Dustflyer

My Feedback: (13)

Chuck,

Many years ago I drove cross-country through Texas and never forgot how different that state was from one end to the other. It's not like driving across New Jersey, more like driving from one part of the country to another. I know it can get dang cold down there, one of the worst ice storms I have ever seen was in Dallas.

Regarding those props, I learned that you can turn all the rpm you want but unless you have sufficient thrust for a particular airframe you aren't going anywhere fast. If rpm means everything how come pylon racers don't run 4 inch props turning 40,000 rpm? Not enough thrust. They learned from trial and error that the props they use are optimum for the airframe.

Clearly the 7.4 inch prop is too small for a Diamond Dust. The speeds I achieved did not even reach the 85% efficiency level of Mark Mallory's formula, in fact I fell well short of that.

I played around with Andy Lennon's thrust formula and it turned out a 9X10 at 16,400 rpm created 50% more thrust than a 7.4 X 8.25 turning 21,400. There has to be a way to tie those thrust and speed formulas together but I don't have the math skills to do it.

How does a P-51 Mustang Reno racer go 400 mph? An enormous variable pitch prop and the torque to turn it! I wonder how many inches of pitch that thing is set at for high speed, 30, 40, 50 inches?

The problem we have with model airplane engines is that they are rpm, not torque machines. We have to work at lower pitch ranges whether we like it or not. The trick is finding that "sweet spot" of diameter vs. pitch for the particular airframe we are using.

ChuckAuger

My Feedback: (12)

Well, I might not be doing anything great by running 12" of pitch, but I can't see what else there is. If a 7.4" diameter prop is too small, it's too small. So I need to run more diameter, right?? Well, if you start going up in diameter, you have the choice of not turning as many RPM's, or running less pitch.

So I'm dropping RPM's to run more pitch. Your P-51 doesn't turn very fast (compared to a model engine). The giant scale racers are going 250 and they aren't turning very fast, either (comparatively speaking), but they are running lots more pitch.

The guys flying the Whiplash are reporting plenty of thrust with a 9" diameter prop...but unless you have some HP, running a 9" diameter prop is gonna load the engine down if you try to run much pitch.

Also, I think blade shape plays a factor. The skinny blades on the 8.9X9 APC pylon prop I tried just didn't seem to be moving much air IMHO. I think that's why there are so many pitch .25" pitch steps..to allow them to fine tune an engine with a fairly narrow power band. I know it would go from idle to full throttle in about 1/2 a second.

And as far as model engines being RPM vs torque oriented..if I can turn it, I guess I have enough HP.

So I guess I don't understand your point..seems to me I have enough diameter to meet the thrust requirements. Granted, it might not be very efficient until I get up to some speed, but I'm betting it will launch. I have enough power (or torque or whatever) to spin the pitch. Plug a 8.75 X 12 into your charts at 16K ~ 19K and see what it reads.

And also I think my Dust might have been happier with the 7.4 X 8.25 than yours due to a little higher port timing, running faster unloaded. But it's toast now.

pdxpaul

My Feedback: (1)

Dustflyer may be on to something here. Someone once told me that a Quickie will fly faster with a quickie engine, than with a Q40 engine, and that a Q40 will fly faster with a Q40 engine, than it will with a quickie engine. I don't know if that is true or not, but sounds reasonable enough. If correct, it suggests that a quickie has too much drag for the small high rpm q40 props.

As for the Diamond Dust, just by looking at one, it appears to fall somewhere between a quickie and a q40 on the slipperly airframe scale. I think a q40 has less drag, but its just by observation.

One way to prove my theory would check Dust speeds with a Nelson quickie and them with a Nelson q40. I have been told that quickies are in the 160-170 mph range, and q40s in the 180-190 range.

Finally, I went to my first q40 race last summer in Medford and saw the big names race. After seeing that, I realized my Jett 50 DD, although fast, is not close to 180-190.

Paul

banktoturn

Dustflyer & ChuckAuger,

I don't know what the best prop is, but I'll make a couple observations. The engines we use are "rpm machines", because they are single cylinder 2-stroke engines. There is some tweaking that can be done to improve torque, but likely at the expense of maximum power. You can't talk about thrust by itself, you have to talk about thrust at a given ( or desired ) airspeed, because props are very sensitive to airspeed, and you can't outrun your prop's pitch. ChuckAuger's comment about "enough power to spin the pitch" is a great way to look at it. This is what forces the diameter to be small, because the power needed to spin a prop goes up really fast as the diameter is increased. Two potential improvements suggest themselves. Gear the prop down so that the engine can make its best power while driving a high-pitch prop at an efficient speed, and/or move to a single-bladed prop. The C/L speed guys use single-blade props, because they are more efficient. You might be able to get a high-pitched prop up to a more efficient diameter be exploiting the inherent efficiency of a single-blade prop. I don't know what all the trade-offs are.

banktoturn

BruceDana

Has anyone consider a 4-stroke engine to swing the larger diameter / pitch props? Not as much RPM as a 2-stroke, but more torque. Overall horsepower approaches (and may even exceed ??) a 2-stroke of the same displacement at the target RPM where props are most efficient.

Both OS and YS make supercharged engines. The YS FZ91 swings a 14 x 14 to nearly 13,000 RPM. You would need a larger airframe because of weight (27 oz). A 14 pitch prop at 12,000 RPM geometrically is about 160 MPH (100% efficient). I wonder what one of these would do with an 16, 18 or 20 pitch prop of about 10"-12" diameter?

I also wonder if anyone has tried some of the small speed 400 airframes with glow engines that will will turn small props (7") up to very high RPMs. There has been discussion of .28 buggy engines, but I've read the old Conquest .15 (a 6 oz aircraft engine) turned 25,000+ on the ground with a 7" prop. Some posts I read said it would do 30,000 with a pipe. This would be suited for a speed 400 airframe, I would guess.

Dustflyer

My Feedback: (13)

Bank and pdx,

I guess I was a bit fogged in the head because this whole thread was supposed to be a reply to Chuck's post over at the "speed contest" thread. I hit the "new thread" button by mistake.

Here's a reply I put on the original post:


Chuck,

I should have waited until I woke up this morning. I accidently made a reply to your post into a whole new thread.

If I sounded confused that's understandable, I confuse myself half the time. The point I think I was trying to make is that below a certain diameter of prop it doesn't matter how much rpm you turn, you won't have enough thrust to accomplish anything let alone go fast. A Nelson might be able to turn a 3" prop at 50,000 rpm but I doubt if that would even get a pylon racer off the ground.

Your idea of cutting down a fat Zinger prop is pretty interesting, I'll bet you see some good results with that. Like I said, I always believed my Dust was fastest with that APC 9X10 sport prop turning 16K plus on the ground. When I take it out for one last radar run I want to check it with that prop as well as the 8.8 X 9.75 which does indeed have a much narrower blade. It turns a thousand more rpm than the 9X10 with only a 1/4" less pitch. It will be interesting to compare the results.