FlooredCOBRA

I have a quick question about shaving or cutting down props. Say you have an 8 x 6 and cut down to about a 6 1/2" diameter. What would the pitch be then after cut down?

Once it is cut down seems more meat of the prop is now at tips of blade. So what would the pitch be after that is done? Or is it still the same. Any ideas on this or know any way to determine the pitch after this is done? Or is there a way to calculate what the pitch would be after a certain amount is cut off the prop?

Reason why I am asking I would like to be able to know pitch and diameter so I can just buy a made prop at that size. It just takes way to long to cut, shave down, and and balance a prop and find its still to big. Then have to repeat when messing around with different pitches. I think it takes me about an hour to do one prop. I would rather be flying than messing with props. So if anyone has an idea on this or way to figure this out maybe a calculation let me know.. Thanks

combatpigg

My Feedback: (3)

Hi FC, A lot of the guys who have flown pylon for long enough time to go back to wood props carry pitch gauges. The pitch stays relatively uniform on the outer 1/4 of each blade. The extra tip area should be removed in most speed applications, so it is a balancing act between diameter, pitch and blade area. As you work the prop down to the optimum proportions, a piped engine will let you know in a hurry when you are "there". It is easier for me to order a selection of props from APCs' online list than the old cut and try method anymore. In the past I have got good advice right here from the other guys who have already "been there, done that". APC has a mind blowing selection of props, what puzzles me about their selection is they are lacking some small diameter/ high pitch prop combos that I would like to see for extreme speed with a small light plane.

FlooredCOBRA

I know what you mean about the small diameter and high pitch props. I found a 6.5x6.5 APC Pylon prop and going to try it out and see how it works. I am hoping I can just put it on and it work. So far the only way I have been able to get a prop to work was cut down a larger size prop. I have tried several diameter and pitch and so far I have not found a direct bolt on.

If I knew what pitch my 8x6 and 8x7 were when cut down to 6!QUOT! or so diameter i would buy those made already. Well if I could find them that size. If I cant find anything I guess I will continue to cut down my own. It found this works and been doing this the whole time but would like a easy change.

der_steuermann

My Feedback: (2)

Regarding the nominal pitch at the tip after cutting: Unfortunately there's no general rule/receipt for that. In fact prop manufacturer X uses a certain pitch distribution for his props while manufacturer Y does apply a different one. Thus after cutting down the prop the final pitch result at the new tip location will sometimes be quite surprising [:@]

In addition the labels quoting dia/pitch often differ significantly from the REAL pitch measured at a certain blade location. Let's take the 8*6 prop. That announced pitch of 6" should be valid at the outer 20% to 10% of the prop radius. But quite often you will notice up to 1” less [:'(] of stock pitch at the tip..

So there's no rule of thumb! It is the process of trial and error helping to achieve the best result for your speed project.

Another disadvantage of most commercially available props worth mentioning is the airfoil thickness being too much when talking about 'true' speed props – resulting in too much drag and reduced top end performance. Even the props belonging to the APC D1 pylon series are too fat due to the utilized prop material (let’s call it ‘APC-plastic’ ).

In conclusion these miserable circumstances 'forced' me to make my own speed props - of course in carbon fiber technology (MUCH better tensional strength and torsional stiffness). Now it is possible to determine the custom design (pitch distribution, airfoil distribution, blade area and blade geometry) right from the beginning - based on experiences from certain airframe/engine/pipe combos during a test stage.

bob27s

My Feedback: (19)

When we check and modify props for racing, the only real way to know the pitch is to check it with a pitch gauge. I have an old Prather gauge. I think DarrolCady.com still sells the Edmunds version of this. If you require consistant measurements, it would be a good investment.

In fact, for wood props especially, the pitch shown on the lable is often not what the blade measures at. So its hard to even trust a 'stock' size. Often enough I have found different pitch on each blade. Thats ok if cutting the prop, but for a stock prop.... it goes in the paint stirring pile.

As you may be aware, most of our QM40 props are in the 7.5"x8 range, but they start life as 9x7.5 blanks. An off the shelf 7 or 8" prop is usually too thin, too narrow at the tip, and the hub is too small. We usually leave the back of the blade intact at first, and work on the diameter and airfoil shape. The back is only touched to adjust and match pitch.

What application do you have? I have a few carbon/epoxy QM15 prop blanks I might part with. I believe they are 8x6 blanks, that require finishing to profile and diameter. I'd have to confirm the size.

FlooredCOBRA

I been using running a MVVS GRRT FAI .40 for these props. It turns good rpm between 22,000 to 24,000 rpm range depending what prop I used. It runs really well but needs small diameter to get up on pipe. This engine worked very well on my Diamond Dust since it really dont take much to pull it through air so it dont suffer from lack of thrust from small diameter props.

I used 7.2x8.6 and 7.4x8 before and it did not like them until cut down. So far the best working ones has been cut down 8x6 8x7. Yet after cutting down they are more a 6 1/2 or 1 1/4 diameter.

I was just looking for an easy bolt on prop that would work. If I knew what my cut down props pitch were I could have something to go by. I think I better get a pitch gauge and go by that.

combatpigg

My Feedback: (3)

MR FIREWALLED COBRA< I didn't have my head screwed on earlier or I would have given you this link to carbon speed props. [email protected]. He has an awesome assortment of props, called,"ELIMINATOR" props. I have done business with him and he delivered the goods, and on time too. You sure are pretty hard to please if you can't live with the 7.2-8.6! Steve has a 7-8.5 "Z1" prop that might fill the bill. His phone# is 763 531 0604 in Minnesota. His biggest prop is an 8.25-10 that you could cut down to a 6-10

FlooredCOBRA

combatpigg, Thanks for the link I will give that a shot.

7.2-8.6 turns but it never gets up on pipe unless I cut it down some. I tried getting as high as I could and going into a dive and it would never kick up on pipe. On the ground it turns just under the rpm where the pipe supposed to kick in. But it wont unload enough for it to work.

combatpigg

My Feedback: (3)

I think something is off with your pipe tuning, I don't have any experience with the MVVS, but with my .32 I kept shortening the header in 1/4" increments until it got happy with either the 8-8 or 7.2-8.6. The .32 runs in the 19000 range.

FlooredCOBRA

I wish I had a header to cut down. In my case my pipe clips directly to rear of engine.

combatpigg

My Feedback: (3)

That's a pretty looking setup, too nice to lay a hack saw onto[X(]! The custom prop approach looks like the best way. Check and see if the liner or head is shimmed. You can regain some misplaced tork if there is too much shimming going on.

Evil Homer

What nitro content are you using on the MVVS?

FlooredCOBRA

I am not sure on the shims set up. It has been a while since I pulled the head off. I am correct I think it has one thin shim in there now. I been using 0-5% max. It is an FAI engine so supposedly low nitro is way to go.

I tried between 15-10% before and I was eating plugs very fast. Also it did not run to consistant with the higher nitro. Dont get me wrong it will run with higher nitro but it dont like it. With 0-5% it runs very smooth and consistant and no power was lost going to lower %. I put tach on just to see what difference the Nitro made and no power was lost. So I got that little itch out of back of my mind that I would loose power. Now I know.

combatpigg

My Feedback: (3)

From what I've learned about .40s, .008" is as tight as you want to go on the squishband. It is easy to check through the glow plug hole with a thin piece of solder. It doesn't sound like shimming is at fault, how long will the cylinder stay pumped? If it doesn't hold for at least 5 seconds, then that is where some ooomph is going. To run this engine with higher nitro you would want to open the chamber up to a similar size as other hot 40s. It just seems like this engine should resonate all day with the 8-8 or 7.2-8.6

der_steuermann

My Feedback: (2)

Before messing around with different props on that Diamond Dust here’s some first hand experience.

Your MVVS GRRT engine features a very high timed (low torque) liner liking to rev above 24K while never quitting the tuned pipe resonance. This of course is possible only with small pylon type props like 7.2*7.5 for instance, for which that FAI F3D engine has been designed for. And in fact that engine loves these props pulling a F3D type racing plane with authority...

However, the Diamond Dust airframe is a very different ball game. It is much more draggy than a F3D pylon plane – resulting in a real bad situation for speed freaks like FlooredCOBRA. You have to use quite small diameter props when ‘playing’ with pitches in excess of 8.5” on that airframe with that engine. We’re talking about 6” diameters. In general such a small prop is overtaxed with the aerodynamic of that DD frame... [:@]

In addition to the small prop dia when using the DD frame you’ll run into some trouble due to the big pitch.

Say 8.5”+ pitches: These large prop pitches will not work effectively below a certain flight velocity. To be able to reasonably make use of the challenging degree of efficiency of a such high pitched prop a speed plane should be able to fly faster than 200 mph - in our GRRT engine case study.

But this boundary condition sadly collides with the restricting aerodynamic layout of the Diamond Dust frame. [:@] A speed airframe must always allow the prop to unload easily after the first powerdive!

So what to do? I personally utilize the same engine/pipe and a custom made 6*9.25 carbon fiber prop with great success. I have to emphasize that this engine/pipe/prop combo only works on a very slim airframe – being much less draggy than the Diamond Dust. The 6*9.25 prop violently unloads into the 27K on such a slick airframe. So the most effective strategy for real good speed numbers (220 mph +) simply would be an airframe change.

Bottom line: For your DD/ GRRT combo I suggest a prop size of 6.75*8.5

der_steuermann

My Feedback: (2)

BTW, that pre-mentioned 6*9.25 prop has not been cut down from a bigger prop (too fat hub, too thick and wide airfoil, unfavorable pitch distribution, etc) but is an independent low drag speed design.

FlooredCOBRA

der_steuermann, that made total sense and I see what you mean. I might as well be dragging a chute behind it. That would explain why I could not get it to unload in a dive when it was running under the pipe. It did unload if it was running on pipe and I went into a dive.

I really like the simplicity of the Diamond Dust and the odd shape to it. I guess I will settle for what I can use then. But what do you guys think about the new Cliped wing Diamond Dust version. My question is is it any less draggy than the first version? If so this would be a great help with my engine pipe set up.

Rudeboy

Cobra,

APC has 7x7.5 and 7.25x7 props... The MVVS "should" turn up really nice with the 7x7.5, but if you still have problems staying on the pipe, the 7.25 may do the trick. And if the 7.5 is a problem you may indeed want to check your piston and liner fit...

Remember that these engines turn up to close to 30K "on speed"... so even a fairly "low" pitched prop like a 7" will go fast.

You really don't need an 8.6 pitch to get some speed at 30K.