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Flap limiting speed 83 kts exceeded

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JonathanMilbank

Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area

Posted: Mon May 31, 2021 9:01 am Post subject: Flap limiting speed 83 kts exceeded

One of our group members got distracted during climb out after take off, while the flaps remained extended to 18 degrees. Shortly his speed reached or slightly surpassed 100 knots, so the exceedance was about 20 knots before he quickly stowed the flaps.



After carefully examining all the flap hinge plates, the fibre-glass surrounding where they are embedded, hinges inside the fuselage and surrounding structure, flap root pins and the flap root structure generally, flap push-pull rod and lever, no evidence of damage could be found. Subsequent flight confirmed that the flaps seem to be in perfect working order. It also seems that the whole construction is commendably robust.



So my questions are "How is the flap limiting speed determined by the designer and what if any factor is allowed in calculations for possible overloads?"

John Wighton

Joined: 18 May 2010
Posts: 239

Posted: Tue Jun 01, 2021 12:12 am Post subject: Re: Flap limiting speed 83 kts exceeded

Jonathan,

Well done for 'coming out' over an overspeed occurrence - we have all done it to a larger or lesser extent and the questions you pose are very relevant.



I am not speaking for Europa Aircraft (current or past) or for the Club, however, generally, the flap limiting speed is determined having conducted a stress analysis of the whole system.  The failure mode that causes the first failure or lowest reserve factor (RF) is associated with a speed within the V-N envelope.  A suitable factor is applied (which in any case will include at least 1.5 for LIMIT to ULTIMATE and, most likely, additional factors such as 1.15 fitting factor (at joints) or even 1.33 (for regularly dismantled items).  For composite structure failures, there are yet more factors that take into account the environmental influences (e.g. temperature, humidity) and variability of build standard.



Regarding loads, as air pressure is proportional to the square of the airspeed the loads can rise very quickly.  For your overspeed occurrence, the flaps would have seen a dynamic pressure of approx 1.45 (i.e. 100^2/83^2 = 1.45, hence 45% overload).  By definition this is somewhere between LIMIT and ULTIMATE load levels, hence some detrimental deformation may have occurred.



The tg Owners Manual states "Vfe Maximum flap extended speed is the highest speed permissible with the flaps in the prescribed extended position".



PoH states Vfe as 83kts.  Vfe (max speed for extending/retracting flaps) implies the actuator is the weak link. Vf would be a flap limiting speed.



You have done exactly the right thing, you have inspected the system for  deformation.



It is likely that no damage has occurred at all.  



If damage has occurred, the Europa actuator end attachment holes may have ovalised very slightly (rattle tested?) but I suspect the flap end-pin and bearing (and the attachment into the flap) are 

over-engineered and will be unaffected.



Maybe you could add an additional disassembly/inspection to the flap actuator end fittings when the aircraft is under it's next scheduled maintenance.



I suspect Bud may also have some useful input!



Regards

John

_________________
John Wighton
Europa XS trigear G-IPOD

JonathanMilbank

Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area

Posted: Tue Jun 01, 2021 11:56 am Post subject: Re: Flap limiting speed 83 kts exceeded

John,



Thanks for your prompt response. If by "actuator" you're referring to the electric motor which I believe was originally from a Ford car window motor, I'm happy to report that I don't have one. My aircraft was converted by the late great Neville Eyre from monowheel to tri-gear. I took his good advice and retained the cockpit lever which is now only connected to the flaps; this also saves weight and complication.



So having checked everything very carefully and also the flap push-pull rod connected to the lever, I'm quite confident that no damage seems to have occurred.



Thanks again.

Jonathan

budyerly@msn.com

Joined: 05 Oct 2019
Posts: 281
Location: Florida USA

Posted: Tue Jun 01, 2021 5:00 pm Post subject: Flap limiting speed 83 kts exceeded

Jonathan,



I'm tied up at the moment and you will all be relieved for a short and fatigue influenced answer on flap over speeds.



John W gives some sage advice.  Inspection after any overspeed or overstress is the key to continued airworthiness.  Pull the bracket covers, open up the fuselage D tube and inspect all attachments.  Look for cracked glass, delamination, and bolts that have elongated.



Some analysis from the top of my very tired head.



In the US the Vfe is supposed to be the speed at which maximum design G can be loaded momentarily with the flaps extended.  This is an iterative problem.  Once Vfe is set or calculated, then there is another requirement that the V speed must allow for a 1.3 times overspeed (per se) without exceeding design structural limits.  That was always concerning to me in my studies.  The flap is an airfoil that is fixed to the wing of course and the maximum speed of a flap setting has to be at the limit times the safety factors.  Therefore, we see in many aircraft a Vf1, 2 etc. based on the increment of flap extension speed limits to address the process.  So, we see Vf1 of 10 degrees is highest and 20 degrees a little slower, and 30 degrees is quite low.  The Europa was designed for one flap setting.



It is interesting that the flap drive of the Airbus style externally hinged flap, that our little Europa has, actually takes very little force to extend and retract.  (The plain flap extension load is horrendously large in comparison.)



These hinged flap designs were brilliant and worth the drag of the brackets.  This extension /retraction force is evident in the mono where extension and retraction between 60-90 KIAS has virtually no difference in gear extension effort.  Yes, the brackets are draggy, but the loads are low and the flap efficiency is quite good.  One builder even installed a manual flap extension handle (although I don't know who it was,  he was a Flight Crafters client.)  Aerodynamically, the pitching moment, lift and drag of the flap is calculated and the hinge point of the flap and the drive pin are set to reduce the load on the drive mechanism, simplify the gap flow accomplishment, and simplify construction.  All brilliantly executed.  As a result, I don't worry as much about the flap drive, I worry more about the flap attach brackets in an overspeed.



Looking at the flap sum of forces for this type flap,  The force on the flaps when extended to nominally 27 degrees is basically a slotted flap with a lift coefficient of somewhere around 2.0 assumed.



So, the force of lift vertically on the flap is Cl*1/2* (density)* V squared in fps nominally at 83knots and a roughly 7.5 square foot flap is   (2x1/2x.00237x19643x7.5)=348 pounds of force upwards.  The drag on the flap will be roughly 7 pounds.  The pitching moment of the flap will be another couple of pounds.  The moments are summed through the flap mechanism attach point and the resultant flap drive pin position load is a small number.  Sorry, I’m not going to do actual calculations.  In a picture it kind of looks like this picture attached.  More math in public to follow:



The load at say 1.3 times 83 knot Vfe is nominally 108 knots.  Re-plugging numbers the forces go up significantly:  2x1/2 x.00237x33269x7.5 +590 pounds of force upwards.  If I load the aircraft to full G (3.8 Gs) at 108 knots, that 590 pounds times 3.8 or 2240 pounds.



So, some poor engineer will calculate all this out for the structural load.  Figuring 50,000 psi (working hand laid glass strength) with a glass thickness of .009, 4 layers, the glass to flox gap load bearing area of about 1.5 square inches (that flox hole in the bracket), and a one foot hinge moment arm in a shear mode (roughly half the tensile strength) means,  in extremely rough numbers a single bracket will hold 1300 pounds of force times three brackets or 3900 pounds.  Hence the flap attachment is in very rough terms (non exactly measured) about 3900/2240 or 1.74 times stronger than I need, which is greater than my safety factor of 1.5 over the 1.3 V of 108 knots (figuring it very roughly) in a max overspeed.  Again, very rough numbers

JonathanMilbank

Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area

Posted: Wed Jun 02, 2021 5:58 am Post subject: Re: Flap limiting speed 83 kts exceeded

Bud,



Thanks for taking so much trouble to consider this matter while you were so tired. You mention that "One builder even installed a manual flap extension handle", which is what I'm left with after Neville Eyre helped me with my conversion from mono-wheel to tri-gear. It's remarkable how easy it is to extend and retract the flaps, even without the benefit of bungees.

Paul M 383

Joined: 19 Mar 2012
Posts: 97
Location: United Kingdom

Posted: Wed Jun 02, 2021 8:52 pm Post subject: Re: Flap limiting speed 83 kts exceeded

 	  JonathanMilbank wrote: 	
		  One of our group members got distracted during climb out after take off, while the flaps remained extended to 18 degrees. <snip>" 	


Just curious - did you modify your flap drive mechanism to allow selection of 18 degrees as well as the specified 27, or instead of?



Paul

G-PLPM

XS Mono 912ULS

JonathanMilbank

Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area

Posted: Thu Jun 03, 2021 1:16 am Post subject: Re: Flap limiting speed 83 kts exceeded

Paul,



While my aircraft was a mono, the combined flap / undercarriage lever had the standard detents which held the lever either at fully retracted or fully down at 27 degrees. When I decided after almost 20 years flying as a mono to convert to tri-gear, Neville Eyre told me to support the aircraft in order to partly retract the gear and use a clinometer to measure an intermediate flap position of 18 degrees. While the flaps were at that angle, I leaned into the cockpit and marked on the slotted guide plate the position of the lever.



Neville later cut the guide plate in situ to create an intermediate 18 degree detent notch, which happens to lie approximately halfway along the slot. Thereafter we set about removing the mono-wheel swing arm, linkage and other unnecessary components to leave just the lever connected to the flap push-pull rod.



Why Neville specified 18 degrees is something I never questioned, but we use that intermediate setting for take off on the assumption that it reduces drag while still augmenting low speed lift. It also occurs to me that if there existed a graph or table giving different Vfe speeds for varying flap settings, then the Vfe for 18 degrees would be considerably higher than 83 knots; perhaps a lot closer to 100 knots.



That was a long-winded explanation where I could simply have replied 'as well as'.

Paul M 383

Joined: 19 Mar 2012
Posts: 97
Location: United Kingdom

Posted: Thu Jun 03, 2021 5:16 pm Post subject: Re: Flap limiting speed 83 kts exceeded

 	  JonathanMilbank wrote: 	
		  <snips> ...create an intermediate 18 degree detent notch, which happens to lie approximately halfway along the slot...



Why Neville specified 18 degrees is something I never questioned, ... if ... different Vfe speeds for varying flap settings, then ... Vfe for 18 degrees would be considerably higher than 83 knots; perhaps a lot closer to 100 knots.



That was a long-winded explanation where I could simply have replied 'as well as'. 	




Yes it was (), but it answers my question, thanks.



Might 18Âº be the takeoff flap setting for a Trigear? I don't have one but you do and I guess it's in the Owner's Manual? This might then answer your question (on Vfe for 18Âº) ... if you read Bud's reply and do some math(s).



HTH



P



G-PLPM

JonathanMilbank

Joined: 14 Apr 2012
Posts: 384
Location: Aberdeen area

Posted: Fri Jun 04, 2021 1:48 am Post subject: Re: Flap limiting speed 83 kts exceeded

Paul,



You're dead right. I logged into the Europa Club's website and found in the POH on page 5-6 under Pre-take off check list (vital actions) this: Flaps Check 18Â° down. On page 2-1 under Limitations, all that is shown for flaps is Max. Flap extension speed (Vfe ) 83kts.



Being a bit facetious, the 'Max' bit might be understood as max. flap extension is allowed up to a limit of 83kts, implying that lesser flap extensions could be performed at speeds higher than 83kts. Weasel words and thoughts!



However my math abilities don't extend to calculating a Vfe for 18 degrees extension, but thanks anyway for your involvement.

ptag.dev(at)talktalk.net
Guest

Posted: Fri Jun 04, 2021 1:23 pm Post subject: Flap limiting speed 83 kts exceeded

Hi! All.

I recall that flap setting for take off on trike G-PTAG was 18Deg.

Regards

Bob Harrison.   



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