3 Blade prop on a 'B' Box ????

[Cee Vee Ess]

Guys, I have seen a couple of trikes that have a 3 blade prop on a 'B' Box. When I wanted to do the same a while ago, the supplier would not even help me because I told him I have a 'B' Box and he said I need a different gear ratio to operate with a 3 blade. So the big question here is..... why??
So if it is not recommended to have a 3 blade on a B-box, why do some guys have it??
What is the ruling with how many blades you have for a specific gear ratio?

[extra300s]

I allso have seen quite a few trikes with 3 blade props on their 2.58:1 ratio gearboxes. Who can give us a good answer concerning this?

Thanks
Karl

[Cee Vee Ess]

46 views and no advice from anyone yet? Does anyone have any comments please??

[justin.schoeman]

A quick google got the following:

The major difference in these drives are their "weight ratings." That is do you need a 1/2 ton truck, a 1 ton truck? An A or B drive can handle up to 3,000 kg cm2 of mass weight inertia. The C and E drives can handle 6,000 kg cm 2 of mass weight inertia.

So you have to find out how much your "truck load is" - Rotax Service Information 11 UL 91 explains the process - in real life I have found that a 3 blade IVO, 3 Blade GSC, 3 Blade Powerfin, 4 blade Ultra prop up to 66 inch can be loaded into the "1/2 ton" A or B box - with the Warp Drive 3 blades being too heavy.

Two blade props by the above manufacturers can be used up to 68 inches, if in doubt contact your manufacturer!!!!

From http://www.ultralightnews.ca/articles/d ... ection.htm

[Cee Vee Ess]

And in English????

I thought it had something to do with the speed the blades are rotating at?

[Duck Rogers]

a 3 blade IVO, 3 Blade GSC, 3 Blade Powerfin, 4 blade Ultra prop up to 66 inch can be loaded into the ("1/2 ton") A or B box - with the Warp Drive 3 blades being too heavy.

And in English????

Looks like plain English to me.
Which part don't you understand?

[justin.schoeman]

I am not sure why speed would be a problem. With lower ratio gearboxes, the prop rotates faster, and to keep the tip speed sub-sonic you need a shorter prop - but I don't see why you couldn't use more blades?

In fact I have seen some designs with limited prop clearance spinning short 6 blade props on low ratio boxes.

As for mass moments - it is a measure of the weight of a prop, and it's distribution. The heavier the weight of a prop, and the further that weight is distributed from the center of the prop, the higher the mass moment is. Basically, a lot of weight in the hub has almost no effect on the mass moment, as it is very close to the center. Blades however have most of the weight towards the mid-point, and tips, and add a big chunk to the mass moment. And each additional blade adds even more.

The mass moment basically tells you how difficult it is to accelerate/decelerate the prop. The higher the mass moment, the more force it takes to accelerate the prop, so on each power pulse of the engine, more torque must be transmitted through the gearbox. So the mass moment of the prop determines how strong the gearbox must be built.

So the C and E boxes are twice as strong as the A and B boxes - which is probably the main reason why they don't recommend a 3 blade prop.

[Cee Vee Ess]

If the prop allows - you have a finer pitch set on the blades then the total force is reduced, is it not?
Still scratching my head!!

[justin.schoeman]

Try push an uno along a level road - it does not take much force to get it moving, or to stop it again. Now try the same thing with a Landcruiser. You will curse and swear, and eventually get it moving.

The same for props - weight matters.

The prop does not turn at a perfectly constant speed. With each power stroke from a piston, it accelerates a little, and then slows down slowly until the next pulse. The 'heavier' the prop, the more force it takes to accelerate it each time - and the more force is transmitted through the gearbox.

Also, with a spinning prop, there are also gyroscopic forces. The 'heavier' the prop, the bigger the gyroscopic forces.

(I put 'heavier' in quotes, because with rotating bodies, it is not just the weight, but the distribution of the weight that is important.)

Most engine manufacturers don't place hard limits on prop diameter, or prop pitch. When the specify a prop, they will give max RPM, max mass moment those are the two factors that determine the strength of the crank/gearbox.

[Cee Vee Ess]

Quote ----
Few things are as underrated and misunderstood about small aircraft propellers than MOMENT-OF-INERTIA. Low moment of inertia is critical to the safe, smooth, and long lasting operation of your two-stroke engine and reduction unit. Inertia, by definition, is the reluctance of a mass to accept changes in velocity. When your engine is idling the pulse tries to accelerate the prop and the compression tries to decelerate it. High inertial loads in this situation increase compression and hinder the stroke causing many engines to idle poorly. Most people fiddling with timing and fuel mixture trying to solve this problem never suspect high MOMENT-OF-INERTIA to be the culprit. High MOMENT-OF-INERTIA in a propeller is also bad because the propeller becomes a more effective gyroscope with a higher tendency to remain fixed in space.

This has three effects.

It puts more pressure on the bearings in the final stages of your reduction unit as you turn your aircraft in flight.

It causes more adverse turning tendencies due to gyroscopic precession.

It reflects vibration that can be felt by the pilot instead of absorbing and dissipating it.

Low MOMENT-OF-INERTIA in a propeller also means faster access to your available horsepower, reducing spool-up time when you hit the throttle. In the right flying situation a POWERFIN Propeller can pull you out of a jam faster than a propeller with high inertia.

Low MOMENT-OF-INERTIA in a propeller means faster spool-DOWN time, letting a CLUTCHed prop stop sooner!

Rotax has set a limit for how much inertial mass is allowable on the B, C & E Gearboxes. On the A/B-box they want propellers with no more than 3,000 kg/cm2. A 72" – 3-blade Ivoprop Ultralight propeller is about 4,500 kg/cm2. The same Warp Drive is over 10,000 kg/cm2. POWERFIN Propellers are 2,500 kg/cm2 for a big three-blade model and 1,800 kg/cm2 for a two- blade model. POWERFIN is the only U.S. manufacturer to have addressed this issue and met the challenge of designing a very durable, low inertia propeller for the Rotax gearboxes. ------- Unquote.


Still digging... I am busy liaising with a guy called John at our local Ivoprop distributer. Trying to get the inertia rating on a 'Light Ivoprop' for a specific length.
Seems like Powerfin prop is a very light prop and is most suited so far for a 'B' Box.

[extra300s]

Very interesting. Great stuff CVS

[Cee Vee Ess]

Yes it is indeed..

Apparently Ivoprop do not publish their moment of inertia figures because apparently their largest market of props are over the limits of the gearboxes they are mostly used on and say that those have had many hours on the gearbox.
I am told that I must calculate the moment of inertia myself and apparently it is quite easy.
Has anyone done this before -- where do I find how to do it??
Has anyone got any data on a 64 inch Light Ivoprop???

[Cee Vee Ess]

I have emailed a different Ivoprop supplier in SA, a guy names Dale at Alpi Aviation. He is very helpful and he mailed the manufacturer in the US to find me some answers.
Turns out that the UL Ivoprop is widely used on 'B' boxes in the US. The prop is at the limit of the gearbox and is use able on a 'B' box. Being at the operating limit and having higher gyroscopic forces while in a turn does put some strain on the torsion shock absorber in the gearbox.
I don't feel 100% comfortable using a prop on the gearbox at the limits???
So now to compare, does anyone know what the moment of inertia forces are of a normal 2 blade wood prop like a P-prop or a Geo Killey?
Below is the email I sent and the reply from the manufacturer::


+++++++++++++++++++++++++++++++

Dale, I have a Rotax 503 with a 'B' Gearbox. 2.58:1 reduction. This gearbox
can only handle up to 3000kg / cm2. I am looking for a 3 blade ground
adjustable propeller. I like the ivoprop for it has a very quick adjusting
mechanism. I need to know if the ivo ultralight propeller is not too 'heavy'
for my gearbox. Thanks
Laurence

++++++++++++++++++++++++++++++++++++++++++++++

These moment figures do not take into
consideration the fact that our
propellers are flexible. The flex absorbs
alot of these forces. There are over
40,000 Ivoprops on Rotax B boxes, many up
to 3 blade 72 inch diameter with no ill
effects. Also there are a lot of 4 blade
62-64 inch diameter on powered parachutes.
Our 3 blade Ultralight is lighter than
other 2 blade composite propellers.

ROTAX SPECIFICATIONS
'B' BOX 3000 KG X CM2 (CENTIMETERS SQUARED)
'C' BOX 6000 KG X CM2

3000 Kg cm2
UR 3 blade 62
UR 2 blade 72

RIGHT TURN VIEWED FROM THE REAR:
UR 3 BLADE 62" = 3000 Kg cm2
UR 2 BLADE 72" = 3000 Kg cm2
LEFT TURN VIEWED FROM THE REAR:
UL 3 BLADE 64" = 3000 Kg cm2
UL 2 BLADE 72" < 3000 Kg cm2

[Cee Vee Ess]

Spoken to a propeller maker in the meantime and he has a very easy way of telling if the prop is too heavy for the gear box.
For a 'B' box your prop should not weigh more than 3.5kg and for a 'C' or 'E' box your prop should not weigh more than 4.2kg.
And something interesting. to determine your propeller tip speed:

Rev of prop x diam of prop (inch) x 0.00436 = tip speed (ft/s)
your propeller tip speed should not be more than 761 ft/s on a composite prop and not more than 815 ft/s on a wood prop.

The above is all his advice.

[Wargames]

your propeller tip speed should not be more than 761 ft/s on a composite prop and not more than 815 ft/s on a wood prop.

Reason?? Surely he should mean speed of sound, because only then does the aero properties change.