Mishandling a forward slip

[Colonel Sanders]

I don't want Colonel Sanders to make fun of my theoretical post

Ok, but do you really care what someone thinks,
if their favorite food is Kentucky Fried Chicken?

I mean, for all you know I have the complete box
set (in both DVD and Blu-Ray) of all seasons of
Honey Boo-Boo on the mantlepiece.

[tkdowell]

I don't want Colonel Sanders to make fun of my theoretical post

Ok, but do you really care what someone thinks,
if their favorite food is Kentucky Fried Chicken?

I mean, for all you know I have the complete box
set (in both DVD and Blu-Ray) of all seasons of
Honey Boo-Boo on the mantlepiece.

Hahaha maybe not THAT Colonol Sandler's opinion, but yours, yes, I certainly do.

Do you really???

[AuxBatOn]

How's that? If the aircraft doing exactly what I'm asking it to do - there is no loss of control, aistream separation over wings or control surfaces, how's it is in a stall? Remember, we are not asking the wings to provide much lift in a slip, hence the angle of attack is shallow, despite the optics.

Example - when you attempt to maintain your altitude, and stall the wing(s) - it looses lift and altitude. You lost control of it minimally in vertical axis until you get it back flying again. That's a stall.

I operate well beyong stall AoA sometimes, but still have nose authority (in the 45-60 AoA range). Am I stalled? You bet.

[trampbike]

We could even play semantic and say that drag forces pointing upward are part of the total lift couldn't we?

Only if we wanted to be wrong. It's non-trivial to point out that being semantically correct is not the same as being trivial. It's also both semantically correct and non-trivial to point out that lift acts at right angles to drag, so drag forces aren't in any way ever part of the lift, be it total, partial or otherwise-qualified.

If we wanted to be correct, we could say that the vertical component of the drag force supports part of the weight of the aircraft.

I get your point, but still think it's more semantics than anything else. Wouldn't you say that an airplane flying knife edge at a constant altitude is experiencing lift (although not from the wings)? That lift pointing up holding the aircraft weight is basically the same force that slows and airplane when slipping, only in the latter case its orientation makes us call it drag. Aerodynamic lift and drag are physically pretty much the same thing, it only depends on your reference system...

As for pertinence: it's extremely pertinent in a discussion in which "how hard" the wing is working to generate lift is a relevant theme, to point out that the lift generated by the wing is in fact not equal to the whole weight of the aircraft. Even though all forces are in balance and the net acceleration is zero.

When some drag is opposing weight (as is almost always the case), I think it's fair to add it to the total lifting force experienced by the system.

[photofly]

You are, of course, quite right, in principle. But since we've already got a definition of lift, I think we should try very hard to be consistently unambiguous and use a different word for "the force that opposes that weight of the aircraft." It's amazing how much confusion is caused by just using words at cross purposes. It may be semantics, but it's important semantics.

[akoch]

Hey Akoch,
I am curious as to what you use to determine the aircraft's acceleration.
TD

Hi TD, no it is all good and good points.

I use ASI, there is no other instrument available to me. As I mentioned before, my static port is integrated into the body of the pitot assembly. So there is no appreciable ram air effect when side or forward slipping. At least it is identical on the pitot port and the static port cancelling each other since the ASI works on the pressure differential.

Probably due to the location and construction of the pitot tube assembly I don't get ASI reading change when rolling in and out of a slip. So no correction is required, and it is also mentioned in the PoH. I know it is different in some other aircraft I flew, but mine is easier that way.

[akoch]

Chalk and stick agree. You're not stalled. Akoch wasn't stalled.

Aircraft rotated in yaw (to the pilot's right) and bank (to the pilot's left). It still has its longitudinal axis horizontal. Now while descending towards the camera the wings are edge-on and it has zero angle of attack.

photo copy 2.JPG

Photoly, this is exactly the case.... Thanks!

[trampbike]

You are, of course, quite right, in principle. But since we've already got a definition of lift, I think we should try very hard to be consistently unambiguous and use a different word for "the force that opposes that weight of the aircraft." It's amazing how much confusion is caused by just using words at cross purposes. It may be semantics, but it's important semantics.

Then we are in total agreement. Would you happen to have graduated in physics?

Photoly, this is exactly the case.... Thanks!

However, in photofly second picture, the aircraft did not have its nose up at all. Your were saying yours was quite high above the horizon.

[akoch]

I think he has nose level on the first picture, and good 25 degrees nose up on his second one as far as I can tell.
But I know for sure that even slightly dropping the nose results in acceleration in my aircraft. It has to be well above horizon, the actual angle depends on how much bank you have and required speed control. I'm happy to go flying with you if you're ever around Pitt Meadows.

[photofly]

However, in photofly second picture, the aircraft did not have its nose up at all.

I think he has nose level on the first picture, and good 25 degrees nose up on his second one as far as I can tell.

I wasn't modelling Akoch's flight. I was just convincing myself that you could make a steep descent and not have a large AoA.

The nose is actually level with the horizon in both pictures. In the first picture it's facing the camera; in the second it's turned to camera-left (pilot-right) and then banked camera-right (pilot-left).

The point I was making to myself was that if the airplane was descending towards the camera lens in both cases, you could change the AoA from very high, to zero, by an appropriate combination of yaw and roll (and no pitch). If it can vary from high to zero, it can be anything in between also.

[Colonel Sanders]

I operate well beyong stall AoA sometimes, but still have nose authority (in the 45-60 AoA range). Am I stalled? You bet.

Yes, but you are flying a FBW fighter jet with two afterburners
which a replacement for (e.g. F-22) costs a third of a billion
dollars.

I'll take two please. Put them on my VISA.

[trampbike]

I wasn't modelling Akoch's flight. I was just convincing myself that you could make a steep descent and not have a large AoA.

I know, I was simply responding to akoch's "that is exactly the case". He mentionned earlier that the nose was pointing VERY high in the situation he described. I think however, it was an illusion caused by the slipping attitude. It easy to think that the nose was well up in your second picture, although we know it isn't.

[akoch]

The "very high" note was referring to the extreme example where I intentionally attempted to stall it in the slip. Stick full aft, what attitude would you expect to see all things considered? I did not measure it via any objective means, just by the look out of the window. And it was much higher angle than a typical power off stall, more alike of a 35%-power-on stall (which in my case is almost helicopter-like sensation).

In a normal slip, I bank to say 30 degrees and immediately following that with full top rudder, applying a bit of stick pressure to keep it from accelerating. Now, where the nose goes in this configuration? For all intends an purposes, for me it is up and optically it is above the normal approach attitude. It is substantial enough that it is not subjective. And I think that if photofly did not move the camera between shots, same is reflected in his second picture - the nose goes (relatively) up due to the yaw motion to the right.

[Beefitarian]

I operate well beyong stall AoA sometimes, but still have nose authority (in the 45-60 AoA range). Am I stalled? You bet.

Yes, but you are flying a FBW fighter jet with two afterburners.

I'll take two please. Put them on my VISA.

A year ago I'd be excited and ask for a ride but now I know that's as likely as getting some good Chicken from a Kentucky Fried franchise because there's no such thing.

So in conclusion, what's FBW?

[trampbike]

Fly-by-wire

[Beefitarian]

I was miss informed. I thought the 18 was not. Thanks tramp.

[photofly]

Stick full aft, what attitude would you expect to see all things considered?

If you grab a model aircraft and hold it up in the air in front of you you can get a good feel for what's going on.

Take the line between the model and your eyes as the flight path, i.e. it's moving directly towards you. The AoA is then a function of how much of the underneath of the wing you can see.

Raising the nose means you can see more of the underside of the wing, which means you've increased the AoA. On the other hand, yaw to one side and roll to the other, and you see less of the wing, which means you've decreased the AoA.

Now you can play one effect off against the other, and achieve any AoA you want with any nose-up angle you want, by applying sufficient (yaw/roll) sideslip. It's really worth getting a model out to see how that works.

In real life the limits are going to be rudder authority and elevator authority. As far as the model goes, you can achieve zero angle of attack with the nose pointing straight up, by rolling the aircraft (around the now-vertical axis) so you can see only the edge of the wings.

So I'm now not surprised you could have the nose way up in the air; it makes perfect sense.

[akoch]

This make sense. And I would add that we are still asking the wing to provide some lift - it is not a free-fall, just an aggressive descent where lift is not equal to the weight of the plane but still enough to arrest free-fall and provide for a controlled descent which we modulate with the elevator pressure (assuming constant bank and yaw angles for simplicity).

[Expat]

This tread is really too technical for me. A forward slip for me is having the right hand on the throttle, the left one on the yoke, the two feet acting intelligently, and the eyeballs on the runway button. The rest is just simple coordination. It is like stopping your car on the white line at a red light. It just happens...

Dropping jumpers is the best way to practice your slips! Nothing beats slipping from 14000 ft to beat the jumpers to the tarmac!

[akoch]

LOL

That's right! But when you are starting out, you do have all kinds of weird questions. Since realistically you are concerned that you're doing something really wrong (no experience yet to draw from). I guess it is just curiosity and certain healthy fear. When I understand what is going on, I find it is a lot easier to deal with it.

[trampbike]

This make sense. And I would add that we are still asking the wing to provide some lift - it is not a free-fall, just an aggressive descent where lift is not equal to the weight of the plane but still enough to arrest free-fall and provide for a controlled descent which we modulate with the elevator pressure (assuming constant bank and yaw angles for simplicity).

In photofly second picture, the fuselage would be the one providing pretty much all of the lift. And yeah, unless you are accelerating, the total lift of the aircraft is equal to the weight, even if you are descending.

Apologies for jumping on you previously when you were saying that in a slip you are not asking much from the wings as far as lift production goes. You were right (the wings could even be at 0 AoA), but for the wrong reason: it's not because you are descending, it's because the fuselage produces a substantial amount of lift.

Great topic btw.

I was miss informed. I thought the 18 was not. Thanks tramp.

I'd be curious to know if there is any fighter design that got into production after the F-16 that is not FBW. AuxBatOn?

[photofly]

it's because the fuselage produces a substantial amount of lift.

I still think we're being wishy-washy by using the word "lift" when we should be saying "vertical force" or other words with the same meaning.

[Beefitarian]

Great topic btw.

I was miss informed. I thought the 18 was not. Thanks tramp.

I'd be curious to know if there is any fighter design that got into production after the F-16 that is not FBW. AuxBatOn?

Agreed on topic rating. I've been thinking the F-18, 15 and 14 all were some sort of mechanical/hydrolic system for control surfaces.

Single engine not withstanding I like the 16 better than the 18. Yeah I know it's moot/fantasy not having driven either but...

[akoch]

May I say a blasphemy then? The one we were taught in grade nine school, perhaps physics is different on this side of the pond

The weight of the aircraft (as any other object) does not change, given we are staying around (close to) the planet Earth. W=mg, where m and g are both constants in our case.

The lift change all the time. We have excess lift that make the plane go up, or insufficient lift to keep it at the same altitude. It descents. Lift equal weight = aircraft does not change altitude.

The delta between the gravitational force and the lift generated (vertical force counteracting the gravitational force) by the wings is responsible for the vertical movement of the aircraft.

Really, this is just the basic second law. In a slip you descent because the lift provided is less than required to keep it at constant altitude.

Am I crazy?

[trampbike]

Am I crazy?

Nope, but wrong, here is why:

We have excess lift that make the plane go up, or insufficient lift to keep it at the same altitude. It descents. Lift equal weight = aircraft does not change altitude.

This is only true while you are accelerating either upward or downward. Once in a steady climb or descent, you are not accelerating aren't you? Therefore, all the forces acting upon the system are cancelling each other. (Sum of all forces) = mass*acceleration

What happens when you pull on the stick? You are not asking your airplane to climb, you are saying "I want a higher AoA". Inititally this higher AoA will produce an excess of lift, hence making you accelerate upward. This higher AoA brings more drag with it thought, and also part of the weight vector now points opposite to where you are going. The airplane will therefore stabilize in a steady climb, but guess what, it will be moving slower than when you were straight and level.
You don't continue to climb because you have excess lift, you climb because you have more power output than what would be necessary for a straight and level flight at this speed and AoA.

We could also climb by generating more power but keeping the AoA steady. What will happen? Initially, the airplane will accelerate forward, but since you keep the AoA constant, it will experience an excess of lift, hence accelerating you upward. What happens then? Same poutine as with the first example, only now you will stabilize in a steady climb, at the same speed and AoA as what you had in straight and level before you pushed the throttle.

Little note: since most SE light aircraft have the elevator in the slipstream of the prop (which make the elevators more effective), what I described above is not exactly true, pushing the throttle while not moving the stick will make you stabilize at a SLOWER airspeed than what it was, and pulling the throttle will make you descent faster than supposed... An AoA indicator would then be a better tool than just trying to not move the stick!


Keep this in mind akoch: you climb/descent because you have too much/not enough power for your speed/AoA.
You accelerate (in any direction) because there is a force imbalance. As soon as you are steady, there are no forces imbalances.

Power really controls your altitude while the elevators control your AoA/speed.