Vx

[newcrew]

good afternoon all

does anybody know any piston / turpoprop fixedwing aircraft where Vx is flown with any flap deployed.

NB no interest in short / soft field take-offs only pure Vx

thanks for the help

cheers

[Old Smokey]

Each Flap setting will have it's own Vx and Vy. Both are related to Lift and DRAG, with the higher flap settings having the lower Vx/Vy, with Vx/Vy increasing as flap setting decreases. The actual Angle / Rate of Climb will be decreased in all cases when any flap is used.

I know of no aircraft where a flap setting caused less, or even equal drag than the clean configuration, thus the best Angle / Rate of Climb will always be achieved with zero flap at the appropriate Vx/Vy. If one such aircraft does exist, why not leave the flaps out permanently?

There are many instances where pilots conduct their initial climb, often fairly lengthy, with Takeoff Flap selected and at Vx for that flap setting. This will be so where critical obstacles are present. Although the climb angle would be improved in the clean configuration at the clean Vx, it necessitates an acceleration from the ''flapped' Vx to the clean Vx. The required acceleration could only be accomplished at a REDUCED climb angle, thus, the overall climb gradient would be less at a time when obstacles are critical.

Once clear of obstacles, climb angle may be reduced, allowing acceleration to the 'Flaps Up' speed.

The same principal applies equally to a Cessna 172 as for a Boeing 747.

Hope that that clarifies things a bit.

Old Smokey

[newcrew]

slope of the thrust curve

Old smokey thanks for the reply

A fellow instructor assures me that Vx is obtained with flap in both a C150 and a C 152 I will check this as soon as my roster allows.

If the slope of the thrust curve was very steep would it not be possibble for an aircraft to have a Vx with flap?

Anybody know of such a type

[davidjh]

Hi newcrew

I only know of C152 (not sure about 150) that uses 10 deg. flap for Vx climg. I have been told that possibly a Twin Otter might also use flap for climb. Can anyone confirm this?

[Empty Cruise]

Newcrew & davidjh,

as OS stated, there will be a Vx for both F10 and F0. Therefore, the POH may state that you should perform Vx climb at F10 - under the assumption that the typical C150/152 operation with Vx will be performed with the intention of clearing close-in obstacles, where F10 will give you a better net gradient from brake release till clear of a, say, 100 ft. obstacle. So the POH in this case "mind-reads" the pilot.

Therefore, from a procedure point of view, Vx climb is performed with F10 (Vx10). But from a pure performance point of view, the gradient obtained with F0 is greather than with F10. A review of excess thrust curves for the two configs will confirm this So if you're looking for PURE Vx (no other variables involved), you are looking at the maximum thrust vs. the lowest drag, provided by Vx0.

Regarding turbine aircraft, most are only certified for take-off with some flap deployed, so if you have to clear obstacles below acceleration altitude, you climb at Vx15 (e.g.), then accelerate & clean up after clearing the obstacles.

Brgds,
Empty

[newcrew]

is it unanimous then

no other comments or information from everybody out there?

anybody

cheers

[Oktas8]

I agree with Empty Cruise.

Since the C152 has been mentioned, I'd like to point out that, in practice from the pilot's perspective in this aircraft, there is no discernable difference between Vx F0 and F10 - same for Vy for that matter. If the difference in gradient between the two is so minor as to practically immeasurable, Cessna would not bother to specify F0 and F10 procedures. A matter of theory versus pragmatism, and pragmatism winning.

Cheers,
O8

[411A]

<<<If one such aircraft does exist, why not leave the flaps out permanently.>>>

This trick was tried by a TWA 727 crew some years ago, at high altitude, with rather poor results...a bent aeroplane, and a wild ride on the way down.

I did not operate earlier models of the Lockheed Constellation, but the 1649 had very high lift fowler flaps, and improved all engines Vx climb rate could be achieved with the lowest amount of flap extension (flaps 5, I think), at the same speed (V2+30) as with flaps up.
The 1649 had a newly designed wing, which was very efficient.

A very nice aeroplane.

[hawk37]

Two thoughts I’d add

1. Lets not forget that thrust decreases with speed increase, both for a jet engine, but especially for thrust produced by a propellor. Hence while deploying even a small amount of flap or slat would increase drag for a properly designed aircraft, it seems plausible to me that if now flying slower at the new minimum drag speed, this could mean that there is more excessive thrust. Hence, a better possible gradient than with flaps/slats up.

2. No one has mentioned the slip stream effect of a propellor over the wing. While non existent for a jet propelled aircraft, this added “wind” over the wing produces extra lift. Quantifying just what this does to values of Vx and Vy I’ll leave to the experts.

Hawk

[BizJetJock]

It is quite possible for an aeroplane to have its best angle of climb with flap deployed, although almost certainly at a lower speed than clean.
The increased parasite drag of deploying the flaps is relatively low due to the low speed, and by changing the aerofoil section induced drag may well be reduced. This results in a new drag curve with a new lower VMD. The drag at this speed may or may not be less than the drag at the clean VMD, but coupled with the increased thrust mentioned by HAWK37 it is quite possible for the speed of maximum excess thrust (VX) to have a greater excess available than at the clean VX.
OLD SMOKEY asks why if this is the case we do not leave flaps out permanently? Quite simply because the parasite drag increase with speed is very rapid compared to a clean aircraft, therefore there is a serious penalty on cruising speed. In addition there are all sorts of structural complications from the lift distribution at high speed, hence we have VFE limits dramatically lower that VNE/VMO.
As for examples, a quick perusal of the performance section of the flight manual of any Citation (and many other light jets) gives the result that the second segment climb gradient (Flaps 15, gear up) is often significantly better than the (clean) final segment gradient for the same conditions.
I should clarify this by saying that the "often" is at low altitudes and temperatures; as it gets "hot & high" the reducing thrust available and TAS effects make it revert to the intuitive situation.
Have a good Xmas!

[newcrew]

NET THRUST

i belive the thurst of the arguement now relates to the thust increase vs drag (decrease / increase)....

guys any actual examples (can we restrict to piston and turboprops...?)

awaiting all comments

[hawk37]

BizJet,
Interesting thoughts, but I’m not convinced. I’ve seen 727 drag curves, and if I remember correctly, deploying even a bit of flap increased the value of minimum drag, not decreased it. I can’t think other aircraft would be any different.

I’d expect Smokey will jump in shortly. I think you bated him a bit with your response to his post..

But I’m not happy with your citation data:
1. Does the performance preamble anywhere say the final segment gradient speed is for best gradient? Because, as you know, final segment speed requires not ** LESS ** than 1.5 % gradient. Hence the manufacturer can choose any speed that meets this requirement.
2. I just can’t believe flaps 15 gives better second segment climb than clean. Take a look at the flaps 15 max take off weight for second segment limits (2.4%) and compare it to flaps up. This will tell us which configuration has the lesser drag. Can you get back to me on that?

And as for the “hot and high” situation reverting to the intuitive, I can’t see why minimum drag has anything to do with pressure altitude or temperature, except when compressibility comes into play. The reduced thrust available AS SPEED INCREASES occurs both when cold and low, as well as hot and high.
Merry Xmas to all
Hawk

[bobrun]

The AFM for the DHC6 Twin Otter gives Vx and Vy for F0 only (with Vx=87 KIAS and Vy=100 KIAS, no flaps).

HOWEVER, Vyse is obtained with F10 (Vyse = 80 KIAS at MTOW), and confirming/selecting F10 following an engine failure after lift-off is the AFM procedure (F10 should already be selected, as it is the usual T/O flap setting, unless performing a STOL take-off).

[hawk37]

Bobrun,

Does the flight manual actually say that Vyse ** is obtained with flaps 10 ** ?

Or perhaps that Vyse ** with flaps 10 ** is 80 kias?

Certainly it’s common for the manufacture’s go-around/missed approach procedures to be flown with some flap/slat, since immediately retracting them has it’s pitfalls.
And certification approach climb requirements are done with slats/flaps in the approach configuration. Frankly, I’ve never heard of selecting flap on any aircraft in order to get better sustained climb rate or angle.

As per Newcrew’s question, is it just me, or are there others out there who question this?

Hawk

[Empty Cruise]

Hmmnnn, the renowned T'wotter...

Again, my gut feeling (FWIW) is that the EFATO scenario and flaps 10 have another background than perfromance...

...could it be that Vmca has been certified with F10, and that STOL T/O - using lower speeds, I presume - infringes the 1,1xVmca requirement for min. V2? Have absolutely no idea, since I have never had the pleasure of flying the t'wotter

Not willing to give up on firm belief in thrust vs. thrust req. - me old teacher would probably be proud

Brgds,
Empty

[palgia]

Maybe the reason why you might want to have some flaps in a Vx climb scenario might be because the added safety benefit of a greater stall protection margin outweighs a presumably-small performance degradation...

Just my WAG.

palgia

[Old Smokey]

BizJetJock,

With regard to your quote - "a quick perusal of the performance section of the flight manual of any Citation (and many other light jets) gives the result that the second segment climb gradient (Flaps 15, gear up) is often significantly better than the (clean) final segment gradient for the same conditions.", I would be staggered if this were not the case.

(1) Second Segment Climb is conducted at TAKEOFF Thrust, which provides for the maximum possible excess thrust.

(2) Second Segment Climb is conducted at a speed (V2) that is fairly close to VMD for the Flap setting.

(3) The Clean Final Segment is conducted at MAXIMUM CONTINUOUS Thrust, which, in terms of excess thrust, is significantly less than Takeoff Thrust.

(4) I am not familiar with the Citation, but it is quite typical that, for this class of aircraft, Final Takeoff Speed is that for the Maximum Rate of Climb (OEI En-Route climb speed), i.e. significantly above VMD, where climb gradient is NOT a consideration.

(5) Flight Manuals give Net performance data. and, for a 2 engined aircraft, 2nd segment climb is degraded by 0.8%, whereas Final Segment (En-Route) climb is degraded by 1.1%. This will help in making the 2nd segment look better.

For any aircraft, best angle of climb occurs where, for the configuration, the maximum excess thrust is available.

For the Jet aircraft, this is slightly below VMD (but for all practical purposes at VMD). OEI VMD is less than All engines VMD due to Rudder Drag, and engine windmilling Drag, in a speed range where the slight reduction in Net thrust is only slightly lower than static Net thrust. It's not until about M0.3 that thrust decline from static becomes noticeable.

For the Propeller aircraft, thrust decline with increasing forward speed is MUCH more marked (Power = Force X Velocity), and maximum excess thrust will occur somewhat below the 'pure' VMD for the configuration FOR MOST PROPELLERS. I find it almost inconceivable that, for a Constant Speed Propeller, it's efficiency would be greater at a lower speed. (Propellers will vary).

I do not know of, and cannot conceive of, any aircraft which has lower drag with even minimal flap extension - my earlier remark of "If one such aircraft does exist, why not leave the flaps out permanently?" was made very very tongue in cheek, if one does exist I'd be interested to know of it.

Let's take some hypothetical figures, Required Obstacle Clearance Gradient is 5%, Flaps 10 Gradient is 6%, Flaps Up Gradient (Still at Takeoff Power/Thrust) is 8% - Fairly typical figures. Now, if Takeoff Flap setting is 10 degrees, you MUST maintain Flaps 10 and Flaps 10 Vx until obstacles are cleared. What if you decided to avail yourself of the improved gradient available in the clean configuration? You can only achieve this by reducing climb angle allowing the aircraft to accelerate to the Flaps UP Vx, say, we fly for 1000 M at Flaps 10 and a 6% climb gradient, reduce to 3% for another 1000 M to reach the Flaps UP Vx, and then achieve an improved 8%. The Net gradient for the first 2000 M because of the acceleration is 4.5%, LESS THAN the required obstacle gradient, and Splat!!! - You've made the front pages of the papers ( and caused considerable speculation on Pprune as to the 'probable cause'). If some Flight Manual / Operations Manual states "Improved obstacle clearance is available by maintaining Takeoff Flap Vx after Takeoff, they're absolutely correct, for all of the reasons stated above. They're not telling you that you will achieve an improved gradient, you will NOT, they're telling you that obstacle clearance is improved.

All of my current Performance Engineering work is for Jet aircraft, but in bygone years did a huge amount on Turbo-Prop aircraft. In EVERY case, the Flapless takeoff for these aircraft offered the best All engines and OEI climb gradient performance after Takeoff. Of course Prop / Turbo-Prop usually means short runways, so Flapped Takeoffs become necessary because of Field performance, with 2nd segment performance suffering accordingly, BUT, if a Flapped Takeoff was necessary, the Flapped Vx (or V2) had to be flown until clear of obstacles - RIGIDLY!

Dammit Hawk37, you were right, I did get drawn in.

Fly Safe,

Old Smokey

[bobrun]

I also agree with what was said above.....no flaps = better climb gradient performance.......

but still not sure why the procedure on the twin otter is to select flaps to 10 degrees following an engine failure.

On T/O, if more than 10 degrees of flaps had been selected it would make sense to initially raise them to 10 degrees following a failure.

But if F10 had already been selected for the T/O, why not raise the flaps in increments all the way up?

Any twotter experts out there?

[hawk37]

Bobrun,

Exactly whose procedure is it to select flaps 10 with an engine failure? Can you quote from the manufacturer’s checklist? If not, whose **approved** checklist has this procedure?

Is it part of a section dealing with arrivals and/or departures? In which case, perhaps to SELECT flaps 10 is meant to REDUCE the flaps to 10 degrees from 20 or higher?

Hawk