Topic: The side-sliping with flap extended restriction

[Rod Blievers]

Gents, thank you all for your contributions thus far.

I’m rather reminded of some seemingly illogical restrictions that can creep into airline flying – the best example being with my last employer. There, on the B747-200/300 “Classic”, use of the speed brake was absolutely frowned upon except when automatically deployed on the ground. There were dire tales of what this would do to the aircraft’s structure and aerodynamics, despite every other operator in the world using them whenever the pilots felt like it, and further despite there being no such restriction in the Boeing Flight Manual. Apparently, this was traced back to some long retired management type’s personal dislike of the buffeting the “boards” produced. Over many years these stories and warnings had become accepted as fact.  I rather suspect the situation with “no side-slipping with flap extended” is similar.

I have also been canvassing opinions here (in Australia). I’ve consulted with two engineers, one of whom has undertaken the majority of Chipmunk restorations in this country and I consider to have encyclopaedic knowledge of Chipmunk engineering (and he flies too). The second gent is the Chief Engineer of a Chipmunk operating Aero Club, who maintains several other Chipmunks and flies his own example as well. I have the utmost respect for both of their opinions

One gent felt that side slipping with full flap was not recommended because of possible tailplane/elevator “blanketing” resulting in unexpected pitching down moment, although he though half-flap was OK. The other (who actually then consulted with an experienced Chipmunk flying instructor before responding to my query) simply knew of no aerodynamic or structural reason why this restriction should be imposed. I asked him specifically about Roy Travis’ concern about overloading the flaps with the possibility of a cable failure – his response was that as long as airspeed limits were observed the flaps and cabling would not be overloaded.

Over to you again….

On the subjects of myths that become truths after years of repetition, I was delighted to see Duncan’s comments about DH Mod H 104 (the broad-chord rudder) – many people still believe this was fitted to improve the Chipmunk’s spin recovery, yet reading the DH preamble this was clearly not so. Well done, Duncan.

May I too finish with a quote? “The truth is what is loudly proclaimed and stoutly maintained”. There, that feels better…

Cheers,

Rod.

[Roy Travis]

Rod, I think you have summed up the thread very well. Clearly there is no restriction stated in any flight manual or instructor manual that we have been able to uncover.
In my reading of the replies, the notion that side slipping with flap is a no no seems to be far more prevalent in the Southern Hemishere and leads me to the thought that it has grown up through a differance in our training proceedures. Certainly that is where mine originated.
Anyway the discussion has been stimulating. I am off  now to fly my beloved Chippy.
See you at Groongal Rod, or maybe Narandra?

[Rod Blievers]

Both events, Roy! See you there...

Cheers,

Rod.

[chipmunkflyer]

Gentlemen.. might I be allowed to add some information to this discussion

The Type Record page 24 dated November 1948 for the Chipmunk T.10 shows:-

The flap area/side = 11.0 sq.ft   The Flap span = 95 ins
Maximum flap delection = 30 degrees   Vf = 85 mph
                                                        [my conversion 73.78 kts]

Design Cases

Ref. B.C.A.R. D.15 Iss 2(iii) ..... a gust of 25 fpm along the flight path, backwards, at Vf produces the maximum normal and chordwise forces and maximum torque on the flaps at a deflection of 30 degrees. The aerodynamic coefficients are obtained from NACA Technical Report 633.

The maximum design forces on the flap are 676 lb.normal, 137 lb chordwise and 1420 lb.in torque.

My stressman tells me that the force on the flaps will be the same in level flight and in the sideslip if the airspeed is the same - a variable headwind
may cause greater or lesser force on the flaps. It is also probable that the greatest strain on the system is when the flaps are deployed because this is carried out at or nearer to Vf than the speed on the approach.

The figure for Vf is not only for the flaps but for the whole system and according to research a factor of 1.6 -1.8 is built in thus raising this to 113.6 and 127.8 kts using the current 71kts figure.

AP2095 4th.edition Pilots Notes General.. page 14 quotes:- the speed limit for lowering the flaps and u/c arises from calculations, either of the strength of the parts to withstand the air forces, or of the power of the operating mechanism. These calculations normally assume that the flaps are used only during take-off and landings or for straight and level flight and gentle turns.

During many years of research of the Chipmunk I cannot recall any incidences of flap cable failure .... modifications to various pulleys etc.have been introduced to assist system operation but not to the cables themselves.

Whilst I agree that asymmetric flap at any stage of flight is very inconvenient it is probably more so in a Chipmunk bearing in mind the operation of changing hands in an endeavour to resolve the situation.

Lastly, I would point out that side slipping is part of the JAR-PPL syllabus and as instructors I believe that we should give our students the ability to control the aircraft and the situation during an engine off landing.

I am awaiting the diameter of the flap operating cables and their tensile strengths from my friendly engineer to compare with the above figures -
unfortunately they are not quoted with the other controls in the Type Record

Best

Rod

[Nigel Stevens]

I 've just measured the flap cable diameters as I happened to be under the rear seat, the single cable from lever to pulley pair, the two cables under the seat and the cables in each wing both operating and flap return give measurements of 1,92 to 1,95mm. in diameter.
Nigel

[Brent]

It has certainly turned into a thought provoking discussion.  I don't have all that much time in Chipmunks, but logic and basic aerodynamics would seem to indicate that in as much as the flaps are in effect mechanically independent from each other, so long as the flap limitation speed is adhered to there should not be any increased loads experienced.  I don't think the flap cares much whether the wind is squirting from the side or straight ahead so long as 73 kts. is your limit.  At our speeds, the IAS is the governing factor as far as aerodynamic loads are concerned.

Blanking of the tail is not a factor for the Chipmunk as it was said to have been be with the Miles Magister...........at least not with my Chipmunk.... so far.  (Don't tempt fate Brent)

As far as admitting that a side slip is the result of misjudgement, that may be so, but how is that any different from adding a little power if the misjudgement is leaving one a little short?  To me sideslipping is merely another option for me to save myself from yet another of my errors in judgement.  Not only that, but I can also make my approach that little bit steeper and tighter, which can be helpful on occasions!

Brent