Overloading an engine
 

What happens when you over prop an engine in such a way that the max RPM is 20% lower than with a typical prop?

Does it damage the engine? If so, does the damage occur immediately, or does it happen over time with repeated flights with the larger prop?

Thanks,
Juice

RE: Overloading an engine
 

Horsepower is lower when running an engine outside of the power band and the engine will usually run hotter. Little damage is done unless the engine seizes but it generally is not a good idea.

Bill

RE: Overloading an engine
 

As Bill says, it depends on how hot it gets and for how long whether there is any damage or not.

RE: Overloading an engine
 

The RPM figures that manufacturers give may not be realistic for the rest of us, so use your judgement on what is the right curve for your engine. Some people don't realize that the highest torque is below max RPM. It depends on what you are after.

RE: Overloading an engine
 

Our club had a low noise limit and it was common to fit larger props to reduce rpm and hence get inside the noise limit.

Not a problem unless the cooling is marginal.

RE: Overloading an engine
 

Thanks for the answers guys. I'm going to try it for one flight, just as an experiment. From what you guys are saying, it should be okay as long as I make sure the engine is kept cool.

The engine is an RCV 90SP (the weird one with 2:1 gear reduction to the prop). The prop hub is custom made and features removable and adjustable pitch propeller blades. The propeller blades come from Soloprops ([link]http://www.soloprops.com[/link]).

I've been flying with a 16x11x3-Blade configuration at 5200 RPM. Remember it's geared down... the prop is spinning at 5200 RPM, but the engine is chugging away at 10,400 RPM. Next time out, I'm going to try flying with more impressive (visually) 18" blades set to 10" pitch. I expect to get 4400 - 4500 RPM with that prop configuration. That's more than enough to take off, and it'll fly at half throttle.

Check out this video of this setup on a test stand...
[link=http://www.vimeo.com/1443554]RCV 90SP + 18x10x3-Blade prop[/link]

Thanks,
Juice

RE: Overloading an engine
 

Sounds like a winner

RE: Overloading an engine
 

First one must understand the concept of a “hot” and “cold” glow engine. If the ignition timing of a glow engine is correct it is neither “cold” nor “hot”. If the timing is too fast it is hot, a very undesirable condition because it will cause the engine to overheat which in turn would advance the timing even more leading to thermal runaway. IF your engine were running perfectly and you changed to a much bigger prop it will now be running hot because of pre-ignition. You would now have to “cool” the engine, to do this you can reduce the amount of nitro, use a cooler glow plug or ad a head gasket. I have successfully ran my OS91FX with a 15X8 APC prop, tune pipe and 15% nitro but I used a F plug and a extra head gasket. At 9300 rpm it made very little noise and had a very smooth power curve.

RE: Overloading an engine
 

Piet Le Roux asserts: I am curious why this is true - why would a bigger prop lead to pre-ignition?

RE: Overloading an engine
 

Since the timing is set by the glowplug/fuel/compression, for a given prop, you optimise the correct fuel to air mixture, if you leave all others alone. A lean mixture will ignite earlier than a rich one - it is the same as advancing or retarding the ignition on a spark-ignition engine.
If you put a bigger prop with a higer loading, the engine will take a little bit longer to make a full revolution. If the mixture is set for a perfect ignition timing with a smaller prop, it will ignite too soon and lead to over-heating.

Leaning/enrichening the fuel mixture adjusts the timing on our model engines, that's why they run hotter on a lean mixture setting and cooler (retarded ignition) on a rich mixture setting.

RE: Overloading an engine
 

A glow engine ignition timing is related to glow plug, compression ratio, and mixture. As RPM drops with everything else the same there is more between ignition and when the engine reaches TDC. If too much of the mixture burns before TDC the engine gets very hot. Ignition point can be delayed by richening the mixture, reducing compression ratio, or installing a colder plug, or a combination thereof.

RE: Overloading an engine
 

Hi!
Look at it this way: the bigger prop takes longer to turn than a smaller prop...now the ignition takes place when the the piston is on its way up...even further down than before using a smaller prop...what do you think will happen when the flame front hits the top of the piston..??

RE: Overloading an engine
 

All summer I've been flying my H9 Corsair with an RCV-90SP and a 16x11 3-blade prop. The engine likes this prop and I can get a peak 5400 RPM (2:1 gear reduction built into the engine).

I finally tried over-propping my RCV-90SP engine. The motivation is to try my custom made 3-blade hub with 18" blades.

Here's the video of my attempt:
[link]http://www.vimeo.com/1918254[/link]

The question again is... If I make sure the engine does not over heat, is there any long term negative effects of over propping an engine? I can really get used to flying with that nice big prop. :D

Thanks,
Juice

RE: Overloading an engine
 

As long as the engine is propped to spin above its peak torque RPM, at full throttle, you should have no problems at all.

If you prop it BELOW that RPM, it would lug and possibly detonate (that could damage it).
And when flying at speed; because the torque rises AND load is reduced simultaneously, the engine will 'unload' more than usual, possibly causing the mixture to become too lean...

You don't want that either, believe me.


Oh... You don't know what RPM the peak torque is!? Use a good prop calculator and do the math with known prop loads.

When you increase the prop load, RPM is gradually reduced... At some point the RPM drop will be greater than you expect...
This is because you have gone below the peak torque RPM.

RE: Overloading an engine
 

The real problem is ignition timing that is too advanced for the load on the engine.

Overloading an engine can advance the timing to the point of preignition. Preignition causes the engine to run hotter than normal, or above the temperature that the engine can safely handle.

There are other ways of accomplishing the same thing. Running too much nitromethane, too lean of a mixture, too much compression, etc. If preignition is too severe, it can lead to detonation. Detonation will destroy your engine fairly quickly.

As others have mentioned, cooling airflow in adequate amounts are mandatory for normal operation when everything is perfect. Flying a model with marginal airflow to the engine and with one of the above causes of preignition present can ruin your engine fairly quickly. Remember to have a large, directed air intake (internal baffles) with an air exhaust opening two to three times the area of the intake opening. This is the rule of thumb that has been proven to be true over the years with engines that are not being driven into preignition or detonation.

If you absolutely must run a larger than normal prop on your engine, for whatever reason, you can do several things to reduce the likelihood of preignition or detonation. One is to reduce the load on the engine by reducing your propeller's pitch. If that is not acceptable, reduce the nitro content in your fuel. Or, switch to a colder glow plug. Last but not least is reducing the compression of your engine by adding a headshim. These measures are only necessary if the engine is exhibiting signs of overheating (preignition/detonation).

It will not harm your engine to employ the previously mentioned tactics if you're not sure your engine is experiencing preignition or detonation. The engine will just produce a little less power. However, if your engine improves with power production with these changes, you are simply confirming that the engine was overheating due to advanced ignition timing and you did the right thing.

Good luck and tell us how things turn out for you.


Ed Cregger

RE: Overloading an engine
 

I don't think it's possible to have over advanced timing with any load given the way we tune our glow engines for peak (or a little below) revs. Any hint of over advanced ignition will cause a loss of power and therefore loss of revs which would immediately be thought to be too lean a mixture. Not long ago I put a 13x6 prop on an ST45 just to see how it'd run. It was flat out at 9100 but showed no signs of stress and tuned as easily as usual.

RE: Overloading an engine
 

I think it is possible to have the timing advanced too far as I have also noticed the same thing as Ed from time to time. I suspect that when overloaded the mixture is already so rich that any richer doesn't do much for the timing, thus the peak is at a point the engine is too advanced and either detonating or about to detonate, such that next time in warmer weather it does detonate.

RE: Overloading an engine
 

prop 14x12 / 16x8

Above is the prop recommendation right from Webra's specs for the P-5 .91.

Since I love to tinker and experimant I ran the P5 with an APC 14x12 which it turned at 8,350 rpm on Fox 5% fuel wtih 20% castor.

I then ran the 14x12 on my MVVS .91, please don't cringe Dar, it handled it just fine at 8,050.

I then tried the 14x12 on my SuperTigre .90, same fuel and it could only manage 7,000 and very quicly got hot.
I'm not sure what this tells me but I've seen several people come on here disappointed in their P5 .95 when they ran 14x6 and came up short on rpm compared to the old Webra .91s which would turn a 14x6 10.000 rpm.

The bottom line is, if you run the P-5 with a 14x12 and the SuperTigre with a 14x6 they both do their intended jobs very well. I suspect the MVVS .91 would work best with a 14x8.

RE: Overloading an engine
 

Dave,


I won't cringe, since MVVS specify up to a 15x10 on the .91...

Also, I am not surprised at its slight disadvantage on the same prop, compared to the Webra 5 port engine.

The webra, which has a 17 mm crankshaft, has a larger gas passage and it runs better with a muffler, while the MVVS is more inclined to use a tuned-pipe... And with a large prop it will need a long one.

With a long tuned-pipe, Mike Billinton got pretty good results with the Super Tigre, in [link=http://www.supertigre.com/engines/supg0235-man.html]this MAN test[/link], although RPM numbers are hardly mentioned.

RE: Overloading an engine
 

Detonation, preignition, and the like aside, there would also be more side loading on the piston of an engine that is being over-lugged. This can lead to advanced wear/scoring of the piston/liner. Ross

RE: Overloading an engine
 

Sorry Dar, I did not mean to indicate that the MVVS .91 has less power, I don't think the MVVS has a whole hour on it yet. The MVVS .61 picked up about 500 rpm, (as a Diesel), over a season as Pe' said it would. I'd bet the .91 will do the same.

RE: Overloading an engine
 

No problem at all, Dave.

No need to apologize...[:o]


We've slipped off the subject of this thread, I believe.

The MVVS does gain a great deal more power than other engines, as more time is logged on it.
This is true of all MVVS ABC engines.


The MAN test I referred to indicates the ST has its peak torque value just below 6,000 RPM, which is very low for a model two-stroke engine of this size. But even though it spins it faster than what we think is peak torque, it appears to be overloaded... Could your engine be so different from the one Mike Billinton used for his 1996 test?

Could it have different timing numbers that shift the torque curve up and it now peaks higher than the RPM achieved on that prop?

These numbers are advertised in the test and you can if your engine's numbers differ...


An interesting test would be to see if a 14x10, or a 14x8 prop of the same make (if you have them...), would make the RPM jump at a certain point...


If the 14x10 is spun at 8,500 RPM, for example (a gain of over 1,000 RPM), it would be an indication that you had just crossed to the safe side of peak torque...

The MVVS and the Webra seem to be in the safe area anyway.

RE: Overloading an engine
 

I went to the LHS to pick up a colder glow plug. I explained to him why I need it. He ended up talking me out of it, explaining why it wouldn't work for what I want to do. He gave a couple reason's:

1) 4-stroke engines (like the one in question, RCV 90SP) needs HOT glow plugs to keep the filament lit up for all four strokes of the piston. A cooler plug would lose it's glow between ignitions.
2) Cooler glow plugs are used in 2-stroke engines for high performance applications, like racing. It wouldn't work to put in a cooler glow plug and just lazily fly around.

I'm going to try lower nitro after my current jug of fuel is done.

For now, I'm trying to get my head around this peak RPM thing. How can I find it? My propeller is custom made and I do actually have the ability to adjust the pitch (on the ground only). I have some RPM numbers at various pitch settings. Can anything be determined from these numbers?...

18x13, 3840 RPM
18x11, 4400 RPM
18x10, 4450 RPM

16x13, 4760 RPM
16x12, 4960 RPM
16x11, 5200 RPM

The typical RPM for this engine is between 5000 and 5400 RPM. This number is about half of a typical engine because it has built-in 2:1 gear reduction.

Juice

RE: Overloading an engine
 

Wrong, a four stroke engine only needs a plug that is hot at idle, even then it may not be critical. There are brands of four stroke plugs that are colder than others. A K&B 4C plug is about the coldest four stroke plug I have found.

Wrong again, any time you increase nitro you may need a colder plug. Especially if the engine has a high compression ratio.

IMO most hobby stores give poor advise except for the most rudimentary. There are plenty of experts here, 90% of them give better advise than you LHS.

That said, why is it so important to be peaked?

RE: Overloading an engine
 

Four-stroke glow engines need a glow plug with enough mass to stay lit between power strokes. This does not take the heat range into consideration.

The heat range of the glow plug is a completely different matter than the persistance (how long in time-units the glow plug element will stay sufficiently hot to fire the next fuel/air charge).

Had I known that the engine in question was an RCV four-stroke, I would probably have answered your question in a different manner, though I'm not backing away from what I stated previously. It is still true, in general.

I'm not at all current on RCV engines, however I seem to recollect that one of the customer complaints concerning them was the exceedingly long break-in period required before the engine was usable. Once broken-in, they appear to operate quite well, so it goes.

Since this engine does use a rotary intake/exhaust valve (rotating drum, IIRC), it is possible to damage the engine without using the specific glow plug(s) that are recommended by the manufacturer (over reach, damaging the drum valve). I would contact the manufacturer or distributor about your concerns, since the RCV engines are different enough from other engines that they have their own idiosyncracies and tricks. Good luck.


Ed Cregger