Re: [ULF-1] Why is Vne 80 km/hr

From: Member 9
Date: 2011-05-14 22:13 UTC+01:00
Message: #0268

Dear all,

the cambered WORTMANN high lift wing section behaves almost
like a symmetrical airfoil with a plain - down deflected - trailing edge
flap. It produces a rather high (zero lift) nose down pitching moment, which
increases with velocity squared, causing high torsion loads on the wing 
D-box.

>From about 70 km/h onwards, the airflow on the lower side of the wing
separates, which leads to considerable performance losses, simultaneously
reducing downwash angle and dynamic pressure in front of the horizontal
stabilizer. The latter can't do its (complete) job anymore and needs the
help of the (up deflected) elevator, thus resulting in further performance 
losses
and rapidly climbing control stick forces.

Both, torsion strength of D-box and stick force, limit the ULF-1 "never
to exceed speed" (VNE) to 80 km/h. This value includes a 10 percent speed
margin, and a safety factor of 1.5 against structural failure.

Regards,

Dieter





----- Original Message ----- 
From: <[email]>
To: <[email]>
Sent: Friday, May 06, 2011 7:51 PM
Subject: Re: [ULF-1] Why is Vne 80 km/hr


>
> Hi Richard,
>
> I think you may have misunderstood the polar curve.  It
> shows the sink rate through the speed range, rather than absolute
> values for lift.  This enables glide angle to be determined at various
> speeds (and head/tailwind components too by shifting the origin) and is
> useful for speed-to-fly and so forth.  Forgive me if I'm patronising
> you here - but maybe I misinterpreted your meaning.
>
> Flying fast, the
> wing will generate a lot of lift, causing a tendency to raise the
> nose.  This needs to be countered by lots of down elevator.  So with
> increased speeds come increased loads on the wings and tail (and hence
> the bit that holds them together too).  I would guess that the tail may
> be the weakest link here.  There is also the danger of flutter, which
> would not go well on a light, stiff structure.
>
> As to the specific
> value, then yes - Dieter will have a far better answer.
>
> Member 62
>
> P.S.
> Hope your build is going well.  I'm just waiting for my next Member 60 order
> to be delivered ...
>
>
>
> I think that this may be a question for Deiter,
> can anyone please explain what limits the VNE to 80 km/hr? The aircrapt
> is well stressed to stand +6g -4g.
> The very slow airfoill section
> begins to lose lift above about 40km/hr, does this indicate the
> possibility of a high speed stall at some speed well above the 80 km/hr
> Vne? In other words what is the Vne safety margin and what determines
> Vne for the ulf-1?
>
>
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