Having recently succumbed to a ground-loop grimlen I bought a prop. The IvoProp web-site says this:

• If you wish to increase top speed or decrease cruise RPM, switch to a 2-blade prop or cut the 3-blade prop to smaller diameter using a hack saw.

I want to carry on using a 3 blade prop because they are quieter and because I think they look sexier (I am a bit vain). I believe that a two blade prop is more efficient in terms of power applied to thrust given, and even that that a one blade prop with balance weight might be more efficient still.

Wot's this stuff about a smaller diameter prop being 'faster' ???

UK Minimax lore says I need to cut my Ivo down from 62 inches to 60, that being the best compromise determined by the UK LAA. Should I cut it to 59.1" as the original Arplast Helice was?

Wot's the theory?

My hack-saw is in my hand....

Propellers are like guitars. The two identical ones in the music shop play completely differently. The engine has a lot to do with it. My cut down 60 inch by 43 was made for a 503 and cut down for a 447 worked pretty well but it took some experimentation. A 62" three blade is a good bit too big for a 447, I think and I would ask the question whether it is suitable for a B gearbox?

The propeller pitch x rpm calcs should give a ballpark speed figure. But in practice if you reduce the blade diameter, the engine speed will increase. So you could get an increase in speed because the rpm has gone up: with the same pitch the plane should fly faster.

But there's a lot of buts in that concept. When you reduce the prop diameter you reduce the prop disc area, so less work is being done at the same rpm, hence engine speed increases to do the same work (disc area x airflow). The increase in rpm may allow the engine to get 'on the cam' and in turn produce more power, leading to higher airspeed.

As to 3 blade v. 2 blade, in general fewer blades are more efficient, but that's not really significant in our case. Maxes are very slow & draggy, so we're really looking for more thrust at lower speeds. Similarly, if you reduce the diameter you also reduce the aspect ratio of the blade, leading to reduced efficiency. But this is probably overcome by allowing the engine to rev faster & potentially produce more power.

There's a lot of trial & error in matching props to airframe/engine combinations. So maybe the solution is to accurately measure your present performance, then reduce prop dia incrementally, measuring the effect after each change. And don't forget there's the fundamental trade off between climb rate and cruise speed.

I would also carefully weigh each blade and make sure they are balanced afterwards.

Quoted from Antoni

Wot's this stuff about a smaller diameter prop being 'faster' ???

You can increase speed by increasing rpm (with the same pitch) or increasing pitch (with the same rpm).  When you increase pitch the load on the engine goes up and the rpm goes down.  In order to get the rpm back up to where it needs to be you decrease the diameter of the prop.  The end result is a smaller diameter prop with more pitch turning at the same rpm....and the plane goes faster.

Typically, racing airplanes use very small diameter/high pitch props to get maximum airspeed.  

I like ground adjustable Warp Drive props.  IMHO,  3- blade props seem to climb better,  2-blade props cruise faster.

I currently have a 3-blade Warp Drive on my CH-1.

Ivo has an in-flight adjustable pitch prop for ultralights.

http://www.ivoprop.com/inflightultralightmodel.htm

I think the electrical in-flight adjustable Ivo would be a bit overkill (and heavy) for a Minimax. In the UK one was fitted to the weightshift Jabiru 2.2 Medway Raven but it didn't sell at all well.

I believe the 3 blade Warp Drive props are a bit heavy but I might be wrong.

What is true (Tom) is that the 3 blade Ivo is outwith the inertia limits of the Rotax B box, even though its total weight is only a couple of ounces more than the equivalent Arplast prop. Ivos have more mass in their blades and less in their hubs.

In 1996 on the instruction of my instructor, and with only 4 hours flight experience I replaced the 2 blade wooden prop with a 3 blade Ivo on my 503 B box Alpha weightshift. In hindsight the only reason was that he wanted a quieter aircraft in his circuit. No problem, and that combination is still flying.

I do understand the concept of prop pitch and speed being related to a road vehicle gearbox. Also prop disc area being proportional to the square of prop diameter and even (I think) thrust being the cube of the RPM.

What I still definitely don't undestand is Ivo's comment above. Simplified -

...to increase TOP SPEED use a smaller diameter 3 blade prop

Is this concept - whatever it is - relevant to Minimax airspeeds ??

While we're on the subject of Ivo props, I have the 3-blade ground-adjustable ultralight model.  The quick adjustment instructions say "torque on the mounting bolts: 200-inch x lbs."  The notation is different from what I'm used to seeing, which would be "200 inch-pounds" without the X in there.

First, am I correct that he means 200 inch-pounds?

Second, 200 inch-pounds is 16.66 foot-pounds, slightly more than hand-tight.  Does it sound correct?  

Surprisingly, yes. The wooden prop for my 447 Max is 9 foot lbs 108 inch pounds There is a load spreading ring in front of the prop and there are six bolts. It just doesn't sound like much.

Nathan,
I don't understand what the x means but 200 inch pounds seems reasonable based on the instructions for my Powerfin which is 175 inch pounds.
Dick

Bolt Torque:

The used Ivo I've just bought came without any documentation. I was told it had never flown. The hub has heavy knurling on it, and any amount of bolt torque would have left clear witness marks on the blades. None present.

200 inch pounds [16 and two-thirds foot pounds] seemed a lot to me because the last time I bought an Ivo my memory is that the torque specified was lower. That was a 'Mk1' shim-adjusted prop, not the screw adjusted one available now. Glad to see that another person has come to the conclusion that 200 inch pounds is correct, also according to the Ivo web-site, but they don't tell you how to differentiate between the 'ultralight type' and the other types.

...but  "slightly more than hand tight"  ?   I think 200 inch pounds is much more than that!!!

Bolt torques can be deceptive. 23 years ago I bought a torque wrench especially for bolting that shim Ivo to my Alpha. When it arrived at work I held its drive in the vice and felt for the 'click'.

No way. This tool has got to be sent back.

Then I thought - well you like to call yourself an engineer so measure it!

Very humbling.

It was spot-on when checked with a spring balance. It was my expectation of what a particular torque value feels like which was VERY wrong!

What does 'Hand-Tight" mean to you?

Thanks for the replies.  I'm glad I asked!

  

Inch-pounds is the measure for torque.  It is calculated as a force applied at a distance from the center of rotation.  The distance (inches) is multiplied by the force (pounds) as in:   Torque = distance x force...  and that's where the "x" comes from.

Purely as a matter of interest, it used to be a rule of thumb that a man of average strength could apply a torque of 40ft/lbs through an 8" spanner (Wrench ).

"Use the right size spanner and pull it tight." Tight is a setting known to engineers. It comes from knowing the materials, loose and too tight. Iron cylinder heads don't  often need a torque wrench for example but aluminium ones always do.

50 odd years of spannering has left me with a pretty good feel for 'tight' but elastic bolts still freak me out!

My old boss from 20 years ago used to call it the "calibrated grunt" When we had to torque something we couldn't get a torque wrench or adapter on. Never had a problem

Up front, I'll say that I'm really new to building airplanes, so please take the following as comments coming from my mechanical engineering background.  There are at least two good reasons for specifying a particular torque - one for ensuring the minimum torque and one for not over-tightening things.
1.  Bolts need to stretch just a tad (too little to accurately/practically measure) to make sure the friction between the bolt and nut is sufficient to prevent things from backing off.  If you are using castle nuts and your torque doesn't give you the right alignment to run the safety wire, the nut/bolt/washer combination may not be what the doctor ordered.
2.   Tightening prop bolts too much can crush the fibers of composite or wooden props.  I've seen tech literature from one wooden prop manufacturer that uses the distance between the front and rear hub plates as a measure of proper bolt torque.

Regards,
Alan