Magni Rotors - the secret revealed...

Learjet

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Cape Town, South Africa
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Magni M16 Magni M22
It's been interesting (and amusing) to read all the comments and speculation from the various threads about Magni rotor blade construction...

Here is a pic of a cross-section.

The blue section is foam, the brown "Bakelight" looking section is the composite, and just behind the leading edge is triangular shaped lead.
 

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OK, but this is just half of the story (thanks for that, anyhow)

Does this buildup vary throughout the length of the blade or does it remain the same?

Kai.
 
Hi Kai !

Was askin´this when I was down at their factory. Wouldn´t tell me or anybody else. It´s a "company secret". Too little Magni-rotors are destroyed in accs and when, not analyzed throughout the length of the blade, section by section.

But better keep the secret and have no accs instead of having so many accs that the blade structure isn´t a secret any more. :D
 
...But better keep the secret and have no accs instead of having so many accs that the blade structure isn´t a secret any more. :D

Seems to me that all that's required to "lift the secret" is to cut up one blade at various positions along its length. Since apparently LearJet already has one section and this blade is obviously destroyed, why not make a couple of others using the same blade?

-- Chris.
 
Did no one look at the Magni cutaways that Xavier Averso posted here?

Birdy, the Magni and the MT03 are night and day when it comes to control authority! Even though they might share the general looks, there's BIG difference when you fly them. I bet the MT03 comes much closer to your taste of an agile flying machine.-- Chris.

Herein lies the night and day difference.
 

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Chuck
Which blades are you comparing? The bottom one looks like a Magni, I thought the MT blades were extruded! I couldn't find the Xavier Averso post although I remeber reading it.

Mike G
 
Both are Magni blades, tip and root ends.

The Magni blade is overbalanced at the tip, the CG being ahead of the aerodynamic center. That provides tremendous rotor damping at the expense of a heavy stick and sluggish response.

It also tosses a monkey wrench into the theories of some people.
 
Is it the sluggish response that makes it safer ?. (Assuming it is actually safer, although
its track record strongly suggests it is.)
 
One might say it’s the sluggish response that makes it safer; that’s one aspect of rotor damping. It responds more slowly to pilot input and also more slowly to disturbances caused by gusts and whatnot. As the airframe tends to bobble around, the rotor maintains its orientation in space in and provides a resisting moment.

I wish I could lay my hands on the article Bensen authored on the subject of chordwise balance of rotorblades; I think his original article might have been reprinted in the book; “Collected Works of Design Classroom.”

In the article, he related his experiences with excess noseweights. He stated something to the effect that as overbalance is increased, the rotor becomes increasingly stable until a point is eventually reached where the rotor responds to nothing, including cyclic input by the pilot.

I have long suspected that Wing Commander Wallis stumbled on to the same thing.
 
The blue section is foam, the brown "Bakelight" looking section is the composite, and just behind the leading edge is triangular shaped lead.

Interesting indeed. Well worth blowing this picture up for a look.

Looks like a simple single-layer composite skin with a composite shear web between the two sections. UPDATE: I'm not sure about the shear web. That could just be a very heavy epoxy bond between the two fillers? Two different fillers used. I can't actually ID that brown stuff but it's not primarily structural IMHO, just gives shape and maybe, just maybe, adds to strength in shear. The blue stuff is the same foam everybody uses for moldless foam composite construction.

The nose includes both the triangular piece and three rods that are also presumably lead. UPDATE: Notice that the tip weight shown by Chuck -- Xavier Averso's picture, merci bien, Xavier -- has lead rods and the root weight doesn't. Is the Magni rotor variably balanced along its length? Magni acquired this technology from Jukka Tervamaki and continued to develop it.

Nothing unusual in this rotor design that I see. I'd like to see someone who's built a lot of composite rotors comment. Dennis Fetters, you out there?

My guess is that the skin might be thicker close to the hub. Those nose weights are partly for balance, but they may also vary along the width of the rotor, presumably to increase inertia. UPDATE: I was wrong about the skin but the other sections make me think I was right about the varying weights, but then wrong about the reason! The tip is not just rebalanced but very lightly weighted as rotors go. CarterCopter it ain't.

Thanks David, and thanks to the mystery man who took a bandsaw to his rotorblade.

cheers

-=K=-

UPDATE: I'd like to add that while Magni has a generally good safety record, there was one fatal involving a company ship a few years ago I've never seen explained. Has anybody got the report on that? Also, there are quite a few Magni prangs in the South African records -- it's a very popular machine there, and most of the prangs are pilot error.
 
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Hi Chuck,

Perhaps Arliss Riggs discovered the same thing when he mounted rebar all along the leading edge of his wooden rotor blades and then puttied over the same (forming the leading edge of the blade airfoil) before fiberglassing over to give cordwise balance to his blades.

I suspect that Boeing and Sikorsky "over balance" their blades too, partly for leading edge protection but also for cordwise balance.

Wayne
 
Wayne, despite the bib overalls, Arless Riggs was no dummy. It is my understanding he spent his spare time from gold prospecting haunting all the public libraries in Orange and LA counties studying aerodynamics.

A rotor carved from a solid plank will have its CG at ~40% of chord and is sure to flutter without noseweights that bring the CG to somewhere near ¼ chord.

SkyWheels rotors with CG well aft of the aerodynamic center may not flutter but become divergent at high tip speeds. I’ve seen them being run on a test stand where they abruptly go violently out of track at some rpm less than 600; I don’t recall the exact rpm.

Generally, professional helicopter rotorblades all are closely balanced at the aerodynamic center, near ¼ chord. Tip weights are added solely to increase blade inertia and are also positioned so as not to upset the chordwise CG.

Arless’ rebars may have been just enough to achieve ¼ chord balance.
 
sorry Chuck it was a stupid question, the answer is on the photo.
Mike G
 
Chuck
How can you conclude from the photos that it's overbalanced at the tip?
Mike G
 
Mike,

Aluminum has a density of ~165 lb/ft³ and epoxy/fiberglass has density of ~110 lb/ft³. Foam has a density of 2 or 3 lb/ft³.

The tapered spar makes the blade tips nose heavy.

Xavier had measured the CG and posted it here; I don’t remember the exact CG location at the blade tips but it was quite nose heavy.
 
Rappel pour Mike et Chuck,

Je connais vittorio Magni depuis plus de trente ans et j'affirme que cet homme est consciencieux et ces productions sérieuses, toutefois dans la pratique commerciale les Magni's ont tendance a laisser croire que seul leur produits sont bons..!!!
J'ai volé avec de bons rotors Magni et j'ai volé avec de mauvais rotors Magni ( beaucoup de vibrations )
Il n'y a ni secret ni magie, en 1996 j'ai eu l'occasion d'ausculter un rotor Magni , voici les données: Diamètre du rotor : 854 cm ; longueur de la pale : 420,5 cm ; corde : 22 cm ; épaisseur du profil : 27 mm ; poids de la pale :18,7 kg ; CG de la pale depuis le premier trous de fixation : 185 cm , le CG de la pale est a 44% de la longueur de la pale, le longeron est fabriqué en sifflet, d'une largeur de 9 cm a l'encastrement a 4 cm au saumon, au bord d'attaque il y a du plomb sur la totalité de la longueur de la pale sauf sur les premiers 30 cm de l'encastrement, a l'encastrement l'équilibre sur la corde est a 27% , au saumon l'équilibre sur la corde est a 24% , un rapide calcul du lest en plomb donne une masse de 3800 g.
J'imagine que depuis ce produit a évolué ..???
Pour éviter les erreurs des traducteurs informatique , j'ai fais traduire par mon ami Canadien Edwin Cox que je remercie ici.


Recall for Mike and Chuck,

I have known Vittorio Magni for more than thirty years and can state that this man is conscientious and his products are serious ones ; nevertheless, in commercial practice, Magni have the tendency of letting you believe that only their products are any good!!!
I have flown with good Magni rotors and I have flown with bad Magni rotors (a lot of vibration).
There is no secret or magic in this, in 1996 I had the opportunity of sounding out a Magni rotor, here are its parameters: Rotor diameter: 854cm ;blade length: 420.5cm ; Chord: 22cm ; Profile thickness: 27mm ; Blade weight: 18.7 kg; Blade CG from the first fixation hole: 185 cm; the blade CG is at 44% of the blade length, the spar is tapered, 9cm wide at the hub and 4 cm at the tip, the leading edge is lead weighted over the whole blade length except for the first 30 cm from the hub, at the hub the chord balances at 27%, at the tip the chord balance is at 24% :
A rapid calculation of the lead ballast gives a mass of 3800gr.
I imagine that this product has evolved since…???
To avoid automated translation errors, I have had this translated by my Canadian friend Edwin Cox and thank him for it here.
 
Perhaps no more than a coincidence but Jukka Tervamaki (Magni rotor designer) was a Bensen employee in about the same time period that Bensen was researching overbalanced rotor blades.
 
Merci Xavier I remember you posted this before.

Chuck Please correct me if I'm wrong (and I assume I am).

If I understand correctly Xavier's data the C of G at the tip is 2.079" (24% of 22 cm) back from the leading edge and the C of G at the root is 2.338" back from the leading edge. That's only 1/4" difference along about a 12 foot length of blade. Is this enough to say that the tip is overbalanced compared to the root?
Attached is a drawing I put on another thread showing my Magni blade. The C of G that I measured is within 0.3mm (12 thou) of a line drawn between Xavier's root and tip C of Gs. So Xavier and I seem to confirm each others data although we have different drawings and dimensions which might suggest that the design has changed.

Assuming that the aerofoil is 8H12 the centre of lift should be about 31% (about 2.75") back from the leading edge which means it's overbalanced at the root and the tip.

I would have thought that it is the variation in torsional rigidity along the length of the blade that is the biggest impact of the tapered spar rather than the difference in C of G.

I await your correction with interest.
Mike G
 

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The aerodynamic center of the NACA 8H12, Mike, is 26% of chord.

The CG at the more inboard sections is not very relevant because airspeed and lift are low.

It is critical at the outboard sections and is dependent on torsional characteristics. The shear stiffness of woven epoxy/fiberglass laminate is only about 1/5 that of an equal thickness of aluminum.

Certainly, a 2% forward location of the CG of extruded aluminum blades would make little difference but is the only plausible explanation for the very different behavior of Magni rotors compared to others. Unless you’re prepared to believe it’s due to “Harmony”.

But the bottom line is that we’ll not know with certainty until someone fits a set of Magni blades to an MT-03 or ELA or the reverse.

Here’s an article from an Australian site with some simple explanations of gyroplane rotors.

http://www.gyroplane.com.au/index.php?option=com_content&task=view&id=58&Itemid=72
 
Good morning Chuck,

First off, Happy Thanksgiving to you and your family.

Some of the most intelligent men with extensive engineering experience I have ever met, gained their knowledge from practical experience and not from a "traditional collegate education." Arliss Riggs is definitely one of those men.

Arliss' rebar method surely did bring his rotor blade balance somewhere near 25% cord, and as I think of it, serve double duty as a blade spar too.

This discussion is expanding my knowledge of rotor blade design and I thank everyone for all of your inputs.

Wayne
 
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