one makes me wonder how many times I've encountered it before in other places without knowing it.

The easiest way for me to cause loss of control is by changing course. Do it too fast and I'm going sideways. If a wheel catches I'm rolling one foot off the deck into the bargain. It is the roll which most often causes the destruction of the rover. The easiest way I've found to prevent the initial loss of control is to reduce friction progressively from the back to the front of the rover (my most used rover has 16 wheels - upgraded to 20 for Eve). This also helps to keep the rover pointed straight during aggressive braking.

A simple test I use at KSC to help determine the worthiness of a rover is to take it to top or near top speed then hold down the left or right control key. If it rolls, I don't want it. But it can be interesting to see how much I have to change the friction settings to prevent the roll. In the following screenshots the rover is taken to 20.0m/s before holding down the left key (numpad 4). In the first the friction is left at the default 1.0/1.0 and a roll ensues. In the second the friction has been changed to 0.4/1.1 and no roll. I wouldn't advocate such drastic change for Kerbin but it does illustrate how the friction setting can influence loss of control.

The 16 wheeler does about 75m/s at KSC with both Verniers burning. It doesn't even look like it is remotely close to rolling when I apply full lock but every now and then I get to see it suddenly flip upside down on some planet or moon so no matter the design or the setup, bad things will still happen. And I'm glad.

Edited August 28, 2018 by mystifeid

[Apaseall]

I have lateral spin which would be yaw axis. I do not suffer from flipping or rolling in well roll axis. Thanks everyone for the many comments. I will look into the concept of friction distribution fore to aft. Oh and my speed is much higher. My rover is extreme in size. I use OPT K H for my body.

[Dark Lion]

On 8/27/2018 at 4:13 AM, Apaseall said:

...as I am a bit fed up of spinning round when I try to make heading changes at speed...

Without a picture of your rover, I can't be certain of this, but that sounds like it could be a problem with your center of mass. Like with rockets, it's better to "lead" your rovers with the mass to reduce the likelihood of flipping about. However, because traction is necessary to maintain control while under power, I try to put my center of mass just barely ahead of the center of the area between wheels, and as low as possible-- especially in high-performance designs.

Hope that helps!

[Lisias]

9 hours ago, mystifeid said:

But had you in fact increased friction on non-steering wheels and decreased it on steering wheels? If you had, are you blaming your loss of control on that setup? And are you basing your opinion on just one incident?

I was making a joke, and the extensive use of emoticons should had be enough. But, since you want an argument, let's go!  

I used to drive smal truck with 2 tons cargo. I used to drive 4 tons tractors on the mud. I used to drive medium performance cars in not the safest speed I was expected to drive. And in all these vehicles, lessening up the friction on the steering wheels is suicide. In once situation, we were experimenting on different tires on the trucks in order to optimize our funding (aka - saving money!), and we had the bad idea of using a tire of harder rubber on the front wheels (the same we used on the rear wheels). Hard rubber are… harder. But there's also less friction for the same given weight.

Not so bright idea. With a full load, the rear wheels got way more friction than the front wheels, and this leaded to a ridiculous situation: on a U turn, with the truck stoped and every single other traffic rule being obeyed, I turned the steering wheel all to left, engage the first gear and hit the gas pedal (the diesel pedal, to tell you the true). The damned vehicle just gone straight ahead, virtually ignoring the steering.

I had way more friction on the rear wheels than o the front, and by accelerating a bit more than the minimal, the front wheels couldn't steer the car anymore! After math? I drove back home at 10 mph and we never more did anything that could remotely lessened the front wheel's friction. That would had killed me on the field.

In every single situation decreasing friction on the steering wheels is a very bad idea. Aquaplaning a 2 tons gross mass vehicle because you were using sand tires over a curve where a water plumbing had blown is not funny. Not even at 40Km/s (27 or 28mph).

Interesting enough… I could reproduce these situations on KSP (obviously, respecting the limitations of the game engine - there's no aquaplaning on the game).

The rule of thumb is simple: you don't want to lessen your friction on the steering wheels. You need it to… steer (!) the rover!

If you flip over your rover, you are too fast for the height of the center of mass - there's a reason fast cars runs scratching their bellies on the ground.  

If you are breaking your wheels on turning, you have too much weight on them. You need bigger wheels, or lighter vehicles.

If you lessen up the front wheels friction, you are loosing authority on turns and risk doing a involuntary doughnut when breaking. You never design an vehicle with more friction on the rear wheels than on front!  

Edited August 28, 2018 by Lisias
and more typos!

[mystifeid]

6 hours ago, Lisias said:

I was making a joke, and the extensive use of emoticons should had be enough.

Yeah, I don't speak emoticon so you can forget about me even looking at them. If you are talking about real life then of course I agree with you, (I too am a survivor of an aquaplaning incident involving a full 360° spin down a hill at 80km/h - no damage!) but you may find the game engine is no respecter of real life - so...

 

6 hours ago, Lisias said:

Interesting enough… I could reproduce these situations on KSP

Go for it. Always willing to learn.

But I will make this caveat - please, not on a high g world.

On Kerbin I would never bother to change friction settings (and did I need to change friction settings on Eve? - well, yes, but not for any of the reasons we're talking about) but on lower g planets/moons where a more fleeting contact with the surface can play a pivotal role it is often a different story. Places where, without engines, it is possible to maintain speeds of 90-100m/s for long periods. Any faster and my wheels tend to explode and any slower and holy cow, it takes like twenty hours to drive around some of these places. (Eve is looking like about sixty hours and Eve's North Pole, though picturesque, has become my new least favorite place to drive a rover).

6 hours ago, Lisias said:

The rule of thumb is simple: you don't want to lessen your friction on the steering wheels. You need it to… steer (!) the rover!

In KSP we are lucky that we have reaction wheels and we do not need friction to give us yaw authority. While I use my alternate keys for forward/reverse, I always use "A" and "D" to turn. This is because, at high speed on lower g worlds, I am often not in contact with the surface and just beginning a turn can cause loss of contact. Losing contact often involves terrain induced yaw so I have to rely on the yaw control provided by my SAS anyway and using two different sets of keys to turn when things are happening fast is just not going to happen. When my front wheels hit the surface I don't want them making much of a contribution if I am yawed with respect to my direction of travel. Quite often a lot of time is spent above the surface and it's nice not to have to micromanage every landing.

Losing steering authority provided by friction is the whole point of what I've been suggesting and yet by doing this in the simple example above, by preventing the roll it could be said that the steering has improved.

Even using a rover with a very low CoM, the terrain or an overzealous turn can easily yaw and roll it in an instant. Was I going too fast? Obviously. Will I slow down? Of course not. Instead I look for a way to ameliorate the effect on the rover. Each world is different and I constantly change the setup until usually about three quarters of the way round I become confident that I've found the one that gives me the highest speed with the least mishaps.

Does it really make that much difference? Probably not but even saving my rover once or twice per trip is worth the effort.

Edited August 28, 2018 by mystifeid

[Tyko]

Keep in mind that something that works on Kerbin won't work as well on a planet / moon with very low atmospheric pressure

[bewing]

I disagree with Lisias myself. If you watch racecars, they either have oversteer or understeer. Oversteer means they spin out. Understeer means that you try to turn and not much happens. There is a balance point between those two extremes that all racecars try to find. To balance them, the racecars tweak the downforce on the front and back to increase or decrease friction at those points. There is no magic number, but the safest answer is to always be on the understeer side. That makes it harder to go around corners, but at least the car is driveable. A car that spins out regularly from oversteer is not driveable.

[Gargamel]

15 hours ago, Apaseall said:

My rover is extreme in size.

Wait, time out yet again.

 

Which wheels are you using?  The really big ones?  Those steer like a tank does, not like wheels do.   So when you turn right with those wheels, all the wheels on the left go forward, and the wheels on the right either stop or go backward (I don't know which in this case).    So when you are at speed, and you try to turn, you're going to spin out, regardless of how much downforce, or any other force you have.   One set of wheels will not be turning in the same direction that they are traveling!

A pic of your rover, as requested many posts ago, would probably clear up a lot of confusion. 

[Apaseall]

7 hours ago, Gargamel said:

Wait, time out yet again.

 

Which wheels are you using?  The really big ones?  Those steer like a tank does, not like wheels do.   So when you turn right with those wheels, all the wheels on the left go forward, and the wheels on the right either stop or go backward (I don't know which in this case).    So when you are at speed, and you try to turn, you're going to spin out, regardless of how much downforce, or any other force you have.   One set of wheels will not be turning in the same direction that they are traveling!

A pic of your rover, as requested many posts ago, would probably clear up a lot of confusion. 

I have not had any luck with trying to post images here in the past. Also I do keep working on this rover, both in terms of what is inside it and yesterday I removed whole body chunks including a pair of wheels. I believe the wheels are from Kerbal Foundries. I have got into the region of 70+m/s turning now which is a vast improvement. In fact now the issue is not turning at all lol closely followed by over heating which blows the rubber off! That is a bit tricky since this craft is currently not piloted by a kerbal.

[Lisias]

On 8/28/2018 at 4:03 PM, bewing said:

I disagree with Lisias myself. If you watch racecars, they either have oversteer or understeer. Oversteer means they spin out. Understeer means that you try to turn and not much happens. There is a balance point between those two extremes that all racecars try to find. To balance them, the racecars tweak the downforce on the front and back to increase or decrease friction at those points. There is no magic number, but the safest answer is to always be on the understeer side. That makes it harder to go around corners, but at least the car is driveable. A car that spins out regularly from oversteer is not driveable.

I understand what you mean, but I think you are misguided on your conclusions.

RaceCars spins out not by "oversteering", but by losing friction from the back wheels - the centrifugal force is bigger than the friction the wheels can provide. You are misunderstanding cause and effect.

By making the back wheels "heavier" (the most effective way to add friction to the wheels), you need to add "weight" into the front wheels as well, or you loose authority from them on the turns - there's no point on allowing the back wheels to be more "resilient" to turns that your front wheels can't sustain.

On RaceCars, the canonical way of adding "weight" on the wheels is by "inverted" lifting surfaces and ground effect. Way better than make the car heavier by adding mass (for obvious reasons). However, such lifting surfaces add a lot of drag in exchange for the lifting, so you must compromise between your ability to survive turns and your ability to go really fast on the straights.

Keep in mind that the "oversteer" and "understeer" is highly related to the car's current kinetic energy, i.e., varies as you go faster or slower. So you can't define a "oversteer" cutoff as the car must be steerable on all speeds it will handle on the race track - including slow speeds while dodging debris from accidents.