David, I wonder if the perception by some people that crow allows them to fly slower is purely psychological? Crow will produce a nose up force, either trimming the plane nose up or reducing the nose down pitch. Either way the model flier is not having to haul back so far on the stick to maintain the attitude down the approach, and the plane responds more easily to pulling back in the flare. Because it is in effect given a lot of up trim it is trimmed to fly slower, and this makes them think it is capable of flying slower when actually it is simply that they are not comparing like trim with like trim.

Even more to digest:

Lift does not equal weight when descending and that is what this discussion was about, the function of crow. Trying to put full size practice onto our toys does not work we are flying in full size air with very small aerofoil sections as far as I remember the Reynolds numbers do not stack up the same.

Mike

Burn him - he's a witch!

Everyone knows Mr B was a magician!

Really?

Descending in a level attitude with the thrust line horizontal because you have slowed down, at a constant rate of descent, lift = weight. If it did not, the rate of descent would increase or decrease.

Yes Really, lift will only equal weight in equilibrium i.e. not descending or ascending.

Mike

Concur! Drooping ailerons will reduce Vs. But the Bucc has a blown wing and other aerodynamic clever bits to offset tip stall and other unwanted effects. The F4 and the Airbus also have other clever bits to get rid of any nasty tendencies - spoilers, saw tooth leading edges, dihedral breaks, vortex generators, etc. Even the US Navy Goshawk gets rid of the toblerones found on the inboard sections of other Hawk wings - these provide stall warning at the expense of lift, but for carrier landings, lower Vs is important as it reduces the energy of the deck landing. The Goshawk has an alpha gauge and audio stall warning instead of the traditional 'onset of light buffet' stall warning you get in other Hawks.

For modelling, as Mike has said, Reynold Number has an effect, as does the lack of clever aerodynamic bits due to difficulty in building and testing these at small scales. Crow braking works at slowing the aircraft down, but it needs to be set up right for best efficiency. As you say David, perhaps the flaps are drooped slightly further to compensate for the loss in lift from the reflexed ailerons. The variables are almost infinite! I reckon we are all arguing round the subject in general agreement, but choosing one aspect as our 'champion' to put our point across.

PS - Mike, in a steady state descent, Lift does still equal Weight!

PS - Mike, in a steady state descent, Lift does still equal Weight!

If that is the case why does the aircraft descend?

Mike

Because it is already descending. In steady state, all forces balance each other. To commence the descent an acceleration is required, so downward forces must exceed upward forces. Ignore other peripheral forces and assume that the plane is kept in a level attitude and that power is reduced so that it slows down. Less lift occurs so a vertical acceleration commences. As long as lift is less than weight the downward acceleration will continue forever with the rate of descent trying to increase to infinity (or it would try to until air drag or solid ground or the limiting speed of light gets in the way). A constant rate of descent is no acceleration, so the upward and downward forces are back in balance. The descent continues as described by newton's laws of motion, an object will continue on its path until an unequal force disturbs it and causes an acceleration.

Mike - You are correct re. Reynolds Number. Making a scale model of a full-size jet and expecting both to behave the same in 150 knots of wind is not proper. Adjustments must be made; enter Mr. Reynolds. However, the general principles of stability and control do apply...otherwise wind tunnels wouldn't work.

Randy

Wrong, just plain wrong ! Sorry.

The total lift when established in a steady descent exactly equals weight.

TOTAL lift is that produced by the wing AND the vertical component of drag, which is the value of the drag multiplied by the sine of the descent angle. In a typical descent. 5 degrees or so, this value is negligible as the sine of 5 is about 0. 05

Regards,

David.

I have a 10-lb aircraft with fair wing loading of around 3-lb/sq ft, I tested the crow on landings and here’s what I found. With full flaps and leveled, after I deploy crow the plane reacts like it was going to slow down due to increase in AoA so I reduced power and AoA to descent. The descending approach was solid with slightly positive angle until ready for touchdown but I was surprise that the speed was too fast, when I chop the power it stall and fall around 2-ft with touchdown speed higher than normal.

So I decided to use negative crow like flaps and the result is really slower even at slightly negative angle (nose down), this time I need to put down elevator and additional power in order to increase speed. When I chop the power and flare the plane continue to float until touchdown at a very low speed.

I decided to put back to the original setting because I need slight speed at the same time lift on landing that I can be able to use even if the wind is not favorable.

3D Aircrafts are designed to be very light and has low stability in order to become controllable at low speed. These aircraft has higher thrust to weight ratio like helicopter to override rules of aerodynamics for stall angle, lift, etc. in which wings use to fly. These aircraft has a wing loading very different compare to the jets. I’ve seen a lot of 3D balloon landings but when it happens to jet it’s a different story.

Aircraft can be in equilibrium even when descending or ascending, even forward. Acceleration (+/-) can determine unbalance.

Very well explain again...

With you a 110%, there's a fundamental vocabulary problem here, and seems like the "experts" are using the configuration of crow to deny this effect at our given parameters (airframe config and wing loading, landing speed, AOA...etc.)

On this note, those "experts" (on earlier posts of this thread) think that by lowering ailerons alone will reduce stall speed because of the additional lift provided are just plain  .............. , nah, you filling the blank.

You are talking to full size practice, although there’s a big difference in Re because of the size but it works the same, you just need to use correct dimensions and parameters.

Models jets are honest and true aircraft as far as aerodynamics is concern...

What is the function of trailing edge wing lift spoilers?

Hahaha, I couldn't resist. jK!

It is use to increase rate of descend as well as drag and to eliminate lift after touchdown...

Found another function of crow on these thread. Not sure if this can be called crow because of the very small amount of deflection but if it is, crow can be also used to increase cruising speed. I'm not recommending this but it can be also use for higher speed applications other than landing. Above cruising speed of our jets at zero deg angle, the altitude can increase rapidly as airspeed goes higher even though without any elevator input or changes in AoA, that is because of the natural camber of the wings. To minimize this effect other than using elevator or trim (when stab is almost 0-deg) is to deploy a very small crow that is close to eliminate the natural camber of the wings as airspeed progress then reduce trim and throw of elevator if required. In this way the aircraft has higher cruising speed without increasing too much load and drag to the entire wings including stabilizer. The disadvantage is higher stall speed however flaps can be used in small increment for lower speed.

That was a standard 15M Open class glider option, If I remember correctly my Vega had about 4deg up deflection in fast cruise, now that is going back about 30 years.

Mike

EDIT: forgot to mention it was the whole TE that went up, flaps as well.

Again this would not be known as crow but aileron reflex I have flown a number of Ultralights with this feature, and on one it increased the cruising speed by 10mph which is a lot considering normal cruise was only 80mph. This type had a metal fabric covered wing with full span ailerons, with the flaps being dropping the ailerons as we would do with some models - flaperons.
The reflex was set one stage past flaps up.

We also experimented with another design by setting the zero position of the flaps (conventional type, inboard of ailerons) higher than the zero position of the ailerons, so they were reflexed up. It made the aircraft fly at a slightly more nose down attitude in the cruise and again an increase in cruise speed. It is all about moving the centre of pressure of the wing.

Good info by everyone… Now, the different point of view or understanding about the true natures of crow came to a common fact. Hopefully I've got a better conclusion taken from the recent post.

Aircraft has different weight and different wing area. On landing, given with desired alpha and airspeed when an aircraft couldn't establish constant rate of descent for target landing slope, there are two possibilities:

1. Lift produce by the wings is greater than the weight of the plane.
2. Lift produce by the wings is lesser than the weight of the plane.

Considering all lift and drag devices were deployed. To solve for #1 there are 2-possibilities: to add weight or to decrease lift. Solution is to decrease lift.

There are possibilities to decrease lift: reduce flaps, reduce alpha, reduce airspeed, add spoiler and deploy crow. Reduce flaps is good but we need drag, reduce alpha is also good but we have desired angle, reduce airspeed is also good but we need airflow, spoiler is what we need but aircraft has no spoiler so crow is a better option.

Let's also solve for #2, if lift is not enough to support the weight of the plane, there are possible solution. Increase flaps, increase angle of attack, increase airspeed, add high lift device (HLD) or deploy negative of crow. Flaps are already max, AOA is not safe when closer to critical angle, also we have desired angle. Increase airspeed can cause high speed landing, also we have desired airspeed. The only solution left is to add HLD or deploy negative of crow.

Aircraft in between doesn't need anymore crow at all.

Since weight, wing size and airspeed are the factors, I therefore conclude that crow option is based on the wing loading of an aircraft. Some require crow deflection, some are not. Light aircraft require crow deflection. Some require negative of crow or ailerons flaps and these are heavy wing loading jets.

Just to add to the clarification, crow is the deployment of reflex (up) aileron with flap, when deploying just up aileron this is known as just reflex.

Mike

I have just flown from London to Sydney in the A380 (Emirates, very,very good too, particularly liked the bar area, very useful on the 14 hour slog DXB-SYD ! Incredibly quiet too, beginning to wonder if the engines had stopped when climb power was selected after T/O ! ) and was able to see the aileron operation which was rather interesting, particularly after this thread. As the the first stage of flaps were extended at the beginning of the approach, the outboard ailerons (which are each of 3 sections) drooped by about 10 degrees. On landing when the spoilers were raised for lift dumping (max weight on wheels, max brake effectiveness) the ailerons rose by about 25 degrees. As the spoilers were stowed after runway claarance, flaps went back to droop, then during taxi in as the flaps were retracted the ailerons went back to neutral, aligned with the outboard wing trailing edge section. FBW working ! From the upper deck, inboard seat, I couldn't see if the A380 was fitted with high speed, inboard ailerons as are Boeings.

It was interesting to see that the three sections of aileron moved at quite different speeds (I guess they are powered from different hyd, systems.) when they were "active" during the approach but were perfectly aligned when no control input was being made.

Proof, indeed, of how aileron angle can be used to vary lift to obtain best performance advantage.

Regards,

David.