# Variable Pivot Flap

### Help Support Homebuilt Aircraft & Kit Plane Forum:

#### DaveK

##### Well-Known Member
I was wondering if anyone on the board knows much about the variable pivot trailing edge flap. It is a single slotted flap that uses a rotary actuator attached to the leading edge of the flap via a short linkage and a second linkage that is attached further back on the flap that connects further back on the wing then the actuator. The net result is the flap moves down and back. I know it was used on the 747SP. Seems like an attractive system as it is a simple four bar linkage and doesn't have flap tracks or exterior hinges to deal with.

#### orion

##### R.I.P.
I've seen a few folks head down this road over the years but I have yet to see one fly.

#### Dana

##### Super Moderator
Staff member
Sounds like a Fowler flap, with slot.

-Dana

Ever notice the Secret Service and the Nazi SS have the same initials?

#### orion

##### R.I.P.
The variable pivot flap and the Fowler flap are just subsets of a single slotted flap, which in essence is the geometric configuration they all form when deployed. The difference is simply in how that deployment is accomplished. The basic single slotted flap uses a single point external hinge that allows the flap to pivot about a displaced axis. This is very simple for rectangular wings since the inboard and outboard hinges are identical.

The problem gets a bit more complicated for tapered wings since there you'll have some sort of inclined hinge line, requiring two different hinge geometries and a tapering flap. In order to get the proper motion and prevent binding, some folks also recommend using two actuating linkages, one at the root and one at the tip of the flap.

The Fowler flap then is characterized by an aft-ward motion before it initiates its downward deflection. The aft motion is beneficial in that it provides the wing with a slight increase in effective wing area, thus reducing the stall speed a slight bit more than the simpler single slotted configuration. It's this increase in area that provides the Fowler flap with a slightly higher max lift coefficient however the design requirements of deflection and position are identical to those of the simpler geometry. Fowler flaps tend to require a slightly more complex system of deployment, including the properly designed tracks. Here too, more complex wing shapes may require motion control at the root and at the tip of the flap surface.

The variable pivot flap then provides a similar benefit to that of the Fowler flap, but without the complexity of the track and rollers. The end of its stroke is still defined by the same criteria as you'd use for a slotted flap, but now you also gain a bit due to the aft motion (and thus the increased effective lift area) the four bar linkage can generate.

#### DaveK

##### Well-Known Member
It is sort of like a fowler flap but without the complex tracks. The motion is sort of psuedo fowler it can have the ideal geometry at one point and sort of mimics the ideal positions in other from what I can tell. My understanding is the 747SP used them to save weight and reduce drag by eliminating some external items like fairings for the flap jackscrews used in the other 747 models. I was thinking they might be a way to get reasonably descent flaps without the weight and complexity of a full fowler flap. Then again if Orion is right and nobody has every really incorporated them in a light aircraft there might be a good reason. Anybody know the good reason?

#### mikemill757

##### Well-Known Member
The Fowler flaps I'm designing will use a displaced hinge (about 8") and need no tracks, just an actuator. I like what Perer Garrison did on his Melmoth 2 with some small tracks and follower bearings, but it would take me a month of Sundays to reproduce it in my garage/shop.

#### DaveK

##### Well-Known Member
There is a reason it took Peter 20 yrs to build Melmoth 2. I think he described himself as being a master at gratuitous complexity.

I wouldn't mind seeing a picture of your flap setup.
Dave

#### orion

##### R.I.P.
The flap you're describing (with a displaced hinge line) is simply a basic single slotted flap. To be a Fowler flap you'd need the tracks, which move the flap aft before deflecting it downward. This may sound nit-picky but if you're going to use proper and accurate design information you'll need to use the proper terminology or you'll end up with something that's off from optimal.

#### DaveK

##### Well-Known Member
Orion says it clearer then anyone else again. I was wondering how you could have a Fowler flap with a simple displaced hinge.

#### GESchwarz

##### Well-Known Member
The 4 bar linkage in less laterally stable than a simple hinge and is capable of extending further aft by only a slight amount. A pair of 4 bars also have a wiggle that must be constrained by a push rod at each 4 bar assembly, otherwise the flap will rack. So I decideed that the 4 bar wasn't worth the extra complexity and weight. If you want to add a lot of wing area you'll have to go with tracks. There are plenty of STOLs to copy and make sure you have plenty of tail volume to compensate for the increased pitching moment.

After looking at the supersonic FA-18 that must also land on a carrier I decided that if a simple, single axis flap does the trick for the Navy, it's good enough for me. You will also notice that there is a variable geometry slot lip that follows the flap down as it arcs back. This flap is actuated at the inboard end.

Last edited: