Re: [ULF-1] Vne for ulf-1 etc

From: Member 67
Date: 2011-05-12 16:19 UTC+01:00
Message: #0267
Thanks Member 62,

Sorry for the delay replying but work has been calling!



By no means patronising, I'm always willing to learn and please forgive me if I did not make my email very clear. As my son always says; "The sillyest question is the one not asked".

I was not looking at the polar curves for the airfoil but simply at the sink rate versus velocity for the whole aircraft. This tells me that assuming still air the aircraft has its lowest sink
 rate at ~40 to50 km/hr and obviously this is the most efficent forward velocity in steady air in order to achieve the longest duration; this actually occurs at 40 km/hr.The longest distance occures at 50 km/hr in still air assuming the same starting altitude. The best glide slope is nearly 13:1 but will depend upon the all up weight and quality of construction.

The real point of questioning the Vne is because I'm making my ulf-1 into a motor glider and I suspect that it would be quite easy to exceed the Vne if one was not very careful. You may be correct in stating that the tail is the weak link....at this stage I do not know but will look into this.The normal stress analysis methods give a goog feel for static loading but things get altogether more complicated for the calculation of dynamic behaviour in airflow.

The lowest speed that the wing (plus the rest of the aircraft and pilot) will fly is some where about 30 km/hr. however it is possible stall the wing at any speed depending on the angle of attack relative to the airflow and this would I
 suspect be more critical, i.e. prone to happen at lower angles of attack at higher speeds than if say a higher speed airfoil was chosen, hence my reference to "high speed stall". In conventional powered aircraft this can happen inafast tight turn where the outer wing can stall due to its angle of attack and higher velocity.

How is construction going?....a bit slower than envisaged but at the moment I'm working on the wing spars. Like so much of the construction I'm using a combination of Engineered Bamboo and Carbon Fibre. "Stronger and lighter is the motto"

Just for interest the engine I'm using is a ZDZ 420 B2....a flat four two stroke which weighs in at 9.6 kg and produces a maximum of 38 bhp (~200lbs static thrust). Calculations show that the aircraft will sustain flight with about 10 hp so I have plenty in hand without racing the engine, hence my concern over Vne.

Is there anyone else who has either powered their ulf-1 or intends to do so?.....I would like to hear from you.

Best regards,


Richard





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>Sent: Saturday, 7 May 2011, 18:35
>Subject: Ulf-1 Digest, Vol 34, Issue 2
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>   1. Re: Why is Vne 80 km/hr ([email])
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>Message: 1
>Date: Fri, 6 May 2011 19:51:38 +0100 (BST)
>From: "[email]" <[email]>
>To: <[email]>
>Subject: Re: [ULF-1] Why is Vne 80 km/hr
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>
>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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