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L/D and gliding to the runway


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I'm trying to write a script that will return a spaceplane to the runway. The actual landing works rather well already, and atmosphery entry too, now I'm trying to tie them together: lining up with the runway coming from arbitrary directions, optionally circling in order to rub off excessive energy.

One question I have to deal with is setting the right airspeed / glide slope targets. How much leeway do I have? It's one of the things I usually do by the seat of the pants (if at all, admittedly), but now I have to write an algorithm for it...

So, a question to those pilots who fly a lot and pay attention to data displays: could you provide me with a few data points of what works for you? How low and slow can you come in depending on Lift/Drag ratio?

Also, quite generally, what do *you* consider to be a high or low L/D?

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I come in very high, then do an extreme dive just before the runway, then do an extreme climb and dive to scrub all my remaining speed, then come into the runway with a 1 m/s descent from 75m altitude. Doing this, I don't have to care at all about L/D.

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1 hour ago, bewing said:

I come in very high, then do an extreme dive just before the runway, then do an extreme climb and dive to scrub all my remaining speed, then come into the runway with a 1 m/s descent from 75m altitude. Doing this, I don't have to care at all about L/D.

Well, that's one tactic.

Me, I do barrel rolls and pirouettes... :D

 

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6 hours ago, GoatRider said:

You guys haven't heard of a "stabilized approach"? That what pilots do IRL.

Without even looking it up,I can assure you that it isn't necessary. As it has turned out, the stock atmosphere is still quite soupy. I've heard that before, of course, but never really noticed.

Yesterday I tried to steer my plane into a glide slope that would assure an airspeed of 180m/s until close to the runway, which I thought was in the right ballpark for a returning shuttle. Guess what? It had to go into a steep dive in order to maintain that speed. When I started setting fixed slopes of 10, 8, 5 degrees I found that my planes can hold any of these, and even 5 degrees doesn't require much in the way of AoA.

Eventually my shuttle was circling at under 80m/s, 5km up. It's a sailplane!

Now, that was with a L/D of ~5 -- I don't really know if that is much or not. But I guess even planes with considerably less wing can approach the runway at a leisurely pace and will have a lot of time to line themselves up.

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I start my descent at the far edge of the western desert.

I aim for the water before the landmass  with KSC.

Without using engines for the entire descent.

I try to just get over the western mountains.

Then apply airbrakes as needed (not always depends on the craft) while aiming for the start of the runway.

I generally touch down well before the hangar. Apply brakes/airbrakes and pop the chutes.

I try to come to rest parallel with the hangar.

You have to keep in mind that the drag will vary wildly. On spaceplanes you try to reduce it as much as possible for getting into orbit. Then you feel you wish you had more as you pass over the Western mountains.

D.

 

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Optimum lift to drag ratio is about 5 degrees angle of attack at supersonic speed and 2 degrees at 100 m/s...   think the transition is pretty linear.

For the approach from orbit, i usually aim my reentry to hit the ground in the desert continent west of the space centre, then i hold somewhere close to optimum lift/drag angle on re-entry (5 degrees) until it looks like i might overshoot,  then start pitching up to a steeper angle (10-15) degrees.

In terms of final approach to runway,  it's usually just a screaming nosedive with a pullout then some sharp manuvers to try and bleed off all the speed that built up in the dive,  followed by a landing somewhere on the same postal code of the space centre due to overestimating/underestimating the energy needed.     So long as you put it down without damage in the right continent, that's enough.

If by some miracle the screaming dive was not needed or you're landing off-airport and aren't bothered about landing in a specific place,  a stable glide on the last few hundred metres is best i think.   Glide in at 4 or 5 degree AoA and gently flare to landing.    More than 5 degrees on the approach there is a risk you won't have enough energy to flare, you'll either hit he ground at high sink rate or you'll get too high AoA and tail strike.   Once you get above 10 degrees angle of attack there is not much lift to be had from pulling harder just a whole bunch of drag.   If you're reaching 10 degree AoA and are not <2 second from  touchdown its time to panic.

OTOH if you only have 2 degrees aoa on the glidescope you're coming in very fast and the "float / flare" part of the landing could last a very long time on a slick airplane.  You may end up landing several miles further ahead of where you thought you were going (plenty of my spaceplanes have ended up touching down at water's edge when i was aiming for the west end of the runway)

Edited by AeroGav
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On 1/18/2019 at 8:06 PM, bewing said:

an extreme dive just before the runway, then do an extreme climb and dive to scrub all my remaining speed

On 1/18/2019 at 9:46 PM, swjr-swis said:

Me, I do barrel rolls and pirouettes...

On 1/24/2019 at 11:04 PM, AeroGav said:

it's usually just a screaming nosedive with a pullout then some sharp manuvers to try and bleed off all the speed

Oh great, now how do I teach that to a computer?
Here's what I currently have, by the way:

Reentry is easy: if too fast, pitch up. If too low, pitch up. If in danger of climbing too high, start rolling. That's pretty simple, yet I love how it plays out (at about 5:30).

I'm pretty conservative about reaching the Space Center, falling short is so embarrassing, so more often than not the plane comes across the mountains at Mach2 / 20km and needs to rub off some energy before it can land. For the time being, I just set a glide slope from here to runway, and if that seems to be altogether too steep, I plan a circle (entering circle at 8:40, back to straight approach at 11:40, eventual landing at 14:20).

That's what spawned this thread: for the time being, the acceptable slope is hard coded. I'd prefer if I could somehow figure out how well the plane is going to glide and plan the approach according to it's abilities. Only, how do I tell? Would any data I gather during reentry still be meaningful in a subsonic glide? Can I just count the amount of wing and relate it to vessel mass?

 

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2 hours ago, Laie said:

Oh great, now how do I teach that to a computer?
 

Reentry is easy: if too fast, pitch up. If too low, pitch up. If in danger of climbing too high, start rolling. That's pretty simple, yet I love how it plays out (at about 5:30).

I'm pretty conservative about reaching the Space Center, falling short is so embarrassing, so more often than not the plane comes across the mountains at Mach2 / 20km and needs to rub off some energy before it can land. For the time being, I just set a glide slope from here to runway, and if that seems to be altogether too steep, I plan a circle (entering circle at 8:40, back to straight approach at 11:40, eventual landing at 14:20).

That's what spawned this thread: for the time being, the acceptable slope is hard coded. I'd prefer if I could somehow figure out how well the plane is going to glide and plan the approach according to it's abilities. Only, how do I tell? Would any data I gather during reentry still be meaningful in a subsonic glide? Can I just count the amount of wing and relate it to vessel mass?

 

As far as i know the glide ratio of an airplane is equal to its lift : drag ratio,  something that is continually displayed in the AeroData GUI (Alt F12) menu - don't know if you have access to that.

Alternatively,  you could just do some test flights to find the lift drag ratio of your airplane at different flight reigimes, to get some rough numbers to extrapolate from.    Eg.  from 200 to 100 m/s for example.  On a ship with wings attached flat to fuselage the optimum glide rate is about 3 deg aoa at 200m/s and closer to 2 at low speed (under 100).   You could use this to determine if you are under or over shooting.

If you are over shooting,  the easiest thing to do would be to pop some airbrakes .  It could also lower/raise the gear and open/close service bays, depending on the severity of the overshoot..     Doing S turns is much more complex, pitching up or down trades altitude for airspeed and changes your glide ratio,  so makes the math harder.  

Maybe fly the whole subsonic approach at 2 degree AoA,  that way you only need to know your glide ratio at this angle,  and since this gives plenty of energy to flare for a soft touchdown,  but not so much that it touches down so far into the runway it can't stop?

 

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17 minutes ago, AeroGav said:

As far as i know the glide ratio of an airplane is equal to its lift : drag ratio,  something that is continually displayed in the AeroData GUI (Alt F12) menu - don't know if you have access to that.

I have access to both lift and drag numbers.

At the time when I first asked, I was sure that L/D would somehow lead to a good answer, but alas, L/D varies with AoA. So I'd have to pitch up and down a little in order to get at good numbers, something I'm loath to do. I'd have to transition into and out of that maneuver, and probably would need to remain in neutral roll for as long as it takes.

Shortcut needed.

One of the design goals was that the script should be rather generic, so I wouldn't have to update it every time I come with another vessel; or at the very least, it should be able to handle this particular vessel whether it carries ballast or not. So I'm looking for something that I can do within the script (rather than doing test flights and changing the code together with the vessel).

37 minutes ago, AeroGav said:

On a ship with wings attached flat to fuselage the optimum glide rate is about 3 deg aoa at 200m/s and closer to 2 at low speed (under 100). 

Oh yes, I occasionally see you posting plots of L/D vs AoA and airspeed. Where did you get that data? Could I extrapolate from the known current values for AoA and LD?

43 minutes ago, AeroGav said:

Doing S turns is much more complex, pitching up or down trades altitude for airspeed and changes your glide ratio,  so makes the math harder. 

No need to tell me :)
If I could, I'd just be doing S-turns until shortly before the runway. As it is, I need a 10-20km straight run to get things lined up and settled down, and if I take an extra round, that more than doubles the distance.

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20 hours ago, Laie said:

h yes, I occasionally see you posting plots of L/D vs AoA and airspeed. Where did you get that data? Could I extrapolate from the known current values for AoA and LD?

I may have posted aoa vs l/d for wing parts graph, but this won't be relevant to the complete craft - that will depend on how much fuselage drag it has.

What i would do  then,  once AI has transitioned from re-entry to "landing mode",  is pitch up and down to maintain a constant AoADo some test flights to  find out what the lift drag ratio of your craft is at that AoA.         If the distance to runway vs altitude calculations show an overshoot,    trigger the airbrakes but don't change pitch or heading.

If flying a constant AoA glide,  at the beginning you are going to be fast and high in thin air ,   over time the speed will decrease and the air get thicker at lower altitudes.  As the aircraft gets further and further from the sound barrier, lift to drag ratio will improve.     So , what looks like an "on glidescope" condition at 240 m/s will gradually turn into an overshoot as you get nearer the runway, but you can remove that with airbrakes,   unlike an undershoot..

At the last 50 feet above ground level, switch to flare mode for  touchdown.

 

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