RE: 3 blade prop...help!
 

In full scale aviation the three bladed prop is considered superiou to two bladed props. Has nothing to do with efficiency, they know two bladed props has higher efficency, has everything to do with high speed thrust at higher altitudes.

RE: 3 blade prop...help!
 

Uh, uh, uh Sport Pilot -- you know that we've been there before. [sm=thumbdown.gif]

I haven't got time right now --- but I'll be back.

You know very well that virtually no full scale considerations, other than diametrally-limited power transmission, apply to model props, & that what you just said is quite incomplete, if not misleading -- be back to chat later.

RE: 3 blade prop...help!
 

True, but my statement is essentially correct, therefore the world does not accept that two bladed props are superior. Not superior in every model application either. Again efficency is not the only factor; noise, and drag are important factors as well.

RE: 3 blade prop...help!
 

Maybe the "one inch size, and one pitch size" theory just doesn't cut the mustard
in some cases. Imagine a full sized (sixty powered) pattern plane, with a wimpy
10-6 prop on the front....even if it was a three blader. This is the same plane much
later in life...it had the K&B and the three blader on it's first flight.

It flew so poorly, I brought it down right away, because it was in serious danger
of stalling and crashing, simply because of the prop....and I certainly didn't want that.
The normal prop for this combo would be an eleven incher. The prop on the Kaos
now is a 12-8 zinger on a long stroke .61 rear piper. ;)

FBD.

RE: 3 blade prop...help!
 

Bob,


This engine is not and never will be a 16,000 RPM engine.

The manufacturer's HP claims are nonsensical and even they know it...


This engine will be at its best, in the 11,000-12,000 RPM bracket, with an 11x6 two-blade prop.


To know what the corresponding three-blade prop is, just deduct 1" from the diameter and keep the pitch identical.

OR

Deduct 2" from the pitch and leave the diameter identical.

OR

Deduct 1/2" from the diameter AND 1" from the pitch.


So the load equivalent of am 11x6 two-blade is a three-blade 10x6, or 10.5x5, or 11x4.
And the load equivalent of am 11x7 two-blade is a three-blade 10x7, or 10.5x6, or 11x5.

RE: 3 blade prop...help!
 

OK guys this is not going to go Full Scale, there is a Forum just for that discussion. This an RC prop application discussion.

Dave, since I fly mostly with fourstroke and Diesel engines I seldom use 3 bladers with less than 7 inches of pitch, mostly eight. I learned this from an article by Don and Judy Apostalico of Don's Hobby Shop in Salina, Kansas. I wish I could find that article again.

RE: 3 blade prop...help!
 

Look at this engine, and the rating....16,000 rpm's. Who doesn't think and engine
will perform to the manufactures rating ?

http://www2.towerhobbies.com/cgi-bin...?&I=LXBY44&P=0

Here's my OS .40 FX, after one tank of fuel....16,200....it's ready to race.

Fuel....15% PowerMaster sport fuel, noting added. Read it and weep....:)

FBD. :D

RE: 3 blade prop...help!
 

So do you delete posts in the Scale R/C section because they compare their planes to full scale? The main differance is size. There are important considerations that would make a three bladed prop a superior prop.

RE: 3 blade prop...help!
 

I don't respond to Sport Pilot, if he's talking to me, I don't see his posts.

Here's another example....the GMS .47 front carb....

http://www2.towerhobbies.com/cgi-bin...?&I=LXDXL5&P=0

Mine turned 16K flat after one tank on the same fuel. ;)

I have my own break-in and running techniques that assure peak performance.

RE: [Deleted]
 

RCUniverse.com is focused on R/C. Accordingly, topics must remain R/C related.

We do not permit political, social, religious, or other off-topic discussions. There are many places online to express your opinions on issues outside of RC so "RC" Universe is designed to be a place to talk just about RC.
Straight from the rules.

RE: 3 blade prop...help!
 

Ok, so its not OK to compare our props to full scale, but perfectly OK to post models and their engines performance, which seems to have nothing to do with the topic. Or am I getting my threads mixed up?

RE: [Deleted]
 

So when we compare R/C props to full scale props that is not talking about R/C? Why do you allow scale airplanes to compair them to the full scale planes? Unless I have my threads mixed up you deleted a post that I replied to that did not mention full scale at all, not fair to him.

RE: 3 blade prop...help!
 

Dave,


The MVVS Quickie .40 costs a bit more than the GMS.47, or the OS.40FX, but it spins that Master Airscrew 9x6 at no less than 18,300 RPM, immediately after break-in... (This prop will not hold for long at this RPM...)

If you re-read Bob's first post; he intended to run the .46LA with an 11x6, or an 11x7 and asked about 3-B equivalents.
Both are hardly comparable load-wise to that puny 9x6...

RE: 3 blade prop...help!
 

From personal testing I know for a fact that engine doesn't meet its advertised HP, nor does it achieve its peak HP at the manufacturer's advertised RPM. Dave Geirke knows it as well.

That engine reaches its peak HP (~1 HP) at ~13,500 RPM on 10% nitro fuel, at sea level. Prop-loading it for more, or less RPM, will result in a loss of performance.

Guidelines for 2B - 3B conversion are simply that -- guidelines. In the prop size-range that we are discussing, for a particular prop manufacturer, using the same material of construction & with props of the same blade geometry (shape, aspect ratio, etc) --- the approximate equivalency is: from 2B to 3B, reduce the diameter 1" for the same pitch, or reduce the pitch 2" for the same diameter.

In the case of an OS 46 LA developing peak power at 13,500 RPM, a 10-6 2B, or 11-4 2B is about right for general flying. Either of those props will let the engine pull a bit over 13,000 RPM static -- leaving a 400 or 500 RPM unloading margin. Staying with the same manufacturer etc, an appropriate 3B conversion would be a 9-6 or 10-4.

If more speed is desired with that engine, a 9-8 2B is about right, which translates to an 8-8 3B. Obviously, acceleration & climb will be significantly compromised by the smaller diameter props, regardless of any increase in level speed, & regardless of whether the props are 2B or 3B -- but that's life.

If you mix manufacturers, materials, blade shapes & aspect ratios, it becomes very difficult to use anything other than suck-it-and-see testing to determine which 3B prop is equivalent to your favorite 2B. The guidelines become almost useless, other than for vague ball-park guessing.

regarding efficiency --- staying with the same manufacturer etc, as noted above, a 3 blade prop for the same power load will always be less efficient that the 2 blade -- it isn't debatable -- it is simple fact & physics. There is more mass, more wetted area, more frontal area, one more root-transition zone and for the same pitch -- less diameter. All of which simply means more parasitic power losses from drag, plus the loss of disc area in the case of smaller diameter. The engine makes the same HP (same load), but it just produces less usefull work.

However, as soon as you start mixing & matching prop performance factors, you can have some 3B props that actually "outperform" some 2B props -- but you are comparing apples to oranges in those cases & this is where much of the confusion & endless debate arrises.

The more efficient the 2B prop design, the smaller the performance differenceswill be between the particular manufacturers 2B & 3B props. Conversely, the poorer the 2B performs, the disparity in performance of the equivalent 3B is even greater.

RE: 3 blade prop...help!
 

I agree, but I think the issue of which is superior. That depends on you viewpoint. The racer will probably think the blown fuelie is superior, but frugal grandma will think the 1.6 liter economy special is superior.

RE: 3 blade prop...help!
 

Hi!
I agree with most of the things you say but a a OS .46 LA in most high winged or low winged trainer/sport airplanes is not best proped with a 10x6 or 11x4 2-blade prop, not at sea level. A much better two blade prop for any .46 engine is a 11x6 prop, I say this from personal experience...not from any special "rule" of any kind.

Just because an engine develops its max power at a certain rpm doesn't mean it has to be propped to this particular rpm for best performance.
You must mach the engine/prop to the airplane it is going to sit in.
I agree that this engine could be propped with a small 9x6, 9x7,9x8" prop but only if it was in a Viper Q-500 type of model ...but in most trainers and low winged sport airplanes a 11x6 2-blade or 10x6 3-blade is the best size.

Regards!
Jan K
Sweden

RE: 3 blade prop...help!
 

Other than the fact that an 11- 4 works very well with that engine on a trainer, I don't dissagree with you Jaka, but this thread started with a pair of 46 LA's on a large Cessna 310 twin -- performance IS important in that particular case -- especially if it ever ends up on one engine.

It doesn't just have to fly around. A wide-set side-by-side twin had better be able to fly very comfortably at ~ 1/3 power on both engines if it is to have even a hope of survival on a single engine.

RE: 3 blade prop...help!
 

FLYBOY I thought when I got home from my meeting and read your posts ,You'd have picked up my face slapping glove and said ---Speed I have a 11x7 MAS 3 ordered at my LHS But NOOOOOOOOOOO I don't see that post.

RE: 3 blade prop...help!
 

I did not read all of the post and I'm sure this has been said before. The recommended prop for breaking in and flying an OS .46 LA is a 11/6. OS recommends an 11/6 or 7 so a 10/6 three blade should work just fine but you will loose some efficiency. Spinning a little prop at high rpm on scale plane such as this, produces noise and little else. If it was me, I would run 2 bladed props and spend your energy on making sure both of those engines are absolutely reliable on another airplane and I mean absolutely reliable. If you are getting any dead sticks, you better fix the problem before flying your twin. Run those engines 500 rpm rich and don't try to synchronize them with the mixtures. Just make sure they idle about the same, midrange about the same and peak about the same. The set up for both engines should be exactly the same right down the fuel tubing length. Just remember that you are 4 times more likely to have engine failure with 2 engines then one.

RE: 3 blade prop...help!
 

I already gave my prop recommendation for the plane....10-7 MAS.
The reason is I have experience with similar type plane, with similar
power. I have BB Magnum 40's, that are about the same powerwise
as the LA 46's. I had two twins, and have accomplished all the experimentation.

I know for a fact that the props I recommended will not give just satisfactory
but will give outstanding flight performance.

My point about the OS 40 was that it will in fact perform up to the high end
of it's ratings. One of the Internet Myths is that engines will not approach the
high end of their performance rating....unless "an unusable" prop is installed.
This is not the case.

Why do you introduce the MVVS Quickie engine into the equation ?

Another point you and others are missing is the fact that two cycle engines will
continue to make increased horsepower as the rpm increases....there is no
cut off point. This statement, for instance....is incorrect:

"That engine reaches its peak HP (~1 HP) at ~13,500 RPM on 10% nitro fuel,
at sea level. Prop-loading it for more, or less RPM, will result in a loss of performance."

This is one of the most ridiculous statements I have ever read. Any two cycle engine
will produce it's maximum HP at the Maximum RPM you can get it to turn. In other
words, the engine is producing more HP at 16,000 rpm, than it is at 13, 500.

I you have an engine that, for instance, will not spin past 12,000 rpm....regardless
of the propeller, then that engine makes maximum HP at 12,000 RPM. However,
if you change the parameters, like putting a pipe on it, and it turns 14,000 RPM,
the same engine is producing more HP. If you can get it to turn 15,000, the HP
will be even greater.

There is no "set point" for horsepower production on a two cycle engine, that's
why they are so successfull in racing applications....

....the more RPM's you can "get it to turn", the more horsepower it will produce.

Plain and simple. ;)

FBD. :D

RE: 3 blade prop...help!
 

Dave,


I entered this comment about the MVVS, since your photos clearly show models and engines destined for Q-500 competition of some sort, for which this Quickie engine is also usable in some places.

The rest of what you wrote about peak HP and RPM is grossly incorrect!


Horsepower results from torque and RPM and the formula, where torque is in lbs.-ft. is:
Torque multiplied by RPM, divided by 5,252.1 is equal to horsepower.
With torque in terms of oz.-in., it is divided instead by 1,008,403.2.

So as torque increases and as RPM increases HP increases...

Like any four-stroke, every two-stroke engine has a certain RPM, where it breathes most efficiently.
At this RPM the Volumetric Efficiency (VE) is maximized and the torque is at its highest level.

As you decrease the RPM, the value of torque is decreased, in some engines it passes secondary peaks, but it generally goes down.
So, in this situation since both torque and RPM decrease; HP is decreased for sure...

But the exact same thing happens to the value of torque, as you increase the RPM.
The engine's induction tracts and timing become less and less optimized and the torque is reduced.

At a certain RPM, the rate, at which torque decreases becomes identical to the rate, at which RPM increases.
It is at this RPM that HP output is maximized.

If you increase RPM further from that point, the HP output will be reduced.


And, two-stroke engines are successful in racing application, because they inherently make more output per cubic displacement than can four-stroke engines.

RE: 3 blade prop...help!
 

Sorry Dave -- you're just plain wrong. I doubt if you will accept that, but it doesn't change reality.

RE: 3 blade prop...help!
 

I don't know where you get that one -- my OS LA series manual (right in front of my face at the moment) shows a 10-6 as recommended, not an 11-6 -- in fact I don't see 11-7 anywhere in the manual. OS does recommend an 11-6 for the 46 AX & it also did so for the 46 FX.

--- but don't believe me about output & props -- go and read about this stuff somewhere else -- try the March 2001 issue of MAN for a starter. The 46 LA isn't there, but the 40 LA is, & it is essentially the same engine with the same power & torque characteristics.

RE: 3 blade prop...help!
 

Dar,

Everything is correct till the fifth paragraph. After that everything is confulsed. [:o] Begining with the maximized VE. The VE goes down in all engines from 0 RPM and up. Just a matter of how much and most have a point where it decreases falls off a cliff.

RE: 3 blade prop...help!
 

If your Magnums are XL 40's, ya got that one wrong as well.

Unlike the OS 46 LA, your Magnum XL 40's actually do perform best at high RPMs, & are relatively poor performers at the bottom end of their performance curve. If you are propping them with 11-7's, they will be turning ~ 11,000 RPM and will in fact have outputs similar to the LA 46, making just under 0.7 HP at that RPM -- just about right where the 46 LA is as well. However the LA falls off after 13,500 & ~1HP (with a 10-6 APC prop), whereas the Magnums keep puting out all the way up to ~ 17,500 RPM & make a bit more than 1.4 HP at that point --- but they need to be wearing a suitable prop to get there -- an APC 8-6 will do it.

If you put an even larger prop on the Magnum -- a 12-7 for example, its performance falls so much that even a feeble LA 40, rather than an LA 46, becomes a close performance match to it.