Critical angle of attack...
 

It would be neat if manufacturers gave a critical angle of attack for a specific weight. Any way of calculating it on a model plane?

RE: Critical angle of attack...
 

Critical AOA ? I have never heard of that term. The AOA is the same at ALL weights. It also is not possible to measure AOA on a model.

RE: Critical angle of attack...
 

Why is it not possible to measure AOA? It is the angle between the relative wind and chordline. Sure, it would only be a visual estimate, but iy can be noticed with varrying speeds.

You mentioned it is always the same. If by this you mean a wing always stalls at the same angle of attack, then you are right. This AOA is called critical angle of attack.

RE: Critical angle of attack...
 

HalH

Critical angle of attack is a standard term.

For starters go to:
http://www.alphatrainer.com/handouts...f_links/13.pdf

You can go much further than this source, the depth of your search is up to you.

RE: Critical angle of attack...
 

How could you possibly use the information?
Model flight is so transient that processing mechanical information distracts from flying the airplane.
"Feeling" where the plane is at the point where it shouldn't go any further is more important than knowing what that point might be, geometrically.
Relying on some external aid distracts from flying the airplane.
Experience is more important.
.
However, such outside aids have been used, full-scale. The USN had calibration marks on many of its aircraft. The Landing Signal Officer would observe these as the plane approached the carrier and adjust his commands to the pilot to get the plane at the correct attitude.. see the attached... Not airplanes had them, some were merely single stripes.
Later a light was placed inside the leading edge, with red-green-yellow bulbs.. Green was OK, red was too slow...The LSO would command the proper attitude based on these..
Impractical to use on a model... things occur too rapidly and the planes are too small for anything like this to work well.

RE: Critical angle of attack...
 

The only time where something like that could be used is if you have telemetry equipment inside the aircraft that could register such changes and feed it back to you on real time. For example all the research RPVs and amateur on-board camera installations. Otherwise I have to agree with everyone else, its irrelevant if the manufacturer says it stalls at 15 deg AOA or 18 deg.....by the time you realize what angle you are in the aircraft is already doing something else.

RE: Critical angle of attack...
 

Tall Paul,

As LSO's get more experienced, they tend to look much more at the "big picture" to determine aircraft AOA rather than the indicator lights (green=slow, amber="on-speed", red=fast). By looking at certain features on the airplane (for a Hornet: top of the canopy lined up with the top of the right vertical fin, for a Tomcat: nosewheel centered on the lower edge of the right intake), they can get a good idea if the airplane is fast or slow. A good "eyeball calibration" is much more reliable than the AOA system on the aircraft (especially for an airplane that had been "jousting" with a refueling basket). I had never seen the stripe technique before. Very interesting, but it probably wouldn't work so well with today's approach speeds. At night, every airplane has a unique lighting pattern that changes with attitude. The one time that the lighting system has an advantage is "in close" at night, where the LSO's perspective changes pretty rapidly.

The LSO's eyeball is obviously giving him pitch attitude and not AOA. The reason why it works is that airplanes fly a fixed geometric glideslope to the carrier (usually 3.5, 3.75, or 4.0 degrees). The higher glideslopes are normally used with higher wind over the deck, with the result that a given airframe usually has about the same rate of descent for every landing. As long as the rate of descent stays the same, a given pitch attitude always corresponds to the same AOA.

What does this have to do with flying RC airplanes?

1) For final approach, I think your eyeball makes a very reliable AOA indicator.

2) If you fly your approach with the same geometric glide slope on a windy day that you would use with no wind, your airplane would need to be at a higher pitch attitude in order to be at the same AOA. In practice, higher winds tend to be gusty, so it's probably a good idea to fly the approach a touch faster (lower AOA). In other words, you probably can't go to wrong by flying your final approach at the same pitch attitude regardless of wind.

3) If you don't need an AOA indicator for final approach could it be useful "up and away"? I don't think so, mostly because the control system on just about every RC airplane is irreversible. Meaning that if you release the controls, the position of the control surfaces is determined only by the trim settings (unlike most general aviation planes where the yolk and control surfaces are free to move around if you let go of the controls). Linear Aerodynamics suggests that an airplane with an irreversible control system always trims to a single AOA corresponding to the trim setting. So if you're trying to set a particular AOA, it's probably easier to adjust trim rather than try to follow a telemetered AOA indicator.

4) One case where I can see downlinked AOA being useful would be during dynamic maneuvering where you are trying to optimize turn performance (such as air combat).

I fly unpowered RC planes, so I may be completely off-base. I'm often wrong, so you won't hurt my feelings if you correct me again ;).

RE: Critical angle of attack...
 

You have to remember that the AOA is the angle between the relative wind and the chord line. That is not CRITICAL AOA though. Critical AOA is fixed into every wing. When you said that "critical AOA for different weights" is not correct. CRITICAL AOA is fixed and the pilot can not change it. A wing will reach its CRITICAL AOA at the same angle always, at ANY airspeed and any wing loading. In a wind tunnel design, wing A, B, C, etc. will reach the critical AOA at the same angle always, regardless of airspeed or weight. It is a design angle and fixed performance charasteristic built in and fixed into any wing. Critical AOA has nothing to do with weight or speed. But now take that same wing, attach it to an airplane and the wing now has weight to support. Its critical AOA will always be reached at the same angle as in a wind tunnel or on an airplane, but the wing can stall later/slower (at a slower airspeed) if it is loaded with less weight. Higher weight/wing loading increases stall speed and the wing stalls sooner/faster. But in design and actual performance, a wing always has the same CRITICAL AOA, period. An airfoil will handle differently with weight/loading. When in a coordinated turn maintaining altitude, say 60 degree bank, the plane weighs twice as much (2G turn) and the plane will stall at a higher airspeed because it cant support that weight at that particular AOA. It has nothing to do with critical AOA though.

Full scale planes do have published stall speeds for different weights because of this. Lighter means slower stall and heavier the plane will stall at faster airspeed. Its because the wing has to support the extra weight. An airplane will get its best climb performance when wings level, because a turn will increase the load factor on the wing which means the plane weighs more.

So CRITICAL AOA is fixed and can not change once a wing is built. However stall speeds can and do change with wing loading/weight.

If I take the Cherokee up alone and fly at 100 knots I will be able to maintain airspeed and altitude at a specific AOA. Now I put 850 pounds of people and bags in and if I want to fly at 100 knots and maintain altitude I will actually be flying the plane at a slightly higher AOA, because the wing needs more of an angle (more lift) to support the extra weight. It is noticeable too just by the look out the window and visual reference, as small GA aircraft do not have an AOA gauge.

Generally, most GA aircraft wings have a critical AOA of about 16-18 degrees. And that doesn't change if going 100 knots or 200, etc. Thats why the critical AOA can be exceeded in a dive, climb, level etc. regardless of airspeed.

RE: Critical angle of attack...
 

Shoe, there's quite a difference between a prop plane trapping and a jet. The sighting lines apparently were used early on with jets, but soon vanished.
And they were used sporadically on the prop planes, mostly early in WWII, and seldom seen in the latter parts, revived again after, to vanish once again.
Nowadays the plane has an alpha indicator in the pilot's field of view as well as the glide slope information from the Fresnel lens on the boat. And the LSO yakking at him. :)
I've seen glide slope indicators at non-military airfields at the approach thresholds, and one of our S-3 test pilots many years ago described what the USMC called the "POMOLA", Poor Man's Optical Landing Aid".... three flourescent ribbons arranged to show the pilot where the proper glide path was.
I simulated these on our analog video flight simulator at Rye Canyon for the S-3, and they worked quite well.
Did the Fresnel lens also, using a motor from a Brush recorder to move the "meatball" relative to the alignment lights as if it was responding to the plane's off-optimum position on the glide slope. Built up a scale model carrier mounted on a hinge which allowed the model to pitch like a real carrier.

RE: Critical angle of attack...
 

Tall Paul,

Do you have the original photo without the annotations? I know some former LSO's from VA-195 (now VFA-195) who might be interested in the photo (I doubt they would be able to guess what the hash marks were used for).

RE: Critical angle of attack...
 

.
Done!

RE: Critical angle of attack...
 

More sighting lines... Jets came in too quickly for them to be of much use, and they vanished from the fleet.

RE: Critical angle of attack...
 

The main reason I was interested in knowing the critical angle is so it would be easy to see how close your aircraft is to stall, particularly on final.

Lately, I don't pay to much attention to speed as i do to the aircrafts attitude and my power setting when on approach. i started off at altitude, and paid close attention to how high the nose was before it began to fall. i repeated the process with lower and lower flyby's.

now when there are winds, i can comfortably come in at a familiar angle of attack, even though my ground speed appears unnaturally slow. Even though the speed looks slow, i know there is plenty of airflow over my wings as i'm about to land.

i guess i was hoping that manufacturers would give the critical angle of attack to make the "feel out" process easier.

power + attitude = performance. Any instrument pilots out there w/ me? ;)

RE: Critical angle of attack...
 

Turn, Time, Twist, Throttle, Talk, Track[8D]

RE: Critical angle of attack...
 

I've been wondering about Critical AofA, too. The established wisdom is that it remains fixed for a given wing section. But, it seems to me that it may vary slightly according to the properties of the air, ie, its density, humidity, viscosity and temperature. Anyone out there know whether these or other factors influence the Critical A of A?

Springer

RE: Critical angle of attack...
 

No, none of those factors change critical AOA. It is fixed and does not change for all the reasons I mention in post #8. Critical AOA is the same fixed angle always and forever regardless of atmospheric conditions. That's because it is the angle at which the airflow over the wing separates into a turbulent flow which means the wing is no longer 'flying', whether it is attached to an airplane or in a wind tunnel.

RE: Critical angle of attack...
 

The AOA has a border of a dive or a stall.
You adjust it in the air with the elevator trim.
I dont see any means of calculating it in the air, other than eye sight. All the tech info is usless to you if you build a kit, and not needed.
Incidence is used to set the AOA when building, but not adjustable in the air.

RE: Critical angle of attack...
 

http://www.faatest.com/books/FLT/Cha...ackandLift.htm

RE: Critical angle of attack...
 

Hi,
We know about the six "T's"----does anybody know about the six "P's"??

RE: Critical angle of attack...
 

6 P's...oh do tell

RE: Critical angle of attack...
 

How bout

prior planning prevents piss poor performance.... :D

Happy New Year!!

Jim

RE: Critical angle of attack...
 

Like Shoestring said. The critical angle, properley known as the stall angle, is dependent only on the airfoil profile. But bear in mind that the airfoil profile is also affected by the construction technique so if a give TRUE profile stalls at, for sake of argument, 8 degrees AoA it's quite possible that covering sag, spar ridges and other elements of the structure will change that angle by a degree or three.

Weight and incidence angles have nothing to do with the stall angle of a given wing.

In the end it's up to you as the owner/pilot to become familiar with what your model looks like in the air and practice flying the model at the angle that you are searching for. No one had eyeballs calibrated in degrees so such info would be pretty much useless in any event.

The sideways topic hijacking into carrier operations actually turns out to be very pertinent to this subject. Note that the LSO's use the APPEARANCE of the approaching aircraft and EXPERIENCE of what that attitude means to determine if the plane is coming in hot or not. That is the same technique that we modellers need to develop. And since it changes with each model we fly it's important to approach that familiarization phase of a new model with care just like what we are. Primary test pilots that FEEL the aircraft for what works and what does not. Then stick that info into our memories for each time we fly that particular design. So in this regard a number on the plans would mean less than nothing to the effective flying of the model. Instead just more air time and a cautious approach to a new model will develop the look and feel.

RE: Critical angle of attack...
 

While aerodynamics pusists will cringe at the notion, the conditions you mention have a neglible effect on the crititical AOA in practice, so for all practical purposes, the critical AOA is fixed...

As a "gee-whiz" comment, one effect that does make a dfference is the scale of the airplane, The critical AOA varies with the Reynold's number (Rn) of the wing section. The Rn is proportional to the wing's chord length. A full size airplane wing has a much larger Rn than a 1/7 scale model of the same airplane, and its experimental wind tunnel data show it has a higher critical AOA. This is one of several reasons why we can't take a full scale NACA laminar flow airfoil from a P51 and simply scale it down for RC use. :D

Happy New Year!!

Jim

RE: Critical angle of attack...
 

I think there may be some confusion between crtitical AOA (CAOA) and stall speed. While the CAOA is fixed, the stall speed of the airplane is dependent on its gross weight.

When teaching full size flying, I introduce the critical AOA as the wing's max limit, and that in cruise, the pilot will generate whatever AOA is required to maintain altitude. The heavier the airplane, the higher the AOA to maintain level flight.

For example, let's look at two conditions in an airplane at 120 knot level cruise flight with an airfoil that has a max AOA of 15 degrees.

Case 1: For a gross weight of 3000 lb, assume the wing requires 5 deg AOA. We have a 10 degree "cushion" between cruise AOA and the 15 deg. CAOA.

Case 2: If we increase the gross weight to 5000 lb, the level flight AOA may be more like 7 degrees, leaving a "cushion" of 8 degrees before reaching the critical AOA.

If you reduce speed, you must increase AOA to maintain altitude. In case 1, we can increase AOA by 10 degrees before reaching the CAOA, allowing a lower "stall speed" than in case two, where we run out of AOA after only 8 degrees, resulting in a higher stall speed. [8D]

Similarly, if we bank an airplane in level flight, we use up some of the AOA "cushion" to maintain altitude in the turn... higher bank angle, more AOA required, less cushion, so the stall speed increases with bank angle also... :D

HTH

Jim

RE: Critical angle of attack...
 

Jimbbo. Good points there. But I can't help thinking that we've now got a semantics issue. I saw the critical angle as being the stall angle. Obvioulsy YOUR definition is more pertinent if that's what was meant in the original post. I guess we'll have to wait and see what it was that Flydenfeld had in mind.

It's all good info in any event.