Lets start over...a directionally stable airplane is flying straight and level. It gets shot with a gust of crosswind from the left (pilots view). this plane reacts with a negative yaw rate (pointing it toward the wind gust). Some of the propulsive force counters the crosswinds ability to take the aircraft off ground track. the gust then relents. The airplane then reacts with a positive yaw rate. it resumes its initial heading.

a near unstable (again directionally) airplane is flying strate and level. It gets shot with a gust of crosswind from the left (pilots view). this plane reacts with a right translation and little angular rate. the gust then relents. The airplane then stops translating. it maintains its initial heading...but has gotten off track.

a directionally unstable airplane is flying strate and level. It gets shot with a gust of crosswind from the left (pilots view). this plane reacts with a positive yaw rate (pointing away from the gust) which results in higher beta, which results in higher positive yaw rate........spin.

Which airplane would you want if you wanted to maintain a ground track?


do you want to rephrase this?

Woohoo! All these aerodynamicists here it boggles my mind. Is there someone here who actually does the theoretical and practical work for a living or did everyone just get their information from the 'net and from books? This has become a little bit hot but it would now make sense to get someone who is actually an authority on this...especially on low R regimes. It's all great that on one hand we have people who have practical experience but we also have people who have the theoretical know-how. Now, why don't we do a "real-world" experiment and use one of the planes mentioned here already like that Fazer with the SFGs. One look at it and it seems to be more modified than just that. Why not use actual numbers from that plane? I went for aeronautical engineering and truthfully, there are still a lot of things we don't know out there because in "real world (read:economics)" applications, there's no need to give a great amount of R&D like SFGs.

B

That's smart! Lets get DARPA or AFRL or NASA to fund us to flight test these things. I will supply the intertial avionics, and actuator position sensors, telemetry and software to spit out stability derivatives. Someone else can send me their airplane. I would prefer a 150 cc YAK. We will move the c.g around, put on SF generators, etc. This will cost about $50K total including salaries for 3. Or, we can just use a calculator and estimate using our 100 yrs of flying knowledge. I'll do directional trade studies for $1K

Red, in the second case I suspect the pilot would have their hands full just trying to stay in the air, gusts or no. In the third case the aircraft would have lost it and crashed long before a gust had a chance to catch it off guard. The instability you describe would affect the aircraft all the time. Only the first example and near cases of the second example will fly without an augmentation computer on board.

But you're right. Reactions to gusts are the only times an AIRBORNE plane sees any sort of sidewind. But your reaction description misses one point. The plane will yaw in response to the "new" wind direction and as well it will accellerate until it is again in a balanced wind head on mode. When it flys out of the gust it treats the previous "steady state" as a NEW wind gust from the opposite direction. The actual yawing and any sideways acceleration ensures that it becomes one with whatever the momentary wind is doing in short order. It'll always work to restore it's balance and trimmed flying state in whatever air it is currently flying within.

Thanks for explaining singlet. That was pretty simple after all.

Re-stating: A crab angle is an angle that a pilot flies in order to acheive a desired ground track. It is systematically set by the pilot. Variations in crosswind component will of course necessitate variations in the crab angle. This is most easily seen immediately after rotation in a crosswind, and the reverse on landing. It goes without saying that crab angle is an integral feature of deduced-reckoning navigation, and any other form of course tracking with a crosswind.

From the FAA's Airplane Flying Handbook:
"CRAB—A flight condition in which
the nose of the airplane is pointed into
the wind a sufficient amount to counteract
a crosswind and maintain a
desired track over the ground."

Having written all that and possibly annoyed you, I understand that a change in crosswind component can result in a change in airplane heading proportional to the degree of yaw stability possessed by the airplane and the gust gradient. So, if you mean to argue that a change in crosswind necessitates a change in crab angle, I would have to agree, but we would not then be beyond the most basic of aeronautics, and we are NOT discussing stability yet. If on the other hand you want to link the airplane's crab angle, rather than it's sideslip angle, with it's response to a perturbation in yaw, I must strenously qualify that. Sideslip angle, and not crab, is a term in yaw stability calculation. Crab angle does not enter at all, though of course at any point in a yaw disturbance, the instantaneous crab angle could be calculated, if a crosswind existed at the time, though I don't understand what use that would be.

I think I understand your intent in the examples you gave (the parts I snipped). However, without placing a bunch of additional constraints on your final example, it cannot be said with certainty that the airplane will spin, since as far as we know, aurotation due to spinning requires at least a partial stall. Certainly, if no correction is applied, AND a stall ensued, a spin would likely occur, i.e. it depends on a lot of things that were not specified in your example. Departure due to yaw instability is not the same thing as spinning, especially not initially.

If you insist, I will allow the last sentence in the fourth paragraph to serve as a qualification to my statement about crab angles. Otherwise it stands un-rephrased.

<oops, rephrased first sentence of fourth paragraph>

[sm=tongue.gif]

Hold on! I want the YAK. Then again, maybe Selig and friends have already found the answers we're looking for. No need to get the gov't involved in this. We'll just be mired in red tape anyway.

Looks like we are having trouble with words here
a neutral plane keeps going where it is pointed - if anything - a gust or input changes it's attitude or direction -it then keeps going that new way - subject to speed change from attitude. (Newton ,Wayne- 3rd law)
Ground tracking related to stability - that is a new one on me --I see no relationship
Maybe you do -I don't see why.
apparantly your ideas all relate to tracking as viewed from a fixed ground point. N/Y ?
That is what I see based on your post 18 - frankly I am lost in what you are trying to describe
but then I have been lost for a helluva long time

I am about 6 posts behind -and now I see what Red was trying to describe:
Ground tracking and gust interferrence
BTW I have a 42% EXTRA model which I fly along with a bunch of other sizes down to 4 ounce foamies - And I really can't see where input from any scientific body will change how these react -or are percieved to react
most of it is obvious.
Has been for years
no studies needed except one's own desire to do things in an orderly manner and pay attention

No, I didn't miss this point, I just though I had already over simplified this duscussion. Guess we'll just keep simplifying. Yes, your correct that the second two airplanes need augmentation...again, tried to set up the most painfully simple case. Thanks for the added info for our veiwers though.

Mesae, Just because we learn how to calculate stability derivatives using terms like c_n_beta, doesn't mean that the ability or tendency to crab is not an effect of some stability characterisics. I'll ask you this, is crabbing a good thing? (you should answer..yes, it allows the airplane to maintain a ground track in the presence of a crosswind, see... its right here in my faa handbook) then I'll ask you this, Does yaw stability effect the airplane's natural tendency to reduce sideslip angle (you say yes again). Does this reduction in sideslip angle result in a crab angle? (you say yes, if there is a crosswind component) Then I say, doesn't this imply that yaw stability does have a relationship with crab angle, and thus staying on a ground track???? Then I say, that will be $50 for that lesson.

no one said crab was used to calculate yaw stability parameters. crab is certainly related to flight path and heading...I don't think this topic is called "give us a bunch of definitions"

please don't go here. spin, crash, uncontrollable...you are smart enough to get the idea without venturing onto some other topic.

Again, I think the question still lingers, which airplane would you rather have (all other variable the same[this ones for you guys]) to maintain a ground track (say...for crop dusting). Which robust controller would be simplest to design (this one's for those of you talking flight controllers)? I'm not looking for definitions of crop dusting or various autopilot schemes here. I will not give up until someone states that SFG's reduce an airplanes ability to follow a ground track, or gives an adequate response to why they do or have no effect!

You have made all of this too hard for yourself
look at sfg's as simply increased lateral area -at CG-
as to following a given ground track - NO cause /effect relationship

Let the pilot of the Fazer tell you how it flies differently
I have flown this model for 6 years. I know it well. Last year I added some fuse height from the canopy to the tail. It helped a little on knife edge flight by allowing it to fly a little slower. I have just flown several flights on a calm day, trying to do knife edge loops. I was sucessful, but with difficulty. On a clockwise KE Loop, it is slow to climb the left side of the leg, it almost pivots over the top, so I have to let up on the rudder almost to straight, then re-apply it on the downward right hand side of the leg, but when I do, it rolls left and gets unstable.
When I do the counterclockwise KE loop, it climbs nicely, but tucks to belly so much on the left side downward leg, that I cannot put enough elevator in it to keep it from pitching out of the loop.
Kife edge passes take a lot of correction, both wanting to pitch to belly.
I did some flat turns, even flat turns with a little outside roll held. They were impressive. Had to take it slowly. Flat spins are almost unchanged.
I wonder if the rudder and fuse need to have as much area below the centerline as above. If so, more mods are on the way.
It seems to have a wee more slow speed lift, and has less wing rocking at high angles of attack. I may put winglets on the wingtips, and reduce the size of the SFG s just a little. I like how the air does not flow along the wing length as much now. I think I will just enlarge the rib shape at the ends of the wing and stick them on the end. It may look cool, too
When I say tracking is crisper, I mean that the plane tends to fly in the direction the nose is pointed more precisely. There is not as much sloppily sliding through the air, but more like it is on rails. I think I will move the weight a bit more aft, so the rolls will be automatically axial.
Overall a big different in how it flies. There will be some new manuevers in the repitiore now.
Pattern flyer turned 3 D (patD)
[img][/img]

SFGs, like additional fuselage side area (because that's all they really are, except for their additional function of inhibiting spanwise flow, which has little or no effect on lateral stability or maneuvering by itself), have an unknown effect on the ability to follow a given ground track, unless more is known about the SFGs.

And here are the simplified, non-engineering qualifications: If the AC of the SFGs is in line with the airplane's CG, there will be no effect on lateral stability, though maximum lateral lift will be increased. If the AC of the SFGs is aft of the airplane's CG, they will increase yaw stability and partially reduce the amount of extra lateral lift available. Presumably this is self-defeating unless the additional stability is desired for some reason. If the AC of the SFGs is forward of the airplane's CG, they will decrease yaw stability, which may or may not be desirable, depending on the degree of yaw stability shown by the airplane before installation of SFGs, and pilot preference.

One cannot simply say SFGs increase or decrease lateral stability without considering their geometry and placement, any more than you can say that increasing fuselage side area increases or decreases lateral stability without specifying how and where the additional area is distributed.

It is well known in aerodynamics (and intuitive) that cowlings and other lateral area ahead of the CG are destabilizing in yaw, and area added aft of the CG is stabilizing.

<reduced troll-factor>

A classic case of where a little observation is worth a library of formula

*bows*