Flap question for aerodynamics geeks

[Typhoon1244]

The CRJ-200's flap positions are labeled UP, 8, 20, 30, and 45. However, I'll bet that if I went out there with a protractor, I'd find that none of the surfaces really move to eight degrees, twenty degrees, etc. I'm assuming that these are these are theoretical numbers, but does anyone know what they're based on?

 

[SabreFlyR]

The 0 point is usually spelled out in the maintenance manual. Most cases 0 is inline with the wing chord, sometimes it's not, Sabreliner 40 for example. Then the limit switches are set from this point. The type certificate data sheet and maintenance manual for the plane will tell what the maximum limits are and the allowable tolerence.

Of couse it is all theory if the tech's don't set it up right.

 

[Super 80]

On the 727, the flap position was measured on the middle surface. I'd say take a level and a protractor and try to see which one they used. Being aerodynamic engineers, they have to have some factual foundation for the labels.

 

[profile]

The flap positions do not necessarily corrospond to any number more than roughly, if that. In fact, Airbus finally recognized that fact and changed the settings to just "1, 2, 3", or something similar. It is up to the manufacturer on whether the flap settings in the quadrant actually represent real numbers out there on the wing, but the actual numbers should be spelled out in the mx manual.

 

[Super 80]

In fact, Airbus finally recognized that fact and changed the settings to just "1, 2, 3", or something similar.

FRENCH!

 

[TZRav8r]

QUOTE]The CRJ-200's flap positions are labeled UP, 8, 20, 30, and 45. However, I'll bet that if I went out there with a protractor, I'd find that none of the surfaces really move to eight degrees, twenty degrees, etc. I'm assuming that these are these are theoretical numbers, but does anyone know what they're based on?[/QUOTE]

I would be willing to bet that they are in fact angles. The MX manual would tell you which flap section, what locaction on the flap and from what reference to measure from. Ask one of your wrenches.

 

[EagleRJ]

Typhoon, man- you have too much time on your hands!

I'll bet the aeronautical engineers don't get really precise when they're picking flap settings. They probably say "and let's put another flap notch about...here". The flap setting ends up getting called whatever angle it is closest to.

I've wondered why some planes have flap settings that are negative, or slightly above the normal retracted position that is faired with the rest of the airfoil. I think either the 707 or DC-8 have a -1 or -2 flap setting. I know it's for high-mach flight, but aerodynamically, I don't know why it's necessary. Anyone?

 

[enigma]

I'll bet the aeronautical engineers don't get really precise when they're picking flap settings. They probably say "and let's put another flap notch about...here". The flap setting ends up getting called whatever angle it is closest to.

I can't agree. Engineers are so precise that you might say most of them are anal. I'm not an engineer but I did sleep in a Holiday Inn Express recently and I can tell you that flap slat settings are precisely calculated for optimum performance. On the MD80 series, and some other Douglas Commercial birds, the flap setting is infinitely variable. This function, called "dial-a-flap" is utilized to optimumize take off performance. Some companies, AA for example utilize this funcion for every take off in order to utilize reduced thrust every time therefore saving engines. The "dial-a-flap" control wheel allows you to set degrees, and in between. The analysis charts for "dial-a-flap" take offs give the flap setting down to the tenth of a degree. If the flaps/slats are not set precisely as required take off performance will not be up to requirements.
The flap indicators in most airplanes my not be precise, but the flap settings they represent most certainly are precise. Your behind depends upon such precision. You might think that I'm being overly dramatic, but if you've ever rolled for 11000 feet at DEN, or LAS, you will appreciate my position. When you rotate right at nose tire limit speed, there is no room for error.


regards,
enigma

BTW, has anyone else ever accomplished a MD80 zero flap take off? Max pucker factor there man.

 

[avbug]

Control deflections are all part of the most basic type certification data, and they are exact.

Most of what you see in the cockpit isn't necessarily a true, or accurate reflection of what is actually going on, weather that be ITT, control angular deflections, etc. Pilots like to think so, but it's often not the case.

I couldn't tell you weather the published numbers on the flap control on the CRJ-200 directly correspond to the flap angular deflection in degrees, but I would suspect that they do.

The actual flap settings in degrees will be exactly established, and the aircraft will be maintained to that standard. It's part of the basic rigging process.

You can find out very easily be referencing the maintenance manual.

 

[A Squared]

Typhoon,

I'm with the others, if your flap units are degrees, there is almost certainly a place on your flaps where that angle could be measured.... if your flap controls are graduated in some unspecified units, it's anybodies guess what the units are ... but you can be that somewhere in the Mx instructions there are specs for how many degrees of deflection per each "unit".

If you're really curious and have time at an outstation, get one of those protractors with a weighted pointer. (you can get them at hardware stores) place it on a the surface of the flap with the flaps up. Note the angle indicated by the pointer. Extend the flaps to say flaps 30 and measure in the same spot. Chances are good that your second reading will be pretty close to 30 degrees from the first. Who would actually do that though .. right? ummmm, welll ... a story.. first, I come from an engineering background, so enigma's comments may have some relevance if that explains anything. Anyway; A while back one of our planes would seemingly come to a screeching halt when you selected approach flaps, in order to maintain approach speed, you had to grab a whole handful of power. In short, it acted like you had landing flaps when approach flaps were selected. welll, I'd gotten burned by that a couple of times (slow learner) so I got curious. I brought my protractor one day. Out at one of our outstations I measured flap extension angles. I found that, exactly as we'd suspected, selecting approach flaps got you landing flaps. I passed this information on the MX, which earned me a very dirty look when it became apparent that I'd been measuring things on the airplane .... anyway the flaps did get re-rigged and so the problem was solved

 

[Super 80]

AA "Dial a flap" Flap Settings are as follows:

-217 engines: 3, 4, 6, 8, 11, 15, 17, 20, 24

-219 engines: 4, 6, 8, 11, 15, 17, 24

Furthermore, not every flap setting may be available in the data base for an airport and if so, it is not available for use.

In addition, for contaminated runways (water, slush, snow or icy) only flaps 11 and 17 are allowable if authorized.

Landing flaps are either 28 or 40.

Flap settings between 13 and 15 are prohibited (slats transition between mid and fully extended at 14).

The tolerance on the gauge is 1 degree but I can't find the reference right now.

I think the angle measures the mean aerodynamic chordline of the flap to the wing.

P.S.

They used to have an optimal flap program where you inserted 99 into the ACARS to call up a new TPS which would give you the flap setting that had the most leeway from the runway and climb limits, but as far as I know, it's not in use anymore.

Standard flap setting for T/O is 11.

All this information only applies to American Airlines under our Ops Specs. Nothing here should be construed as legal for any other operator to do.

 

[avbug]

Us mechanics are too dense to figure out things like MAC and chordlines. The mx manual will call out a reference, and the flaps will be measured using a protractor at a given point.

Flaps set electronically in a fly by wire system are a database and potentiometer function. The beauty of that is that we don't need to think. Just do. If the potentiometer works, and the linkage isn't broken or worn, then it's a computer nerd's problem.

Your flight training manuals should spell out the actual flap deflections. If not, then the mx manuals certainly do.

 

[enigma]

AA "Dial a flap" Flap Settings are as follows:

-217 engines: 3, 4, 6, 8, 11, 15, 17, 20, 24

-219 engines: 4, 6, 8, 11, 15, 17, 24

Furthermore, not every flap setting may be available in the data base for an airport and if so, it is not available for use.

In addition, for contaminated runways (water, slush, snow or icy) only flaps 11 and 17 are allowable if authorized.

Super80, It's been a long time since I rode an AA jumpseat. Sorry to infer something about your ops. I thought/remembered that your flap setting came along with the ACARS numbers and that it was infinitely variable according to the situation/flex power.

We have airport analysis books that will give optimum flap settings, although we have to get the "true"numbers from dispatch. I've had to use flap settings like 1.7, etc.

Thanks for the info,
enigma

 

[profile]

The MD-11 and -10s have dial-a-flap as well, and our cockpit performance computer can pick anything from 10-25 degrees. We will often see flap settings such as 14.7, etc. The MD-80 must be the same, the limit is not the aircraft but your company's performance computer/data.

 

[uavchaser]

Hey Eagle,

About the negative flap settings. I have flown a couple of different a/c other than the ones you mentioned. The reflexing of the flaps decambers the wing. This unloads the wings to achieve higher speed without the added stress on the airframe. This also brings the pitch angle (nose) up depending on how much flap reflex is applied as long as AOA remains constant. The level body angle allows higher speed in addition to unloading the wings. Correct me if I'm wrong.

 

[Super 80]

Sorry to infer something about your ops. I thought/remembered that your flap setting came along with the ACARS numbers and that it was infinitely variable according to the situation/flex power.

No offense taken, I just wanted to state what the AA procedures for the Super 80 were since I'm current on it. You did pretty good for just sitting on the jumpseat and that's probably been some time with 9-11 and all!

The MD-80 must be the same, the limit is not the aircraft but your company's performance computer/data.

The MD-10 and 11 were actually modern aircraft and bear no resemblance to the ancient and antiquated Rube Goldberg device the 1950's control tab flying ice machine that the Super 80 is.

 

[profile]

The dial--a-flap is the same, though. Is it not just a knob that you roll to various settings? That was my impression, if it has set slotted positions on it, then it is different.