High lift devices

[Nich]

 Most of my SSTOs have a lot of trouble getting off the runway. I could add more wing but that would make it less efficient. I have been experimenting with high lift devices but I cannot seem to make any that pay for themselves.  Is it just me or do lifting  surfaces have a lot more drag then wings?  It also looks like I have to deploy my slats backwards in order to create lift in the correct direction

[Choctofliatrio2.0]

I have the opposite problem. My plane makes too much lift. I try to land it but it just keeps bouncing off and flying again.

[Nich]

Ha ha ha I have had some very heavy lifters not touch down until 50 m/s something not right about a 100 t plane flying that slow

[Tantalus]

Do you get your nose up while on the runway? If not, its propably your aft landing gear too far behind your COM. A picture might help to find out.

[omelaw]

I think slaping one or two drogue parachute will solve it

or aerobrake.

nope, I missread.

Edited February 23, 2016 by omelaw

[SurrealShock]

.

Edited August 19, 2020 by SurrealShock

[Nich]

Here is an example of a craft that can just barely reach orbit.  It has enough wing that it can get off the runway and not fall into the ocean with about 5-7 degrees AoA.  I have added Flaps (rear) and slats (front).  In the first pic you will see that the real world config the slat incorrectly produces down force.  It might increase the lift of the wing but I could not see a noticeable difference.

After correcting this I was just barely able to make orbit.  About 25 less dv.  The plane flew with much less AoA but the acceleration from 130 m/s (runway take off) to 200 m/s where it normally starts flying better (less then 2 degree AoA) was about the same.

 

I know that real planes do not take off with full flaps or any if the runway is long enough.  But short field takeoffs are generally done with 1/3 to 1/2 flaps.  I would argue that this is a short field takeoff because we want as little wing and as much payload/fuel as possible.  You could also argue that because I increased the takeoff lift and total lift I need to do a corresponding decrease to the big s wing using tweakscale to have an equivalent craft.

Edited February 23, 2016 by Nich

[Fearless Son]

Given how large and heavy your parts are, your problem might be insufficient thrust at low altitude.  Those R.A.I.P.I.E.R.s will certainly get you to orbit, but their performance envelope is poor at sea level where you take off from.  You might consider adding more of them, or supplementing them with other engines.  For example, you might add a little more LF/O to your tanks than you normally do and switch to rocket mode for a brief burst while accelerating down the runway, followed by switching back to air breathing when you start gaining altitude.  Alternatively, you might just add more engines.  Yes, this will increase the total weight of the craft and burn fuel faster, but in my experience this actually saves some time and fuel in the long run because you spend less time building altitude and speed until reaching your optimum envelope to climb out of the atmosphere.  

[JoeNapalm]

 

Lift creates drag. That's just aerodynamics at work.

I agree with what Fearless said, and would add that (unless you're dead-set on it being an SSTO), recoverable boosters can help get you out of the sea-level soup.

As someone who builds a fair number of giant SSTOs - and just made a comment that you should go big or go home, in another thread - I can tell you that you're almost always going to do better of you can stay smaller, if being efficient is your driving factor. The only reason to build a gigantic Mk3 SSTO is because you want to - if you have other design considerations, the math is always going to come out in favor of lifting less mass into orbit.

 

-Jn-

[monophonic]

AFAIK KSP does not model airflow interactions between any parts. So no, slats just won't work. Flaps only work by increasing their own lift with AoA, too. Although it could be possible for the control surface module to modify the Cl of the parent part depending on the deployed status of the flap/slat...

[Kertech]

I know it's not SSTO anymore but I usually put on a couple of panther pods on the wings to get the plane to about mach2 (ish) then drop them and fire up the RAPIERS

[tranenturm]

Fleas can be used on the runway to get up to speed but still burn out quickly enough to be recoverable if that's important to you. 

[Moar Boosters]

If your plane has trouble lifting off from the runway, it could be a problem with wheel placement. For it to take off it has to be able to pivot it's center of mass around the rear wheels, and this means they need to be close together. If your craft is locked to the runway but is able to easily climb as soon as it leaves the end of it, your rear wheels are probably too far back. If you move them forward a bit until the plane is close to the point of tipping backwards when stationary you should have a much easier time lifting your nose up whilst still on the ground.

 

It can be easy to smash your engine into the runway if you pivot too much, but you can always add another small wheel near the back which doesn't touch the ground to protect the engine from mishaps. 

 

 

 

 

 

Edited February 24, 2016 by Moar Boosters

[Fearless Son]

Also, though this is a very inefficient use of mass, if you really want to get your heavy SSTOs to lift off the runway, you could add some secondary jet thrust vectored perpendicular to the primary thrust such that it creates an upward force.  Kind of like if you are building a VTOL, but not intended for an actual vertical lift off, just given enough thrust to effectively "lighten" the mass of the craft such that its lifting surfaces have an easier time getting it airborn.  Once you are skyward and building up good horizontal speed, you can shut down the lifting engines and let the primary engines and surface lift carry it from there.  

But like I said, that is inefficient.  Except during take off and landing, those extra engines are just dead weight and potentially drag.  For my money, I would think extra horizontal thrust would be more efficient, but it is still something to consider.  

[jhousen]

On 2/23/2016 at 9:08 AM, Nich said:

Ha ha ha I have had some very heavy lifters not touch down until 50 m/s something not right about a 100 t plane flying that slow

50 m/s is about 110 miles per hour,  not slow at all.

 

Most planes land slower than that. 

[Nich]

Ha ha didn't even realize that lol.  My cherokee I learned to fly in landed about 70 knots or 80 mph.  I got to see a C-17 loadmaster do a short field landing and takeoff and it was truly amazing it looked like it was just floating in the air it was so large and moving so slow.  But most commercial planes land 135-145 knots I am pretty sure.  Feel free to correct me if I am wrong.

[Val]

On 2/23/2016 at 8:09 PM, Nich said:

Here is an example of a craft that can just barely reach orbit.  It has enough wing that it can get off the runway and not fall into the ocean with about 5-7 degrees AoA.  I have added Flaps (rear) and slats (front).  In the first pic you will see that the real world config the slat incorrectly produces down force.  It might increase the lift of the wing but I could not see a noticeable difference.

I know that real planes do not take off with full flaps or any if the runway is long enough.  But short field takeoffs are generally done with 1/3 to 1/2 flaps.  I would argue that this is a short field takeoff because we want as little wing and as much payload/fuel as possible.  You could also argue that because I increased the takeoff lift and total lift I need to do a corresponding decrease to the big s wing using tweakscale to have an equivalent craft.

Judging by the pictures you have 5 RAPIERs to ~87 t. That's 17.4 t per RAPIER. That's pretty much the optimal mass for a RAPIER powered SSTO in terms of payload fraction and ease of launch and ascent to space. (My top entries in both Stock Payload Fraction and the Smallest SSTO to lift Orange tank challenges were ~18 t per RAPIER)

So that part of your craft is definitively OK.

For the high lift devices, I rarely use them, but when I do:

• I only deploy them last second when I rotate to takeoff or reach the end of the runway.
• I make sure they are balanced around CoM, so they don't require pitch correction when deployed. Either by have them equal number of devices in front and behind CoM, or the single pair of devices right on top of CoM.
• I give them none or less AoI than the main wing, both so they don't add drag during flight and to make their deflection less when deployed.
• I usually only use them as Landing flaps.

I'll make a test craft based on your pictures and see if I can figure out why you're having trouble taking off.

 

Edited February 25, 2016 by Val
Landing flaps

[Val]

Here's my recreation. I opted for Tailplane instead of Canards in order to move the flaps closer to CoM. They're still to far back for my taste, though. My craft ended up at ~95 t with 34 t cargo. And yes, this craft is perfectly stable with CoM there. Just remove the cargo and all fuel except for the Mk1 LF and take it for a spin. It flies like a big heavy fighter. Craft file or see below.
Here are aerodynamic forces at 80 m/s. The up and down yellow forces from the control surfaces are from the ailerons and flaps respectively. The flaps give negative lift here because they have no incidence and the nose wheel is a bit shorter.
At 110 m/s I start lifting the nose up to 5° above horizon and record the speed and altitude when I reach the runway end. With flaps undeployed I reached 144 m/s. It did not lift off, it also didn't lose altitude after passing the end off the runway and started climbing eventually.
The second time I also rotated at 110 m/s and deployed flaps at the same time, contrary to my recommendation in the previous post. With flaps deployed I reached 138 m/s and it just barely managed to lift it's wheels before the end of the runway. It continued climbing after passing the end of the runway.

If I'd followed my own advice, I could have reached the end of the runway at 144 m/s and would have had the option to deploy flaps if the craft started losing altitude.

But in my opinion, this craft should be fine without high lift devices. It does need the wing area the flaps provide undeployed, though.

Spoiler

Cargo and all fuel except for the Mk1 LF removed. 28 t Takeoff weight.

 

 

[Rune]

On 23/2/2016 at 9:31 PM, JoeNapalm said:

 

Lift creates drag. That's just aerodynamics at work.

I agree with what Fearless said, and would add that (unless you're dead-set on it being an SSTO), recoverable boosters can help get you out of the sea-level soup.

As someone who builds a fair number of giant SSTOs - and just made a comment that you should go big or go home, in another thread - I can tell you that you're almost always going to do better of you can stay smaller, if being efficient is your driving factor. The only reason to build a gigantic Mk3 SSTO is because you want to - if you have other design considerations, the math is always going to come out in favor of lifting less mass into orbit.

 

-Jn-

Actually, it works the other way around. Your Lift/Drag ratio is a function of your Angle of Attack (AoA from now on). Low angle of attack, and lift is a big percentage of the total aerodynamic forces. High angle of attack, and drag grows faster than lift, to the point that you "stall" (it's modeled derpy, but it works out surprisingly like the real thing). In fact, at really high angles of attack (>30º), lift in fact goes down, while drag keeps going up. With more wing, you can mantain a lower AoA for the same lift, meaning you see less drag at the same airspeed and altitude. High wing-loaded planes can see more drag at low airspeeds for this reason, if they have to maintain a high angle of attack to fly level. You can go the other way and have too much wing, of course, but that is mostly because wings do weight somewhat, not because of drag, or at least not aerodynamic drag. If you want to call it gravity drag...

Also, with the proper caveats about how you can't divide a given engine (so you can only go in discrete increments up or down) your size is totally irrelevant to your flight characteristics. I cannot stress that enough. Rocketry and flight are all about ratios, and given how KSP parts are scaled, everything part-relevant scales more or less linearly (twice the weight is twice everything else, like thrust in engines and lift in wings). If your small SSTOs work better than your big SSTOs, that's because you get the ratios in your big ones wrong(er). Study your small ones, copy the ratios in a bigger one (same TWR, mass ratio, payload fraction and so on), and it will have the same flight profile, with the same dV when it gets to orbit.

 

Rune. Because physics.

Edited February 26, 2016 by Rune

[Val]

And for good measure I took it to orbit, too. I'd forgotten to put fuel in the wings, so take-off weight was ~98 t.

Just under 300 m/s dV in orbit after an 8° ascent. (Build speed to 450 m/s at sea level, then ascend at 8° until AP is 80 km.

 It need a bit of fuel balancing. Replace 4-600 Oxidizer with Liquid fuel.

Edit: I also made a Canard version, craft file.

They fly pretty much identically, but both craft will try to flip over on their backs during reentry, because their CoL is below CoM. They should stay nose first, though.

Edited February 26, 2016 by Val
Canards

[AbacusWizard]

On 2/23/2016 at 8:48 AM, Choctofliatrio2.0 said:

I have the opposite problem. My plane makes too much lift. I try to land it but it just keeps bouncing off and flying again.

Try putting all the wings on radial decouplers so you can jettison all the unwanted lift as soon as you touch down.

[Choctofliatrio2.0]

2 minutes ago, AbacusWizard said:

Try putting all the wings on radial decouplers so you can jettison all the unwanted lift as soon as you touch down.

Interesting idea XD
Don't know how well that'd work for a science jet attempting to reach multiple biomes per mission...

[Guest]

MOAR WINGS

[AbacusWizard]

Oh, well if you want it to be reusable...

 

...then I guess you could bring along a KIS/KAS-authorized engineer to weld the wings back on afterwards.

Edited February 26, 2016 by AbacusWizard

[Francois424]

A trick I have learned is to make sure the forward landing gear pits the nose into a slight elevation.  That way the natural movement of the plane will make it fly.

Second thing is to mess with the Center of Weight, and fuel at liftoff versus fuel at landing.