I-Love-Jets

Hi, there seems to be a “gap” in people’s knowledge here in this forum when talking speed prop sizes for certain applications. For instance someone asked if a modified .45 sized sport engine can swing a 8x11” prop on a Wildhare LR-1 (Speedcat) plane? See

http://www.rcuniverse.com/forum/fb.asp?m=6220714



Q1:
I was also asked if “USA faster than normal airframes” would harmonise well with oversquared props. My counterquestion: What exactly is the US- type?

Q2:
Second question was if the THRUST-HP software is able to calculate oversquared props correctly.
A2:
The answer here is no sadly, since the algorythms only match subsquared and squared prop sizes.

Q3:
Third Q was: At what point does an oversquared prop become a seriously oversquared prop?

Q4:
Is cutting down existing prop diameters desirable?



Maybe there’s a bunch more questions covering this speed prop topic…




In general using very oversquared props such like 8x11 or 8x12 with 10 to 15 cc glow engines can potentially lead to a disaster in terms of effectiveness.

Compared to squared or sub-squared props it is much more difficult to find the “sweet spot“ where dynamical unloading of the engine matches both the torque/rpm engine peak and the optimum propeller aerodynamical effectiveness. Not to mention that an aerodynamically elaborated airframe is needed.



The below linked thread called prop diametre ? shows many pics and also several examples describing the process for determining right speed prop/speed airframe combo:

http://www.rcuniverse.com/forum/m_59...tm.htm#5957428




A3:
It is difficult to determine, at what point an oversquared prop becomes a seriously oversquared prop. Generally we can say that the broader (and higher) the torque peak of an IC engine is developed the better the handling with such props. And of course the more sophisticated the airframe design is in terms of drag, the more efficiently an oversquared prop will work.

For instance in the past, competitors of the German Speed-Cup used 4.2x11” custom made CF speed props for small purpose built speed planes successfully. Actually you can break real 200 mph speed in straight line speed trap measurements with 2.5 cc engines using this “seriously” oversquared prop size. This will only work if there’s still enough static thrust for safe taking off by hand-launch. The engine has to be timed be able to unload willingly from about 28000 statically (partial prop stalling here) to 35000 rpm during flight. Besides the prop design the airframe aerodynamics is one of the most important components for this kind of speed stuff, since it will potentially limit top speed regardless of the engine output and prop choice.




A4:
Cutting down existing props normally does not meet aerodynamical requirements at all.[sm=thumbs_down.gif]

For example APC, REV-UP etc pylon props generally are too fat for serious speed application. So if trimming down the diameter of such props, the new prop tip will generally be much too thick (drag). A loss of engine peak rpm will be the consequence. Thinning APC prop blades generally should be avoided due to structural strength reasons. Then we have to consider that a well designed speed prop features altering pitches per propeller section and changing airfoils as well. For that by cutting an existing prop we will most probably harm aerodynamical effectiveness.

However a promising alternative would be modifying an existing carbon fiber speed or pylon prop to your specs. CF props can be sanded nicely. I strongly recommend to customise only one blade and then molding that blade twice - to establish a new speed prop having downright identical blades. So two molds have to be built in total per custom made prop. If treated with care, several hundred props can be made from that main mold. CF speed props used here in Germany usually are heat tempered within the mold to add (much) more torsional and tensional strength. This refinement is essential especially in combination with 10 to 15 cc engines.



Well made CF props can be very light. They can be very strong even if appearing too thin at first sight. The aerodynamical effect of a very thin blade is positive, i.e. noticeable increasing unloaded rpm peak performance. APCs or other plastic or wood props aren’t really competitive in contrast. The black carbon containing pylon series by APC isn’t strong enough for substantial thinning down measures, because they do not feature continuous strand fibres.




Finally I would like to note that mostly hand laid cabon fibre props are really NOT required if rpms do not exceed

- about 25000 rpm unloaded and a maximum prop diameter of approximately 5” (for up to 3.5 cc engines)
- about 22000 rpm unloaded and a maximum prop diameter of approximately 7” (for up to 6.5 cc engines)
- about 21000 rpm unloaded and a maximum prop diameter of approximately 8” (for up to 10 cc engines)
- about 20000 rpm unloaded and a maximum prop diameter of approximately 9” (for up to 15 cc engines)


So within the above rpm/prop diameter ranges, the commercially available non-CF (pylon) sub-squared and squared props will be well suited (for sport speed).

dirtmagnet

man this guy just ate someones lunch!!!!!!!

NoOneFlysAtMyClub

My Feedback: (32)

Answered some questions I wasn't sure how to ask!!

Leave it to the Germans.

Mike Connor

My Feedback: (5)

1. US- type speed planes are the types typically talked about in this forum. Cleaned up deltas, LR-1, Q-500 and other pylon airframes.

2. It sounds like you are just the guy that could write an algorithm for the load factor of speed props.

3. It sound like you are saying that "seriously over squared props" may begin when pitch exceeds diameter by more the 20%.

4. It seems to be common practice to cut down props in the pylon and sport world. Are you saying you shouldn't even trim off 1/4" or less to get in the sweet spot?

Thanks for your thoughts and knowledge.

combatpigg

My Feedback: (3)

ILJ, thankyou very much for this great article you just submitted. You really are one of the guys who make this forum what it is supposed to be about. Now if we could only get Der Stuermann to come back. I think he just got tired of getting shot down by guys who really weren't serious about extreme speed....or who didn't even want to know how to get there.

Razor-RCU

My Feedback: (50)

***Embarassing drunken ramble***


And yes a .45 can swing an 8x11, not very well I take it but it can.

Cyclic Hardover

My Feedback: (3)

No "Gap" here. The basics are fine with me. Just don't really care about how or why as long as it "does" and it "does "well". Besides its first grade level engineering knowing that theory does not always work out right on actual application.

I have seen a few guys in my day at the airfield flustered and upset because their plane is flying just fine?? But not the way their numbers came out and it bothers them.

HighPlains

My Feedback: (1)

You're right. Posts like your's don't belong here.

Rudeboy

Well, I think we can all agree that just cutting down a prop may not be the best route. But it does work on a pinch. It all depends on what you call "cutting down a prop", doesn't it?
I cut down props often. If you find that your engine is loaded just a tad too much, then taking down the prop tips just a little is most of the time enough to get the engine back in its sweet spot. In Club 20 racing for instance, we almost exclusively use the APC 7x6W prop. But very rarely do we use it the way it comes from the factory. I have a bunch of those props, all slightly modified to get the right load on the engine depending on atmospheric conditions. On cold days, I can nearly use a stock (but cleaned up) prop and still get the engine to unload properly in flight. On hot, moist "crap engine days", I use a slightly lighter prop and a different plug to keep my engine happy without smoking it.

What ILJ means by not cutting down a prop, is that you cannot just take a prop, clip it down to 50% of its original size, put it on the engine and expect it to work.

Great post by the way!

rmenke

My Feedback: (13)

Dirtmagnet:

Sometimes it takes a while to recognize just how hard some of our brethern work at their hobby, and just how bright they have become from years of effort. The European speed guys simply live in their own niche within our hobby, and prove to the world each year just how good they are. On this side of the pond, our go fast efforts usually end in go fast and turn left around he pylons and we do get up around 200mph flat, level and around turns. Their birds are all special one off designs intended to dive-stay togeather on pull out and scoot across the radar at around 250mph and up. I doubt if their specialized birds could stay 10-15' from the ground around 600' spaced pylons and maintain the 250 mph, CAUSE THEY ARE NOT DESIGNED TO DO THIS. When this guy talks about speed, I respectfully listen, and copy it all, and stick it in his file among several other Euro go fast guys. We have one thing in common in this area and thats engines, fuel, props and the desire to be better than the rest. With all that said, I will shut up before what I am trying to say becomes something not intended, by you and everyone else here that I also have great respect for in addition to the desire of outrunning each and everyone. ENJOY

combatpigg

My Feedback: (3)

Good points Roger......also, when is learning from someone elses' experience a bad thing? I say bring the knowledge on. Can you imagine what a run with a 4x11 would look and sound like after the first dive?

daven

My Feedback: (1)

Rather than minor tip clipping, try removing a 1/16 - 1/8" off the TE of the blade (narrowing the blade).

cncswiss1

now THAT was informative.. thank you very much

how are these prop molds made? is it possible to get a prop from someone to laser scan so I can machine a mold? I can scan them and machine the mold, but have zero experience creating a prop, beyond normal sanding to balance/ clipping a bit off the tips to unload better.

it would be neat to make a prop mold for an engine I can get readily, probably the OS91 or similar DF engine.

the molded props look like the have woven fabric in on the outside, is the core a big lump of carbon tow from end to end?


has anyone over in germany played with crankcase length to get a better fuse front end profile (longer less blunt nose)?? or are the harmonics of a longer crank unhealthy for engine life? one would thing that moving the cylinder back 5-8 cm would help make the aircraft cleaner.

Cyclic Hardover

My Feedback: (3)

Never heard this approach. Does it work well? How are you trimmng this?. Sounds like ths would be best on the smaller Q500 stuff?

Rudeboy

Indeed. The outside layer is fabric to increase torsional stiffness.

I would think crank flex is a major issue at these speeds. Making it longer than it needs to be would probably mean that you have to make it a whole lot more massive.

As for reducing prop load: I usually work at making the airfoil near the tip thinner. I don't actually "clip" the prop down a whole lot. The more you reduce blade area, the more you reduce acceleration out of the turns. So my goal is to try to lighten the prop load while trying to keep the thrust up.

daven

My Feedback: (1)

Cyclic,

Yes, it is often used in Q40 (wood blades only).

As rudeboy mentioned, slowly sharpen the TE, start with a little at a time, and test.

freakingfast

My Feedback: (3)

Let me throw another wrench in the works here. Plastic prop+pitch gage+heat gun.
Just don't go crazy with the tweaking

Mike Connor

My Feedback: (5)

I was reading where a guy was getting away with this (so far) on E-props. Don't think I would stand to the side of one above idle.

daven

My Feedback: (1)

People have been heating and twisting the APC black Q40 props for some time. Fine for practice or sport use, but highly illegal for organized racing. In the early days of these props, they were pretty heavy for some conditions and motors, so we would tweak them to reduce the load on the motor for practice up here, and use the stock props for racing in the South. Now there are plenty of lighter props and this is not necessary for our racing needs, but definately doable for sport use.

Troy-RCU

My Feedback: (1)

I have done it to some extent but it can be done wrong and with dangerous consequences. I've seen blades over heated. Mind you, these were on thin nylon pylon props for e-pylon. Usually this was to match the pitch of both blades or de pitch some to unload the motor. The 'Bang-Bang' of combustion motors makes for specialized needs in the hub area and that becomes critical.

Cyclic Hardover

My Feedback: (3)

Hey Dave, just for conversation here, do they have inspectors etc to "tear down" planes after a win or whatever to make sure you all are all "legal" I guess?

daven

My Feedback: (1)

For the most part yes. I've seen venturi's checked for proper diameter, plane weights checked, motors torn down, etc..

If you tried to cheat on a national level, you would eventually be caught. The prop situation the past couple years may be a different story.

Unfortunately there was enough variance from prop pitch from one prop to another, that it was very difficult to tell if someone de-pitched a APC black plastic prop. The initial props were a tad on the heavy side and only the stronger motors and better local conditions would really let you get the best out of the props.

However, this spring, Fred B. at APC got several lighter props approved that work well in bad conditions, with the new LS motor, the Jett Motors, and even older less competitive motors. Twisting props now is completely unnessesary as you have plenty of sizes and variance to work with.

Mike Connor

My Feedback: (5)

Speaking of weight, APC sport props are about 50% heavier then MA props that I weighed. There must be a lot of energy wasted just to overcome the additional weight. I may take another look at the MA S-2 series props for high speed sport flying.

This is not meant to wind anyone's spring or start an APC MA debate.

combatpigg

My Feedback: (3)

In most cases we're asking these engines to rotate a prop 300-600 times a second, rotating mass does effect rpm. Whether or not the MAS prop in question can make more speed compared to APC is easy enough to find out. Since APC arrived, they have taken over pylon not just because they are so pretty. Does MAS have a competition series?

daven

My Feedback: (1)

MAS props are not approved for either 424 and 428 quickie, or 422 Q40.

Fred B., is a very active pylon racer, and recent National Champion. His props have proven to work under the stresses we put them under. Nothing against MAS props, but they are not legal for most National Pylon Events.