Vf vs. Vfe

[Joe]

I did some quick searches (in the forums, online, and in my textbooks) and couldn't find an answer, so I was wondering if anybody could tell me the difference between design flap speed (Vf) and flaps extended speed (Vfe)?

 

[PeterPilot]

I "think" (as opposed to knowing) that Vf can be used when Vfe and Vfo are the same speed. If anyone can cite a reference, I'd appreciate reading it.

 

[tgrayson]

I did some quick searches (in the forums, online, and in my textbooks) and couldn't find an answer, so I was wondering if anybody could tell me the difference between design flap speed (Vf) and flaps extended speed (Vfe)?

Reading through Part 23, it looks like Vf is used internally to specify the design requirements on the flaps. It itself does not appear to be designated as an operating limitation. Vfe is governed by the same requirements as Vf, but is designated as an operating limitation. I presume it allows the manufacture to set a different speed for the pilot to allow some cushion from the value actually used to test the structural integrity of the airplane:

§ 23.1511 Flap extended speed.
(a) The flap extended speed VFE must be established so that it is -
(1) Not less than the minimum value of VF allowed in §23.345(b); and
(2) Not more than VF established under § 23.345(a), (c), and (d).
(b) Additional combinations of flap setting, airspeed, and engine power may be established if the structure has been proven for the corresponding design conditions.


​

 

[Minuteman]

VF is a minimum flap retraction speed.

From FAR 23.345(b):

VF must be assumed to be not less than 1.4 VS or 1.8 VSF, whichever is greater, where—

(1) VS is the computed stalling speed with flaps retracted at the design weight; and

(2) VSF is the computed stalling speed with flaps fully extended at the design weight.

Edit: Crud, beaten for looking for an analog in Part 25 (didn't find one).

 

[tgrayson]

VF is a minimum flap retraction speed.

Doesn't say anything at all about retraction or extension.

 

[TUCKnTRUCK]

Doesn't say anything at all about retraction or extension.

If you look under §23.345(C, D)
(c) In determining external loads on
the airplane as a whole, thrust, slipstream,
and pitching acceleration may
be assumed to be zero.
(d) The flaps, their operating mechanism,
and their supporting structures,
must be designed to withstand the conditions
prescribed in paragraph (a) of
this section. In addition, with the flaps
fully extended at VF, the following conditions,
taken separately, must be accounted
for:
(1) A head-on gust having a velocity
of 25 feet per second (EAS), combined
with propeller slipstream corresponding
to 75 percent of maximum
continuous power; and
(2) The effects of propeller slipstream
corresponding to maximum takeoff
power.


It would appear, that you are correct in that, Vfe is a limit specified to make sure that the Design limits at Vf are not exceeded. For use, flap retract is bugged as Vfri, others i have seen Vclean, of course this speed is weight variable, and not a limitation.

 

[Minuteman]

Doesn't say anything at all about retraction or extension.

That's true, and I am inappropriately mixing uses.

Airbus planes have a "VF" speed bug called the Minimum Flap Retraction Speed, which is not a limitation, but provides a reasonable margin from the stall speed at the minimum flaps-extended configuration. Some Boeing aircraft use a similar scheme for speed tape indications, using a simple VREF for a normal landing configuration+additive schedule ("Flap Maneuvering Speed"). The intent is the same: to provide some separation from the stall speed.

The VF described in the Part 23 is unrelated to establishing a flap retraction schedule, and I am a not so slick. That VF is, as you say, used to proscribe the boundaries for VFE and I should have read further.

 

[tgrayson]

If you look under §23.345(C, D)

Yes, I'm sure those specifications encompass extension and retraction, but my point was that it wasn't limited to that and hence shouldn't be called a "retraction" speed.

 

[tgrayson]

Airbus planes have a "VF" speed bug called the Minimum Flap Retraction Speed, which is not a limitation

That may be what the OP wants to know, rather than the Vf I assumed.

 

[TUCKnTRUCK]

Yes, I'm sure those specifications encompass extension and retraction, but my point was that it wasn't limited to that and hence shouldn't be called a "retraction" speed.

I just quoted the wrong part, VF, Vfe has nothing to do with retraction or extension. Maybe they were thinking about the split between landing gear, and extension speeds in some aircraft?

Also, the OP asked about VF (flap extend speed)... so I am guessing that he's not asking about Airbii clean speeds.