jamesg25

hello all
i'm running a 46ax on a Spad bulldawg weighing about 4plbs right now its running an MAS 11-7 cut to 10-7, its quick but not fast enough, before i put a 60 size engine in it, i'm going to play with the prop. I've tried a 10-8 but that seems to much, what is the best prop for speed?
thanks regards
James

daven

My Feedback: (1)

I like the apc 10x7 with that engine.

Should tach between 14-14.5 k which seems to be the optimum torque band for that engine.

jamesg25

thanks for your input Dave,
I thought that the 10-7 of the MAS series was the best, so i will try APC.
regards
James

daven

My Feedback: (1)

We race that motor locally, and the 10x7 apc works best.

MJD

My Feedback: (1)

If that's the SPAD delta thing, and you want to go as fast as possible, you likely ought to look at smaller diameter than 10". IMHO you need to get your rpm up and run some more pitch. A 10-7 will fly pretty quickly and will have good pull at lower airspeeds, but if yu are looking for the best top speed you probably want to think about perhaps a 9-7 or 9-8 as a starting point. For high top speed you want to prop the engine to run close to the horsepower peak on the ground, then find the prop that runs at that rpm range and that works "best" on your aircraft. Some aircraft are draggier than others so a precise recommendation is not always possible, so for example while an 8-8, 8-9 might work great on a small clean model, you might need a 9-7 on something a bit draggier. Myself, I'd start with the 9-7 and go from there. MJD

MJD

My Feedback: (1)

Hey, I take some of that back. I just found Dave Geirke's review of the .46AX, and it has a completly different powerband than I thought. I am used to many .40-.46's having a torque peak in the, oh, 12,500-13,500 regime, somewhere in there, and horsepower peaks higher up the scale as in 16-17k or so. Seems the .46AX has torque and horsepower peaks very close to each other, with horsepower peaking at [hope I remember correctly, left the mag at home..] 13,800rpm. A 10-7 runs right about there, so if one was to go for higher pitch/smaller diameter for a small clean model, perhaps something along the lines of a 9-8 might be the ticket. But 8-8 or 8-9, which would favor engines whose horsepower peaks are higher up the scale, would take this engine past the horsepower peak.

MJD

daven

My Feedback: (1)

That would make sense in theory, but in reality the 9x8, 8x8 don't go any faster than the 10x7.

I can't explain it.

MJD

My Feedback: (1)

Do you mean specifically on the .46AX or in general?

Could be that they simply run up against a drag wall on many aircraft and the engine's peak horsepower is the limiting factor. I would imagine in that case the aircraft is flying much closer to pitch speed of the 10-7, and the rest of the flight behaviour (launch, vertical etc.) is also better due to the disc area and lower pitch. Unless I goofed the calc, the pitch speed of a 10-7 prop at 13,800 is 91.5 mph. Add to that the engine unloading in the air and the uncertainties of the pitch numbers and it probably all makes sense.

MJD

daven

My Feedback: (1)

Just on the AX.

The 10x7 APC prop turns 14 - 14.5 k on the ground with a quickee airframe.

On Radar, consistent 125 mph passes can be made (averaged for wind).

In theory that is not possible. In reality it is.

MJD

My Feedback: (1)

Reality rules!

Interesting. 14 - 14.5K on a 10-7 for a sport engine seems pretty respectable.

MJD

bob27s

My Feedback: (19)

Blade surface area, blade airfoil, planform, stiffness, airfoil transition all play in to how a prop performs. Keep in mind that its a moving wing, and it makes more sense. The pitch x rpm might give a theoretical speed, but there is much more to it.

I think many of us have carved enough paint stirrers to know that one 7.5x8 prop can be drastically different in performance to the next one (even though they look an measure similarly, and we spent the same amount of time sanding it). An easier example, a wood Zinger 10x6 performs much differently than an APC 10x6 (at the same given rpm - and that will vary depending on prop design/efficiency/load too). Both good props, just different.

And as was pointed out here, the prop & engine have to be matched to how it runs best (do not rely on many numbers you see published - other than as a broad reference - too many variables) and then add in the airframe, and the prop selection is then a compromise of what works best in that specific application.

daven

My Feedback: (1)

MJD,

The only reason I am familiar with that specific engine is that it is what we use as our "1 engine" class for our local racing. I have spent MANY hours testing more props than most people know exist. There are props that are slightly faster (about 5mph), but loose so much speed in a turn that it doesn't make sense for racing. Those props also take off so slow it would make hand launch difficult.

Other props worth trying, slightly faster (not noticeable), but will not be nearly as friendly:

APC 8 3/4" x 9W
APC 8 3/4" x 8.75W
APC 8.8" x 9.25
APC 8.8" x 9
APC 8.8" x 8.75
APC 8.8" x 8.5
APC 8 3/4" x 8.25 N
APC 9 1/2" x 7.5 NN

These props will turn between 15k - 16.5k

good luck, the AX has been a nice engine for us if you keep the bearings oiled.

crasharama

Hi Guys,
We also run the AX in our class of pylon, we have a standard prop, & FAI, no nitro fuel that we all must use.

The Prop is a Bolly 10.5 X 6, it peaks on my engine at 13 400 on the ground with this setup. We consistantly run around the 100mph mark at this.

We have also tried the APC 10 X 6 & is noticably quicker than the regulation prop, & tachs at 13 900 on the ground, on our fuel.

Tony.

sigrun

Me 2 ...also rips on the TT's 46PRO![sm=thumbup.gif]

sigrun

Very 'tasty'. [8D]

Crasharama do you know if the Harrods are still moulding that unit or if anyone (Greg G ?) is offering it in partial kit form? If so, have you got any contact details? Ta.

crasharama

WIll pm you some details....