First completely unpowered landing

[unknown_orbiter]

What I find so funny about this is that I had gone as far as doing interplanetary travel before actually trying to do this. I landed a bit hard at -5 m/s, but no wheel collapse or injuries so I'm happy. The thing that irks me about reentry and landing is the 3 out of 5 times, when the reentry autopilot disengages over KSC or Wideawake, I can not pitch up or keep any velocity... it is very odd because I just am dropping strait down and even with RCS and Elevon enabled, I can't do it. However the rest of the time, I'll come in and have a nice amount of speed to do an unpowered landing on the runway. Anyone know why this is?

 

[markl316]

Don't worry. I was the same way; deadstick reentry is the last real thing I learned how to do in orbiter. I also learned this after interplanetary travel.

Well, think of it this way. An angle of attack creates lift. There is a theory that an angle of attach is the only way to create lift (at least it's true in orbiter. Get into a delta glider on the runway, engage full main engines, and don't do anything else like pitch up, and see what happens).

There's only a certain amount of lift that can be created. Also, with this angle of attack comes much more drag. When flying at low speed, you can increase the angle of attack pretty easily. At high speed, even a slight pitch up will increase the drag drastically (drag is based on the square of velocity, so 4 times as fast means 16x more drag), so even a small change in angle of attack creates a LOT of lift and a LOT of drag.

Now, why can't you go from a 0 degree angle of attack to a 40 degree angle of attack? Because remember what a reentry really is, it's a controlled stall. 40 degrees of angle of attack, there's no smooth airflow over the wings. It's very hard to disrupt that smooth flow of air at 0 ish degrees angle of attack and make it stall without going below stalling speed.

What ship are you flying? If it's a DGIV, then yes, it's going to drop like a rock. I suggest not disengaging the reentry autopilot until about 800 m/s, then just let the nose drop until you're flying normally.

I hope this answers your question, I'm not 100% sure what you're asking.

 

[Hielor]

...I can not pitch up or keep any velocity... it is very odd because I just am dropping strait down and even with RCS and Elevon enabled, I can't do it. However the rest of the time, I'll come in and have a nice amount of speed to do an unpowered landing on the runway. Anyone know why this is?

Sounds like you're stalled at that point, or close to it.

You'll want to push the nose *down* to increase speed.

 

[Arrowstar]

Strictly speaking, there are two kinds of lift: camber-induced lift and angle of attack-induced lift. The former comes about from any camber on the airfoil, and the latter comes about from pitching the airfoil/wing upwards. My guess is that the DeltaGlider simply doesn't have any camber, which is why you can move along the ground for ages without flying if you don't pitch up. Of course, I'm no expert on how Orbiter implements flight, so perhaps lift coefficient curves in the program go through 0 at zero angle of attack...

 

[markl316]

Strictly speaking, there are two kinds of lift: camber-induced lift and angle of attack-induced lift. The former comes about from any camber on the airfoil, and the latter comes about from pitching the airfoil/wing upwards. My guess is that the DeltaGlider simply doesn't have any camber, which is why you can move along the ground for ages without flying if you don't pitch up. Of course, I'm no expert on how Orbiter implements flight, so perhaps lift coefficient curves in the program go through 0 at zero angle of attack...

Well we're both correct...these are theories, and they're still not quite sure what truly produces lift. You can get lift from a flat plate in real life, however, provided that it is at some angle of attack.

 

[Arrowstar]

Well we're both correct...these are theories, and they're still not quite sure what truly produces lift. You can get lift from a flat plate in real life, however, provided that it is at some angle of attack.

Well, no. We do know what causes lift, and we can explain it by the use of Newton's Second Law or Bernoulli Principle. We know through experimental testing that camber on an airfoil causes lift at zero angle of attack, and we know that angle of attack itself causes lift to some extent. These things have all been mapped out, and while "lift" is not a universal law (though it stems from one), I would not venture to say that there are still multiple theories behind lift. It's fairly well understood in many applications.

 

[markl316]

Well, no. We do know what causes lift, and we can explain it by the use of Newton's Second Law or Bernoulli Principle. We know through experimental testing that camber on an airfoil causes lift at zero angle of attack, and we know that angle of attack itself causes lift to some extent. These things have all been mapped out, and while "lift" is not a universal law (though it stems from one), I would not venture to say that there are still multiple theories behind lift. It's fairly well understood in many applications.

My aerospace professor last semester, who is a graduate advisor and who calculated the reentry and landing trajectory for the pathfinder, said that while we know that camber causes lift, and so does angle of attack, we're still not quite sure as to the true reason. There's the theory of Newton's 3rd law (downwash propelles the plane up), as well as the pressure differential theory. It may be one, the other, or a combination of the two. Just saying what my prof said.

And interesting fact of the day: those who think that it's the air having to travel over the top of the wing faster to "keep up" with the air on the bottom of the wing, the air on top actually arrives at the back of the wing BEFORE the air below the wing.

 

[Arrowstar]

I have to disagree. Integrate the pressure distribution around an airfoil and tell me which way the resultant force vector is pointing.

 

[Hielor]

My guess is that the DeltaGlider simply doesn't have any camber, which is why you can move along the ground for ages without flying if you don't pitch up. Of course, I'm no expert on how Orbiter implements flight, so perhaps lift coefficient curves in the program go through 0 at zero angle of attack...

Actually, the reason the stock DG can move along the ground for ages without flying is that the neutral trim position results in a fairly strong nose-down tendency. You can see this if you turn on torque vectors on takeoff. With trim in the neutral position, the nose-down torque grows as you increase speed until you're limited by friction and drag. If you give it even a small amount of nose-up trim, it'll fly itself off the ground quite happily.

The DGIV and XR2 are the same way.

In short, it's a feature of the vessel, not the sim.

Slightly off-topic, this is probably why IRL checklists have "Trim - Set for Takeoff" on the before takeoff section

 

[Arrowstar]

As I don't fly the stock DG often, I wasn't aware. Thanks for the information!

 

[flytandem]

I've always enjoyed watching debates on,... is the lift from newtonian equal and opposite forces reaction, or bernoulli faster airflow slower airflow.

I can hang by my hands from a tree branch. What is keeping me from falling to the ground? One could say it's the strength of the wooden structure ( tree trunk and branch) with tension and compression sides of the wooden branch, or they could say the psi pressure on the inside surface of my fingers (that contact the bark of the branch) is greater than on the outside, or top surface of my fingers.

The newtonian explanation of lift is parallel to branch strength, and bernoulli is the pressure on fingers in the above illustration.

Neither way of looking at it is wrong. They are both needed to get a fuller understanding of what's happening.

If however one tries to enhance the bernoulli explanation of lift by first claiming equal transit times from leading edge to trailing edge then one has made an incorrect assumption.

In addition, I saw in a post above where it's claimed lift is from two types, angle of attack and camber. This is not "strick" but rather creative and shows a misunderstanding of what angle of attack is as far as what could be meant by zero angle of attack. Do not quantitatively measure angle of attack as a reference to the bottom surface. For that matter, the bottom surface could be cambered so what would you do then? What is correct is to raise or lower the angle of attack to find the "zero lift" angle of attack. In the case of a flat bottom surface and cambered top surface, it would have the bottom surface appearing negative at the zero lift AofA. And from the zero lift AofA one raises the angle for a quantitative measure of angle of attack.

There indeed are two things needed for lift. One is airspeed and the other is angle of attack. Roughly speaking for subsonic flight, aerodynamic forces vary with the square of the airspeed, and as far as AofA goes, a more detailed description is required that includes top surface airflow and stall and how it affects lift and drag.

 

[Arrowstar]

In addition, I saw in a post above where it's claimed lift is from two types, angle of attack and camber. This is not "strick" but rather creative and shows a misunderstanding of what angle of attack is as far as what could be meant by zero angle of attack. Do not quantitatively measure angle of attack as a reference to the bottom surface. For that matter, the bottom surface could be cambered so what would you do then? What is correct is to raise or lower the angle of attack to find the "zero lift" angle of attack. In the case of a flat bottom surface and cambered top surface, it would have the bottom surface appearing negative at the zero lift AofA. And from the zero lift AofA one raises the angle for a quantitative measure of angle of attack.

All I meant to imply was that, when one looks at the lift coefficient curve (versus angle of attack) for a cambered airfoil, one will see that even at zero angle of attack, the curve will not pass through zero lift at zero angle of attack. (To see this, take a look at a Cl-alpha curve for my favorite airfoil, the S-1223, page 2 of this document.) This excess lift is camber-induced. That being said, I can see how my saying there are two "types" of lift is misleading, as you are certainly right, there is only one type of lift, lift!

As someone with a fairly strong fluids background (for an undergraduate), I'm personally in the Bernoulli camp when it comes to explaining lift, but I am also aware that a perfectly acceptable explanation of lift may come about through other governing physical law. Interestingly enough, the whole debate is really moot, since one group uses Newton's Laws and the other uses Bernoulli's... which is derived from the Euler Equations, which is derived from Navier-Stokes, which is (partially, at least) derived from Newton's Second Law (N-S being an equation of motion and momentum)! :lol:

 

[Hielor]

As someone with a fairly strong fluids background (for an undergraduate), I'm personally in the Bernoulli camp when it comes to explaining lift, but I am also aware that a perfectly acceptable explanation of lift may come about through other governing physical law.

What about non-cambered or symmetrical airfoils?

 

[Arrowstar]

What about them? They have plenty of uses. The NACA 0012 (or was it the 0009... either way) seems to get a lot of use as a rear horizontal stabilizer, for example. All it means is that they don't produce lift without angle of attack, much like a thin flat plate.

 

[Hielor]

What about them? They have plenty of uses. The NACA 0012 (or was it the 0009... either way) seems to get a lot of use as a rear horizontal stabilizer, for example. All it means is that they don't produce lift without angle of attack, much like a thin flat plate.

<devil's advocate> I mean, Bernoulli's Principle kind of can't apply when the airfoil is symmetrical, right? So it's clearly AoA that matters... </devil's advocate>

 

[Arrowstar]

Explain to me why it can't apply? Bernoulli's principle is an applicable derivation of a governing physical law that allows us to explain lift. Angle of attack is... just the angle of the wing to the freestream. You're not even talking about the same thing.

 

[MeDiCS]

Returning to the main topic, I just did an unpowered landing myself (with the DGIV2), and it was pretty neat except for the final touchdown (-6m/s, ouch). I bled most energy only when near the base.