RCU Forums - Slotted flaps help?

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Slotted flaps help?

I've got a dV8r jet that's ready to fly but was wondering about changing the flap setup. Right now the flaps are huge and simply hinged at the bottom of the wing for max deflection. Seems most of these models experience porposing when full flaps are deployed. The smallest correction makes the jet start to bob until it turns into a lawn dart. Not good, like to avoid that. I was wonder if the flaps are blocking the horizontal stab? Would slotted flaps allow some smooth air to flow over the stab and correct the problem?

Thanks,
Shaun

RE: Slotted flaps help?

Slotted flaps are aerodynamically more efficient, and thus should produce less turbulent airflow over the tail feathers at a given airspeed. Slotted flaps can also produce much higher lift than plain flaps, enabling the airplane to fly slower. As the airspeed lowers, the horizontal tail, which does not normally have the benefit of slots, will have a tougher and tougher time developing the necessary stabilization force. From this viewpoint, slotted flaps could reduce stability and controllability. Yaw stability also suffers, since the vertical tail will also have less effect at lower airspeeds, and the airplane will be more prone to stall and snap roll. I would increase at least the vertical tail area considerably for use with slotted flaps, unless the airplane had a very generously proportioned vertical tail, a rather rare occurence.

RE: Slotted flaps help?

Shaun,

Two things (well, at least two things) are likely to happen when you extend flaps. 1) you will very likely change the angle of attack for which the airplan is "trimmed". Any time a stable airplane is not at the angle of attack for which it is trimmed, it will try to return the trim angle of attack. According to linear aerodynamics, it will do this through its "short period" mode. I think the easiest way to visualize this is to think about an arrow that is moving through the air, but not quite alinged with the direction it is going. The arrow's tail feathers will try to align the arrow with its flight path. The arrow will typically overshoot the flight path several times before lining up. The period of one cycle is the "short period". 2) By extending flaps you will also likely change the aircraft lift coefficient at any given angle of attack. If you take a plane in level flight and make a small increase in it's lift coefficient, the extra lift will bend the flight path up. If the power isn't changed, the plane will slow down as it climbs. Most planes will continue to decelerate below the speed that would keep them level at the new lift coefficient. This means that their flight path will eventually bend back down. On the way down the aircraft will speed up and eventually begin to climb again. This periodic motion is what is called the airpalne's "long period" or "phugoid".

The short and long period motions can continue for many cycles (the long period can go on indefinitely for planes like the P-3), or "damp out" in less than a full cycle. A good pilot or flight control system (or some combination of the two) should be able to intervene when flaps are extended in order to keep porpoising to a minimum.

If you are looking at ways to minimize the amount of pilot input required to minimize porpoising, your best bet would probably be to look at a simple interconnect between the flaps and the horizontal stab. As you point out there may be an aerodynamic connection between the flaps and the stab. The idea is to electronically or mechanically "take out" some or all of this natural connection (a T-38 would pitch up uncontrollably at flap extension if not for a built-in "flap-slab" interconnect). More sophisticated automatic flight control systems can add artificial damping, but these typicall require sophisticated sensors (like rate gyros).

To answer your question, I don't think that slotted flaps would help much by smoothing ot the flow.

RE: Slotted flaps help?

That pretty much explains what's going on with this jet. Guess there's not reason not to try a rate gyro like a futaba GYA350 or 351. Not like I'm in any rush.....there's still a lot of snow out there lol.

Quote:

ORIGINAL: Shoe

Shaun,

Two things (well, at least two things) are likely to happen when you extend flaps. 1) you will very likely change the angle of attack for which the airplan is "trimmed". Any time a stable airplane is not at the angle of attack for which it is trimmed, it will try to return the trim angle of attack. According to linear aerodynamics, it will do this through its "short period" mode. I think the easiest way to visualize this is to think about an arrow that is moving through the air, but not quite alinged with the direction it is going. The arrow's tail feathers will try to align the arrow with its flight path. The arrow will typically overshoot the flight path several times before lining up. The period of one cycle is the "short period". 2) By extending flaps you will also likely change the aircraft lift coefficient at any given angle of attack. If you take a plane in level flight and make a small increase in it's lift coefficient, the extra lift will bend the flight path up. If the power isn't changed, the plane will slow down as it climbs. Most planes will continue to decelerate below the speed that would keep them level at the new lift coefficient. This means that their flight path will eventually bend back down. On the way down the aircraft will speed up and eventually begin to climb again. This periodic motion is what is called the airpalne's "long period" or "phugoid".

The short and long period motions can continue for many cycles (the long period can go on indefinitely for planes like the P-3), or "damp out" in less than a full cycle. A good pilot or flight control system (or some combination of the two) should be able to intervene when flaps are extended in order to keep porpoising to a minimum.

If you are looking at ways to minimize the amount of pilot input required to minimize porpoising, your best bet would probably be to look at a simple interconnect between the flaps and the horizontal stab. As you point out there may be an aerodynamic connection between the flaps and the stab. The idea is to electronically or mechanically "take out" some or all of this natural connection (a T-38 would pitch up uncontrollably at flap extension if not for a built-in "flap-slab" interconnect). More sophisticated automatic flight control systems can add artificial damping, but these typicall require sophisticated sensors (like rate gyros).

To answer your question, I don't think that slotted flaps would help much by smoothing ot the flow.

RE: Slotted flaps help?

Some things I've noticed with models is that flaps tend to be on a switch which, when activated, deploys the flaps at full servo speed, the flaps just pop down. When this happens so fast there is no time to stabilise the model, the plane usually jumps up and down while the pilot tries to control it, untill speed and trim has time to settle in. Sometimes elevator re-trimming is still required.

I believe that if flaps are lowered slowly and elevator trim is properly mixed to flap position, the plane will be much better behaved and the pilot won't have to work to stabilise it.

I notice too, that some models require 'up' trim and some need 'down' trim when flaps are down.

I know some guys who refuse to use flaps, when they are there, because of the way the plane behaves when lowering them. They simply haven't set-up the plane properly.

Cheers from the Barfly.