Jonathan,
I'm tied up at the moment and you will all be relieved for a short and fati
gue influenced answer on flap over speeds.
John W gives some sage advice. Inspection after any overspeed or overstres
s is the key to continued airworthiness. Pull the bracket covers, open up
the fuselage D tube and inspect all attachments. Look for cracked glass, d
elamination, 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 probl
em. 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 maximu
m 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 exte
nd and retract. (The plain flap extension load is horrendously large in co
mparison.)
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 exten
sion effort. Yes, the brackets are draggy, but the loads are low and the f
lap efficiency is quite good. One builder even installed a manual flap ext
ension 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 i
s calculated and the hinge point of the flap and the drive pin are set to r
educe 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 b
rackets in an overspeed.
Looking at the flap sum of forces for this type flap, The force on the fla
ps 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 square
d 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 wi
ll be roughly 7 pounds. The pitching moment of the flap will be another co
uple of pounds. The moments are summed through the flap mechanism attach p
oint and the resultant flap drive pin position load is a small number. Sor
ry, I=92m not going to do actual calculations. In a picture it kind of loo
ks 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 pound
s 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 thick
ness 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 momen
t arm in a shear mode (roughly half the tensile strength) means, in extrem
ely rough numbers a single bracket will hold 1300 pounds of force times thr
ee brackets or 3900 pounds. Hence the flap attachment is in very rough ter
ms (non exactly measured) about 3900/2240 or 1.74 times stronger than I nee
d, which is greater than my safety factor of 1.5 over the 1.3 V of 108 knot
s (figuring it very roughly) in a max overspeed. Again, very rough numbers
.
If I simply up the load the flap area and stuff and multiply by 1.5 times 1
.5 (the super factor the Europa is designed to) based on the normal 83 knot
forces I'm sure it will come out similarly .
So, the extremely rough numbers give me a ball park confidence that althoug
h I am over speeding my flaps, and must inspect my aircraft attachment poin
ts I will concentrate my inspection on the flap bracket attachment to the f
lap and the wing attach points first. Then I will do a retract test and ma
ke sure my flap rig is correct (bent flap tube is a possibility), then insp
ect my flap drive attachments/welds/glass supports to assure that they are
still sound.
Just my thoughts. Don=92t overspeed the flaps if you can help it. I have
had a number of demo/checkout flights inadvertently extend flaps in the 90
Knot range for the last 15 years on poor old N12AY because they just don=92
t realize how clean the plane is. In a 914 if you don=92t make a very larg
e power reduction from the climbing crosswind to downwind level off, you hi
t 90 Knots pretty quick. Failure to monitor airspeed in the final turn can
build speed rapidly also even with full flaps. I can=92t tell you how many
secondhand owners end up crossing the threshold still doing over 83 Knots.
If you make the speed offender do the inspection panel and cover open up
and then put it back together, they pay attention to the airspeed more clos
ely.
Stay within the limits. IF you exceed them, inspect and repair as necessar
y. Don=92t just rattle the flaps and blow it off.
Best Regards,
Bud Yerly
-----Original Message-----
From: owner-europa-list-server@matronics.com <owner-europa-list-server@matr
onics.com> On Behalf Of John Wighton
Sent: Tuesday, June 01, 2021 4:12 AM
Subject: Europa-List: Re: Flap limiting speed 83 kts exceeded
john@wighton.net>>
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 relevan
t.
I am not speaking for Europa Aircraft (current or past) or for the Club, ho
wever, generally, the flap limiting speed is determined having conducted a
stress analysis of the whole system. The failure mode that causes the firs
t failure or lowest reserve factor (RF) is associated with a speed within t
he V-N envelope. A suitable factor is applied (which in any case will incl
ude 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 disman
tled items). For composite structure failures, there are yet more factors
that take into account the environmental influences (e.g. temperature, humi
dity) and variability of build standard.
Regarding loads, as air pressure is proportional to the square of the airsp
eed the loads can rise very quickly. For your overspeed occurrence, the fl
aps 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 a
nd ULTIMATE load levels, hence some detrimental deformation may have occurr
ed.
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) im
plies 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 d
eformation.
It is likely that no damage has occurred at all.
If damage has occurred, the Europa actuator end attachment holes may have o
valised very slightly (rattle tested?) but I suspect the flap end-pin and b
earing (and the attachment into the flap) are
over-engineered and will be unaffected.
Maybe you could add an additional disassembly/inspection to the flap actuat
or 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
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