Problems caused by overpropping a 4-stroke
 

I am getting the magnum .91 to put in a TF .60 sized p-51. I am making it very scale, so a 4-bladed prop is a must. I figure, according to the 1 inch off for every extra blade, I would have to have something that was 11 inches. That seems quite small to me. I was reading that at the slower rpm prop wash doesnt have much effect, so I was wondering, what would putting a bigger 4 bladed prop on there do? I would want it for better looks and more thrust because its bigger and would swing slower? Does the engine get warn extremely fast?

props
 

The engine itself may have problems if you try a larger prop. Anything from erratic idle to very low top end.
As far as the prop goes, the slower your motor turns the better a multi blade prop will work! Thrust is created when a prop BITES the air. On our high rpm small engines, a 2 blade works wonders because there is sufficient time between blades. Now when you go to multi blade, time between blade is cut down and the blade is trying to push previously cut air(in essence cutting the vortex!). This is highly innefficient! More blades, even less efficient. So if you can get your motor down to about 4,800-5,500 rpm allowing the air to recover between prop passes you will regain efficiency.
Byron has a gear reduction just for this purpose on their big planes and also the reason you see 4 blade static props but pilots then put 2 blades on to fly.
You are also going to be handicapped using a Magnum!
Use a 2 or 3 blade and save the 4 blade for static use.

Problems caused by overpropping a 4-stroke
 

There is nothing dumber looking on a nice scale ship than a little ol' two blade prop except an even more riduculously small and narrow three or four blade prop. Unless you are going to use any of the old mostly off the market reduction drives you are headed for trouble particularly as you said "making it very scale" read heavy.

Do yourself a favor, make yourself a genuine exact scale static prop. Easy to do with a couple of big wooden clubs and your airplane will never look better. When its time to fly spin on whatever is appropriate.


John :)

Problems caused by overpropping a 4-stroke
 

You will not have any "handicap" from using a Magnum, and dont let anyone tell you different. I have a friend who flys a Don Smith Constellation on 4 Mag 91's and they are extremley reliable. If they were a handicap, I dont think he would put a mege$$$ bird up with them. However, if you use too large a prop to slow the motor (any motor) down, you can expereince overheating. You need to prop the motor to run at it's most effecient RPM to have good reliability,& power. The "dog" is right about four blade props, in most cases they do not work well.

Problems caused by overpropping a 4-stroke
 

The RCV engines work very well for these scale project. For one, they fit in the cowl very well. For two, they turn props twice as large.

Problems caused by overpropping a 4-stroke
 

Im no expert, but I believe if you overprop the engine severely you will run into overheating problems. I agree with John, make a nice static display prop instead...

Problems caused by overpropping a 4-stroke
 

Actually a 4 bladed prop is not that much less efficient, but ill suited for our purposes. The time between blades has nothing to do with it. For high speed planes a four bladed prop is often more efficient because moving a smaller air stream at a higher speed is an advantage when moving at a high rate of speed. I am afraid that using even a two bladed prop scale sized prop is a problem. I would consider trying the RCV engine if a scale prop is a must.

.90 engines and four blade
 

The RCV .90 would be an excellent choice, I have a RCV .90 on a H9 P51, it turns a17x6/10 Zinger four blade at 5,500. Otto sugested a 17x10 or 18x10 would be even better to hold the rpm to 5,000. I tend to agree. I also agree with Sport that the more blades and less effeciency thing is way over rated. At four stroke and Diesel rpm you can't tell the difference.

Problems caused by overpropping a 4-stroke
 

Yes, the slower it spins, the more efficient the extra blades are working...

RCS 140...
 

I too am building a TF P51 B (1/7 scale) version that I just purchased a RCS 140RE (Moki) engine to use in it. I am in contact with Zinger on the appropriate 4 bladed to use. RCS suggest a 16x8 for break in. I questioned Zinger about their 14x6-10 4 bladed... Is this a suitable match? I hear alot of rumble about props other than 2 bladed as far as performance but I had a Graupner 3 bladed with my trusted ole K&B 61 in a Goldberg Chipmunk and it pulled it around like a scalded ape (to use a phrase I saw here...)! It was amazing so I don't quite understand the argument that multiple bladed (read more than 2) props cut performance... My $0.02.

Justrfb (checking all my options)

Problems caused by overpropping a 4-stroke
 

Hmm. Isn't the major reason multiblade props are used in full scale is situations whn a 2 bladed prop that can absorb the engines power would be so large as to have either ground clearance isues or tip speeds over mach 1?

Problems caused by overpropping a 4-stroke
 

Prop clearance is a major reason for large aircraft. But better performance is also a factor. But our models usually don't fly fast enough for this benifit.

multi blade
 

You are correct about three and four blade props showed up with bigger engines. But RPM is still low. You dont have full scale engines spinning props @ 10k. Also you have variable pitch, Paddle wheel props to catch and bite more air.
We started this discussion with a standard type motor and normally we talk about generalizations. Again, a 3 or 4 blade is no where near as efficient as a 2 blade prop on most 2 and 4 stroke motors. Otherwise everyone would be using them( especially me!) Even the Purr power isn't that efficient! It will fly a warbird scale which not too many people like to fly(it's too slow). If an RCV will run good on a 4 blade @ 5k then go for it. No other 2 stroke or 4 stroke glow will do that.
The Magnum comment is from experience! I've broken in quite a few over the last few years and while the 2 strokes have improved, the 4 strokes can be hit and miss out of the box. A buddy had 3 91 4s's and 2 were ok, 1 was crap. Same on the 60s. All companies are capable of lemons but i see Magnum having a higher rate. For less headaches go with OS. If you feel froggy then get a Saito or YS.

Problems caused by overpropping a 4-stroke
 

People are confusing the term efficiency. The differance between a two blade prop and three blade is very low, 1 to 4 percent. However having the right match for the airplane is a differant issue. To keep the same diameter with a three blade you have to reduce pitch by about 1 inch. If you found the perfect two blade prop this probably wouldn't work as well. The perfect match would probably be to reduce diameter by 1/2 inch and increase by 1/2 inch, but you won't find such a prop.

Don't believe me ask this guy.

http://www.djaerotech.com/dj_askjd/d...fourblade.html

Problems caused by overpropping a 4-stroke
 

Another similar topic.

http://www.djaerotech.com/dj_askjd/d...bladeprop.html

Problems caused by overpropping a 4-stroke
 

My two cents worth: Back to snownskate"s original question:
I have a ancient Sig Quick-built Mustang approximately the same size as the Top Flite Mustang. I weigh about 9 lbs with an os 91 (pre surpass) 4 stroke swinging a 4blade Zinger hub with 16-8 prop blades cut down to a 12 dia. This gives a paddle blade appearance but certainly not scale. The plane flies quite well (big loops from level flight) but no unlimited vertical rolls like some I see. I have put a 14-7 2 blade prop on and the "performance" is very much faster (vertical rolls out of sight). However it doesn't look "scale" or sound "scale" to my likes. Similar results with the Byron Corsair with Sachs 4.2 and 3 blade prop. At our local Giant scale fly in I used to fly "combat" with another Byron Corsair with same engine but the 2 blade prop. His engine turned way faster and his plane was far faster, but to my tastes it wasn't as "scale" flight. Many of the spectators also say my planes look "smoother" in flight. Others say "looks like it needs more power". Boils down to how does the pilot want his airplane to fly!

Problems caused by overpropping a 4-stroke
 

I had a Graupner 14x7 three blade on two different airplanes; a CG Ultimate with an OS 120, and Super AeroMaster with a YS 120. Both airplanes had excellent performance, with the AeroMaster/YS combination being simply awesome. If your engine will swing it, a Graupner 3-blade will perform.

Magnum four-strokes , at least from what I have seen, are like Thunder Tiger helicopter engines; if you get a good one, it runs fine. If you don't, you're in for loads of misery.

Steve

Problems caused by overpropping a 4-stroke
 

Over propping a four-stroke glow engine can cause the engine to over heat. Over heating causes the "timing" to advance. If the timing advances too far, the engine will respond by over heating even more. You can see where this is going.

At some point, the timing can become so advanced that the engine will backfire. This is really hard on the connecting rod and crankpin. Enough of this treatment can cause a catastrophic failure.

A way to compensate for overloading an engine is to richen the fuel mixture. This may, or may not alleviate the over heating problem sufficiently.

If you are extremely lucky, you may find a point where the engine does not over heat sufficiently to cause damage and it still may produce enough power to fly the model. Many times you will not be this lucky.

The answer is to use a larger displacement/more powerful engine, or, utilize an engine that is designed to turn the appropriate size propeller at a usable rpm (RCV). They sound the same, don't they? They are not.

Problems caused by overpropping a 4-stroke
 

Agree with Artisan 100%. By experimenting with fuels and props you can find the largest prop that will not cause detonation. This is the same "knock or ping" that you hear in your car. Very detrimental to engines!! (Wish I had taken a picture of my Yamaha dirt bike in which detonation ate thru the top of the piston. Kinda looked like somebody had been picking at it with an icepick!
One other alternative... if you must use a specific engine and way overprop it, convert to spark ignition. This allows you to set the timing so as not to detonate. A case in point....I have a 1/5 pica spitfire that I wished to use the OS 1.20 supercharged 4 stroke engine. This is an FAI engine intended to swing 14-14 props at 8 to 9000 rpm. I wanted to swing a 18-8 3 blade Zinger prop assy. On glow, at full throttle I couldn't lean the engine past 5000 rpm without detonating. On CH ignition, set at 25 degrees BTDC, I can go full throttle and full lean ~6000 rpm with no detonation whatsoever. I will continue advancing the timing until detonation or no further rpm is noted, then back off a degree or two. Us scale guys will go to ridiculus extremes to make something "scale"!!

Knockin
 

RCW, most likely your Yamaha had too hot a plug in it, back when dirt bikes had two spark plug holes some guys put a hot plug in the extra hole thinking that if the plug they had the ignition wire connected to fouled they would simply move the spark plug wire to the hot one that was not fouled. Well, guess what, the hot plug ran just as hot whether the spark plug wire was connected to it or not and lots of pistons got burned and caved in from this, the very same thing can happen to a glow engine from too hot a plug, overloading or too much nitro.

Problems caused by overpropping a 4-stroke
 

Hobbsy
I think you are right on about the Yahama. That happened more years ago than I want to admit to!! On your comment about more nitromethane....it's seems strange to me that OS recommends 20% nitro, 20% oil fuel for the supercharged engine that is prone to detonation. Wouldn't you think lower nitro content would be better, all other things being equal? I agree the higher nitro fuel is going generate more power and therefore more heat in the engine, but nitromethane has a slightly higher octane rating than methanol and shouldn't knock as easily, right??? I am still in the break-in mode with the supercharged engine and experimenting to find the best fuel for this prop size. I am adding 3% castor to all fuels I'm trying in that engine until it is well broke in. Anybody got any suggestions on fuels for high load conditions such as overpropped 4 stroke engines (other than don't do it)???

Nitro talk
 

I bought one gallon of 20% nitro fuel to try in my two YS engines, there is so little difference between the way they run on 15% and 20% that I personally don't think it is worth it. I only use WildCat fuel now which has about 1.6% castor in it. There are some guys on here who are a lot more knowledgeable about nitro and its effects than I am. I don't run any engine without a little castor in the fuel. By the way I rode a 125 Maico for several years but was too old "30" when I started.

Problems caused by overpropping a 4-stroke
 

How it responds to nitro depends on the compression ratio. If it is not responding it may be detonating. It matters not if it is supercharged. AA fuel dragsters have been running superchargers and 100% nitro for years, but their total compression ratio is below 5 to 1. Recently the NHRA has banned 100% nitro because of the large number of engines blowing up. The ignition technology has allowed the mixture to get so rich and so much fuel poured into the cylinders that the engines can literally explode. This isn't just the blower blowing up from a backfire or detonation, just the mixture getting too rich and the ignition firing at the wrong time. They must now use a minimum of 10% methanol. The number of engines blown up has been greatly reduced.