CraigG

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This topic surfaced on another thread and the OP requested it be taken elsewhere.

My understanding is that crow, (when both ailerons deflect up) cause the ailerons to act as spoilers. Spoilers decrease lift and increase drag. On full scale aircraft, they’re used to slow down and/or increase the rate of decent, but normally not on approach/landing (until after touchdown).

With r/c jets, they seem to help the approach/landing on jets that do not slow down well and/or tend to float on landing. The KingCat is a good example. It’s a tradeoff between a slightly higher stall speed and a more manageable approach and landing. Apparently most guys feel they can do a better with crow than without it on the KC.

Not so much agreement about using crow on various other jets, such as the UB. Some guys say yes, some no. Just wondering what others have to say.

BTW, where does the term "crow" come from anyway?

Craig

Billy

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I thought when the ailerons were up slightly, it gave the wing some washout which made it a more controllable at lower speeds.

I use crow in my Viperjet too.

Billy

rhklenke

My Feedback: (24)

Good questions.

In that same vein, I've always heard that reflexing the ailerons up acted like "washout" and kept the wing from stalling at a given angle of attack. Thus, does adding crow to the ailerons also reduce the tendency for an aircraft to tip stall? Not that the Kingcat or UB need that as they already have washout built into the wing for that purpose, but if you had a plane that liked to tip stall, would adding crow to the ailerons help avoid tip stalls on approach?

I've always believed that the term crow came from the glider community where they use it all the time to help in spot landings. They deflect all of the surfaces up and down almost 90 degrees so that they can point the nose way down and "lawn dart" onto the spot without skidding across the ground. I also always thought that they got it from the way that a crow can spot land on a dime by spreading its feathers out, creating lots of drag to stop.

Bob

InboundLZ

My Feedback: (10)

My understanding is it serves two functions. It allows a lower approach speed (lower stall speed) and allows for a higher AOA. This has been my experience.

I believe "crow" is a term coined by the sailplane guys as they use it all the time in thier sailplaines/gliders as Bob indicated below....

highhorse

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Nah, the total lift available from the wing is degraded, actually requiring a higher approach speed. For the offsetting benefits which make crow worthwhile in some apps, see below.

highhorse

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Craig, your understanding is perfect. Some jets or jet/engine combo's just dont want to land without an undesirably flat approach because there's too much residual thrust/and or the airframe is low drag (flaps alone not providing enough) and/or has a low wing loading....The Eurosport is one jet that achieves this thru the use of a speed brake rather than crow. Others have big honkin gear doors just for this purpose.

An added benefit of crow, as some have pointed out, is that the effective washout at the tips makes for retained controlability if the inner portion of the wing begins to stall. The wing doesn't "let go" all at once or even worse, as is common in some planforms, at the tips first.

Also, crow is also used in some cases to add drag even when the idle approach path is reasonable, simply because the added drag then requires power during the approach. This is because (especially older) jet engines are slow to recover from idle, creating a lag in response when power might be MOST needed on final or during a go-around. Added drag in these cases, requiring some power to maintain a comfortable approach path, ensures that the engine is in a responsive rpm range and that the power is available without delay.

In full scale this need for "spooled angines" is achieved with fuselage mounted speed brakes (vs spoilers) deployed during approach (common among fighters) or with lots of flap drag designed into the wing (common among other types).

Regarding the origin of the term, I'm certain that it's original reference within the glider or some other community was to crows manipulating thier own wings in order to achieve spot landings and that the practice could just as well have been named after any number of other birds.

1320Fastback

In the sailplane world crow is the use of Ailerons and Flaps.

I have not seen flaps mentioned to be in use here but it's probably safe to say they are in use with jets.

If you don have flaps but reflex the ailerons up then this is called Spoilerons.

highhorse

My Feedback: (2)

Yup same reference in this thread, reflexed ailerons with flaps. Not just jets tho, first plane I saw using crow was a stick!

David Gladwin

I think you have that bit wrong Craig. Upset ailerons do NOT "spoil" and cause the airflow to break down as do proper flight spoilers. They, upset ailerons, reduce the mean camber of the aerofoil, retaining normal laminar flow thus reducing the CL for a given A of A.

On the VC10 (long since out of service with BA but still used by the RAF as tankers) we used aileron upset (both about 5 degrees up) at weights above 131 tons to reduce lift on the outer wing to provide wing bending relief once flaps and slats were retracted, the airflow remained, as always on the VC10, smooth as silk.

Other than that you and Highhorse have it spot on !

Regards,

siclick33

Does use of crow always increase drag? With the small deflections used I would have thought that it is possible that drag is actually reduced.

Dave Wilshere

On many aircraft the raised aileron will allow/cause the nose to raise (like the Classic Flash) and this adds drag. Different aircraft behave in different ways, some already have washout (tip TE higher than LE) and adding raised ailerons will reduce the lift severely (wing area/wing loading will affect how this affects!) Its a major thing to cover every possible affect/upside/downside.
Generally on models its done to add drag though.

Dw

CraigG

My Feedback: (40)

Correct David, true spoilers are hinged forward of the wing trailing edge and spoil the airflow rather than change the airfoil camber. The net result however, is similar (loss of lift and increased drag) to reflexed ailerons. That was my point but thanks for the clarification.

Craig

highhorse

My Feedback: (2)

Well, he didn't say they were spoilers or that they became spoilers. He said they act as spoilers. They reduce lift when reflexed. Close enough, I think. .

marquisvns

My understanding probably concurs with some of the above, and here it is:

Both ailerons deflected upwards at no more than 10 degrees, with or without deployment of take off flaps will help increasing the contrast of wing incident between inboard and outboard, therefore, an effect of washout is achieved to increase slower airspeed handling.

Both ailerons deflected upwards say 35 degrees or more, will work as crow as we know it to slow down an airplane, also with or without deployment of takeoff or landing flaps. But do anticipate the plane to pitch up, my tail heavy F-22 will likely be able to do a loop with upward deflected ailerons and some throttle, so down elevator mixing might be a good idea.

Regards,
Barry

roger.alli

Thanks for the education on this guys, Rob, hihorse Dave and others. I was considering experimenting with crow my Bandits, but from the above posts understand that it may not be that useful.

As I see it, properly designed flaps will do everything that crow will do, (increase drag, create washout etc) plus the added bonus of lowering the stall speed. All good things during approach.

I can see crow would be useful if your flaps were not effective enough, but Bandits have huge flaps, and my Super has an air brake.

Roger

crhammond

My Feedback: (21)

I find my Boomerang XL a lot easier to land with crow. I don't know the technical reasons why it works but the effects are very easy to see on the big Boomer:
the nose sits higher without need to add elevator so I don't have to work to hold a slightly nose high AOA
the plane is slightly slower than with flap alone (drag?)
the approach is steeper for a given thrust input (i.e. it sinks better probably due to areduction in lift from the wing)
overall it feels more stable - I even prefer full flap and crow in a cross wind when most people use no or little flap

Without crow theBoomerang XLwill sit in ground effect for a long time and tends to move around more in a cross wind. With crow I can carry throttle down to the runway - without it is pretty much aglider approach from far out which makes transitioning to less forgiving jets kind of tricky. The crow does not feel to me like it changes the stall speed on this plane. It definately makes it more abrupt when it does happen (hard to stall a Boomer XL but I have done it in a stong crosswind - luckily with just a hard landing and no damage.)

On my Super Bandit I have not used crow... it sinks plenty and is rock solid without any. The trick on that one is fly to the runway on a few clicks of throttle, cut throttle over the threshold (but not too high up!), flareand hold the plane in ground effect with the nose up until it stops flying. Letting offthe pressure too soon or forcing it down makes it want to drop or shift weight to the nose resulting in a verybad pogo. I keep some elevator in until the plane is very stable on the ground too - i.e. keep the weight off the nose until it is not lifting any more so the weight transfer is gentle. I would not want to try crow on aplane with this much wing loading as I need all the lift I can get at approach speed. I think the speed brake on the SB helps reduce the tendency to hop vs the ARF - and the extra weight actually helps too as it will stall (stop flying) at a higher speed once the mains are on the ground.

The UB being a lower wing loading sounds like it is safe to use crow (several guys posted in the other thread they were using it successfully.) But if it does an nice approach like a Bandit/Super Bandit it probably isn't needed.

FalconWings

My Feedback: (57)

I thought the function of crow was to claim vengance over the death of his wife and son, at a very dark, grungie, and otherwise pointless gothic and humid environment.

Gordon W

I’m glad you mentioned that. The pogo would be as the result of carrying too much speed on touchdown. Additionally, if you got this on tarmac you might well have had to refurbish the tips of the elevators, and maybe even the wingtips, nose and underside beneath the tailpipe if the pogo was bad enough. I have that tee shirt.

I’ve mentioned in other threads that a few mm of aileron droop will lower the landing speed, increase drag (induced drag from the increased lift) and allow a steeper approach with some engine on. But I generally get shouted down and no-one ever wants to try it. So here goes again

I had a Bandito 90mm EDF jet (4ft span fast 8lb weight Bandit lookalike) that would pogo at the drop of a hat on tarmac and slowing it down for a smooth landing was always a struggle and all too frequently not achieved. Happily I worked out a cure and suffered no more landings like that. I still have a Fox Composites Hawk (about 50oz/sq ft loading) that would do the same thing until I applied my cure. The cure in both cases was not to use crow, which made no difference to that characteristic, but to droop the ailerons.

The nose-up effect of crow ailerons needs counteracting with extra down elevator trim. On a low wing low-tail Bandit/Bandito-style model with anhedral tailplane, the down-trim coupled with the nose-up attitude needed to slow the model can put the elevator tips in contact with the tarmac with regular gravel rash resulting.

Using drooped ailerons causes a nose-down pitching moment which reduces the amount of down-elevator trim required, and also lets the model land in a more level attitude, the reduced nose-up requirement being due to the extra lift from the drooped ailerons.

The two main reasons guys come up with for not drooping ailerons are increased risk of tip stall, and risk of more adverse yaw.

The first is a minimal risk as the flaps already induce massive washout. The second is easy to counteract by coupling some rudder throw with aileron when the flaps/drooped ailerons are selected.

FWIW I’ve been using drooped ailerons as flaperons for landing strip-aileron-equipped aerobats continuously for 30+ years, with coupled rudder for steering during the landing phase. This began from experiments with control-line-style coupled flaperon/elevator for aerobatics and ended up as a standard feature for taking off and landing this type of model on short bumpy grass strips. The only time you might find significant adverse yaw is if there’s enough down-flaperon to prevent the up-going aileron from actually going up at full travel, and my long experience is that at this point the main effect is that at landing speed the ailerons produce very little rolling effect and only a little adverse yaw which is readily counteracted by the use of coupled rudder (but see the caveat below). I have tried setting the flaperons as spoilerons from time to time, but never liked the effect as much as with drooped flaperons.

There is one caveat. If your model rolls adversely with rudder (as does my Boomerang Nano but I don’t know if the other Boomers do the same), then clearly you won’t have the option of coupling rudder to the aileron channel to counteract adverse yaw in the circumstance described above at the point when all aileron effect has disappeared due to aileron droop.

Just my experience. Make of it whatever you will, or maybe even try it out on a spare model.

Gordon

GrayUK

I can not agree with that, i have the same Hawk, with crow it is beautiful to land, with landing flap only, it is not so nice.
I am a convert to crow, as are many of my friends, just works really well, and the Ailerons being up (about 5mm), tends to remove the need for any elevator trim with landing flap.
I have the same setup on the Goldcat 44, once again, tried it without, it would not land, just kept floating on, crow solved it in a moment!

Drooping the ailerons is dangerous from my experience, causes early tip stall.

Paul G

Gordon W

So have you tried it on your Hawk?

Gordon

marquisvns

Seems like the Raptor uses the ailerons to pull high G's with full afterburner:
http://www.airliners.net/photo/USA-A...8c07775f30fc97
http://www.airliners.net/photo/USA-A...8c07775f30fc97
http://www.airliners.net/photo/USA-A...8c07775f30fc97

and slightly drooping ailerons with full flaps (a full span flaps?, still with varying degrees of incident, in and outboard) for landing:
http://www.airliners.net/photo/USA-A...8c07775f30fc97
http://www.airliners.net/photo/USA-A...8c07775f30fc97

Barry

HarryC

My Feedback: (1)

Funny how people think that lowering aileron will cause the wing to stall but it doesn't occur to them that lowering the flaps didn't cause it to stall! Ailerons going up or down when the wing is near the stalling angle do not cause a stall and do not cause washout either. Those effects happen much further forward on the wing than the aileron. So called aileron reversal close to the stall is not the downgoing aileron causing the wing to stall but is a mis-reading of the much greater adverse yaw that occurs which then leads to a yaw-roll coupling in one direction that is greater than the aileron induced roll in the other direction, hence the appearance of aileron reversal. The out of balance flight may also promptly leads to a stall on the inner side of the yaw so the whole effect is generally mis-read as the downgoing aileron causing the airflow to stall. Putting both ailerons down, as Gordon suggest, does not cause that part of the wing to stall sooner, and will reduce the stall speed. What it does do, as Gordon makes reference to, is increase the adverse yaw when aileron is used and that can be a danger which leads to a tip stall but as Gordon says just use coupled aileron rudder or use your left thumb very carefully. But small downward deflections of the ailerons as minor flaps is not likely to lead to such problems.
H

InboundLZ

My Feedback: (10)

Seems we have our fair share of areonautical engineers amongst us... I make no claim to this high distinction. What I do know is what works for me and in my mind crow let's me fly slower with confidence. How it works matters not to me so I am sticking with my original statement...."crow allows for a lower stall speed and a higher AOA"

I will now retreat to my world... :-)

RCJetBazz

crow will get your plane to slow down quicker,
washout will give you the confidence in slower airspeed.

gooseF22

Dave, Nor do I.. but I play one in my other life... LOL>..

However, Im a reflex convert in low wing loaded models because I can control the landing much better in almost everything. I even add a little reflex to my HABU.. without flaps. try it and the model settles in nicely with minimal elevator input. It allows me to set the mains down, and slow the nose so not to bounce. from what I have seen, most guys bounce and get into trouble when the deck angle is flat, and they drop the nose to keep it coming down and actually pick up speed. remember pitch angle controls your speed at constant thrust (landing).

When I added some reflex to the kingcat, it made it 1000% easier to land and not bounce. Even the nano has 1/8 inch reflex.. makes it easier to rotate smoothly on both takeoff and landing. Keeps them from leaping into the air as well.