Mühlbauer Constant Speed MTV-1-A/L160-03 Prop Mystery!

Nobody will just open his Mühlbauer Prop for fun, just to know how it looks like inside! Well, it was an opportunity to get a broken prop to try it!

Just look at the pictures:

Picture 1: this is how it looks like when you take off the spinner.

Picture 2: taking off the cables and the electro motor.

Picture 3: Motor is off, now open the cover. The cover comes up when unscrewing with a screwdriver.

Picture 4: opening the case, you discover the magic of this system!

Picture 5: take off the top ball bearings, the drive (Spindel) and the leading tubes.
Now the bronze cube is now ready for extraction. In order to get it up you must rotate the 2 blades by hand. This is a quite sophisticated cinematics. Compliment to Mr Mühlbauer who developed this system. Don´t ask me now how I did, but in any case I managed to get the bronze block out of the hub!

Picture 6: After extraction of the bronze block, let´s take out the blades.
You must remove first the outside spring ring maintaining the blade in its position.

Picture 7: Then you push the blade inside the hub and the bearing balls get free.
Just pick them up with a grip device.

Picture 8: when all the balls are extracted, the blade is free and you just pull it blade out of the hub.

Picture 9: here you see the inside ball bearing cage. The cage is broken in 2 halves to enable the insertion into the hub. The outside ball bearing cage is pressed inside the hub.

Picture 10: Here you see how the inside ball bearing half is set in, locking the blade against the outside bearing cage.

Picture 11: Here you see the fat-free foot of the blade, the anchor-screws maintaining the wooden blade in its shoe and the pin enabling the rotation of the blade inside the hub. Look at the inside ball bearing cage pressed in the hub.

Picture 12: here you see how the system works. The spindle rotates through the electromotor, driving the bronze block up or down.
The black semi-cylinders are stuck in the pins of each prop blade which force them to a rotation. The rotation of the blades forces the black half-cylinders to slide along in their cage. The pitch of the prop is controlled by a rev meter monitoring the electro motor according to a pre-set engine rev. setting.

Assembling: This is indeed the same work the way back but there are 2 difficult steps.

  • Inserting the balls in the cage: I tried several times alone and did not manage. With a second person holding the blade and rotating it carefully and using enough fat to stick them, it was then easy to insert all the balls again.
  • Inserting again the bronze block with its 2 half black cylinders. We had to try several times to find the correct position in oder to slide the complete system into the hub. It looks difficult but after a few trials you find the way !

Now you know how to dismantle and re-assemble your MT constant speed prop by yourself. I did not take off the wooden blades out of their shoes because this is only possible by destruction!
You can do it you have somebody who can manufacture new blades and is able to insert them with the necessary symetry.

Remark: The Mühlbauer construction has no ajustment device. This means there is no way to adjust the ball bearing pressure. The centering of the blade occurs through the centrifugal forces on the outside ball bearing cage.
The black O-ring on the blade helps to maintain it in its position and avoids the fat to be ejected.
Therefore there is no way to make any assembling mistake. Whether you manage to rebuild the system without using a big hammer and it will work again … or you do not manage and you hang your prop over your club bar!

Patrick Faucheron
CFI Germany – December 2002

Check your Prop Bolts!

Yesterday 2 friends landed back home after a 5 hours flight with an RF5 with a fixed pitch wooden Prop.
The engine made a very unusual noise during taxi. The crew did not notice any thing in the cockpit.

A bit later, another pilot takes off with the RF 5 for a sightseeing flight. Nothing special with the engine!
Only the people on the ground wonder about the abnormal noise, like an old diesel tractor!

Again, after landing everybody looks at and listens to the engine like a doctor: everything seems OK but the drumming noise is frightening! Probably a rod bearing damage or maybe a main crankshaft bearing damage!
Very unusual, but it can happen!
The engine was taken down for an emergency delivery to the clinic!

After removing the prop, what a surprise! 3 propeller bolts out of 6 were broken and the other 3 were loose in spite of the safety wiring!
The mechanic couldn’t believe his eyes! He re-installed the 18 months old prop last April, using a torque wrench (with 18 N/m torque) and secured the bolts as requested by the maintenance manual.
In spite of that, 3 bolts sheared within a very short time. Look at the picture, we are now at the end of August 2002! There is only one explanation: during the past 8 weeks with very hot weather, the prop wood shrunk and the bolts got lose until 3 out of 6 of them broke within a few flight hours!

It was a lucky chance that this RF5 did not lose its prop in flight!
It is not a bad idea to check the torque of the wooden prop screws regularly, especially in hot summers!

Germany 23. Aug. 2002

Sauer 2500/92 CV mit Mühlbauer MT 155/105-1A fix Propeller

SAUER 2500 ST et hélice à pas fixe MT 155-105.

Il faut noter une autre certification intéressante, celle du RF5 avec moteur Sauer 2500 / 92 CV avec une hélice à pas fixe Mühlbauer MT 155/105-1A. Ce moteur était déjà certifié mais uniquement avec une l´hélice Mühlbauer à pas variable hydraulique, donc coûteuse et lourde. La question „sine qua non“ était de remplir les conditions d´émission sonore très sèvères en Allemagne, puisque le nouveau réglement pour tout nouvelle certification exige la norme ICAO chapter 10, moins 5 dbA. Ceci n´est possible qu´avec un très fort taux de montée d´au moins 3,25 m/s en pleine charge , donc un moteur puissant à bas régime et une hélice silencieuse (ou bien un moteur avec réducteur comme le Rotax).

Donc, les conditions pour le RF5 de ne pas dépasser la limite de 65,6 dbA ont été remplies de justesse. Le certificat d´abattement de bruit n´est plus un luxe car sans quoi, les taxes atterrissage deviennent exhorbitantes (Speyer 12 Euro sans certificat). Certains aérodromes sont interdits aux avions sans certificats ou bien sujets à des restictions le week-end (interdiction de décoller entre 12 et 15 heures).

Les autres avantages de cette certification: un moteur très robuste car basé sur le carter du VW refroidi par eau ( dénomination Wasser-boxer) et non pas sur le VW 1600 du Mexique en magnésium souvent l´objet de criques de carter prématurées. Le 2,5 L pèse 6 kg de plus mais l´hélice en bois pesant elle même 8 kg de moins qu´une hélice à pas variable, l´ensemble est même moins lourd. Fini les casse-têtes des RG périodiques d´ hélice hors de prix ou les coûteuses réparations fréquentes sur les hélices constant-speed électriques.

Les performances sont inchangées : en croisière le RF5 file à 180 kmh à 2700 tours et 21 inch de PA avec une consommation de l´ordre de 14 litres. Donc un compromis raisonnable et surtout financièrement abordable. Un conversion avec Sauer 2500 et hélice en bois ne dépassera pas 15.000 Euros. Cette certification devrait être homologuée d´ici fin Octobre le dossier étant déjà au LBA.  (2002)

It is necessary to note another interesting certification: the RF5 with the Sauer 2500/92 CV engine with a fixed pitch Mühlbauer MT 155/105-1A propeller. This engine had already been certified, but only with one of the Mühlbauer hydraulic variable pitch propellers, which is expensive and heavy. The „sine qua non“ condition was to meet the severe German noise level limitations, since the new regulation for all new certification is set at 5 dBa below ICAO annex 10 standards. This is only possible with a very high rate of climb, of at least 3,25 m/s at full load. This means a powerful engine at low RPM and a quiet propeller (or an engine with reduction, like the Rotax).

Therefore, the conditions for the RF5 not to exceed the limit of 65,6 dBa were barely met. The certificate of noise abatement is not a luxury, for without it, the landing fees become exorbitant (in Speyer, 12 Euro without certificate). Certain aerodromes are prohibited to planes without certificates, or restricted during weekends (prohibition to take off between 12:00 and 15:00 hour).

Other advantages of this certification: a very robust engine, for it is based on the case of a water cooled VW (denomination Wasser-boxer) and not on the magnesium VW 1600 from Mexico, often object of premature case cracking. The 2,5 L weighs 6 kg more, but as the propeller weights 8 kg less than the variable pitch propeller, so the whole unit is actually lighter. Finished are the headaches of the periodic expensive propeller overhauls, or expensive frequent repairs of the electric constant-speed propellers.

The performance is unchanged. In cruise, the RF5 slips by at 180 km/h at 2700 RPM and 21 inch of MP, with a consumption of about 14 litres, thus a reasonable compromise, and especially accessible financially. A conversion with a Sauer 2500 and wooden propeller will not exceed 15.000 Euros. This certification should be approved here around the end of October as the file is already with the LBA.

PF   2002


Rotax (912) powered RF 5


Le bruit courrait depuis un moment qu´un inconnu du CFI allemand avait installé un Rotax sur son RF5. Nous l´avons cherché et finalement trouvé. Son nom : Armin Taffelt habitant en Bavière dont la particularité est qu´il est le premier propriétaire se son RF5 D-KIHO depuis 1969 avec le privilège de posséder une petite piste privée. Or les arbres en bout de sa piste, de seulement 300 métres, poussaient de plus en plus haut si bien que le Limbach 2000 ne suffisait plus pour un décollage en toute tranquillité… Devenu retraité, Armin a décidé en 1999 de remotoriser son RF 5 avec un Rotax 912-A et une hélice Hoffmann hydraulique HO-V352 F-S1 / pales S170FQ. Le suivi technique de la conversion a été fait par le Landesverband Bayern (ce qui correspond à la FNA de Bavière).

Le bâti-moteur, en berceau assez sophistiqué, a été construit par un atelier homologué LBA ce qui lui a coûté 5000 Euro. Le moteur est légèrement avancé pour permettre l´installation des récipients de refroidissement d´eau et huile. Tous les accessoires sont d´origine Rotax sauf le système d´échappement fait par Heggemann et adapté au RF5. Les capots ont été entièrement refaits. Ils sont un peu moins larges mais le capot inférieur a un ventre comme un SF 25 „Falke“ avec un gros trou circulaire pour l´aération des radiateurs situés sous le moteur. A premiére vue il aurait été possible d´installer le Rotax 10 cm plus haut et ceci aurait évité le ventre nécessaire pour cacher le radiateur et l´échappement. La forme typique des capots RF n´est plus reconnaissable, donc c´est une question de goût. On aime ou on n´aime pas !

Le KIHO vole avec un CDN provisoire depuis Septembre 2000 et a effectué à ce jour 62 heures. Armin indique 100 mètres pour le décollage, 190 kmh á 4800 Tours avec une consommation de 15 litres à l´heure. A 5600 tours ( plein gaz) le RF5 atteint 220 kmh. Le décollage simultané avec 2 autres RF5 a témoigné un taux de montée extraordinaire d´au moins 4 m/s. Le gros problème restera la certification. Il va sans dire que ceci prendra encore beaucoup de temps et coûtera quelques Euros … Armin est le propriètaire de la construction du bâti-moteur (homologué) des moules et des capots. Il s´agit là sûrement d´une innovation très intéressante car le Rotax 912 a fait ses preuves de robustesse et fiabilité, mais seulement à partir du moment où la conversion sera certifiée. Quant aux coûts Armin ne se prononce pas mais il faudra bien compter au moins 25.000 Euros !

As the word spread that someone of the German CFI had installed a Rotax engine on his RF5, we snooped around and finally found him. His name is Armin Taffelt, living in Bavaria, whose particularity is that he is the original first owner of D-KIHO RF5 . He owns it since 1969, and has the privilege of having a small private runway. The trees at the end of his field, only 300 m „short“, kept growing taller and taller, to the point that the Limbach 2000 was no longer strong enough for an easy takeoff… When he retired in 1999, Armin decided to remotorize his RF 5 with a Rotax 912 TO and a Hoffmann hydraulic propeller with a HO-V352 F-S1 hub and S170FQ blades. The technical follow-up of the conversion was made by the Landesverband Bayern (what corresponds to the FNA of Bavaria).

The engine mount, a quite sophisticated cradle, was built by an LBA certified shop, and cost 5000 Euro. The motor is displaced slightly forward to allow room for the installation of the coolant and oil tanks. All accessories are Rotax’s original except for the exhaust system, which was made by Heggemann and adapted to the RF5. The cowling had to be entirely redone. It’s a little smaller but the lower half has a belly like a SF 25 “ Falke “ with a wide circular hole for the ventilation of the radiators, located under the engine. Apparently it would have been possible to install the Rotax 10 cm higher, and it would have avoided the belly to hide the radiator and exhaust. The typical shape of the RF5 cowling is no longer recognizable, therefore it’s a question of taste: Either you like it or you don’t!

D-KIHO flies with a temporary Airworthiness Certificate since September 2000 and has flown 62 hours to this day. Armin quotes 100 meters for the takeoff, 190 km/h at 4800 RPM with a consumption of 15 litres an hour. At 5600 RPM (full power), it reaches 220 km/h. A simultaneous takeoff with 2 other RF5’s proved an extraordinary rate of climb; at least 4 m/s. The biggest problem will remain the certification. It goes without saying that it will still take a lot of time, and will cost a few Euros… Armin is the owner of the certified motor mount structure , the molds, and the cowling design. It is indeed an interesting innovation, as the Rotax 912 has made quite a name for strength and reliability, but it will be feasible only from the moment the conversion is certified. As for the costs of the conversion, Armin’s isn’t saying, but it will be necessary to count with at least 25.000 Euros!