Propeller Thrust

[coosbaylumber]

I often work with older models. On occasion, the plans cite not only the powerplant, but the preferred propeller to be used.

Considering using today's glow engines, and other things now available, would it be proper to derive some sort of equavalent to the older equipment to obtain performance like original? Thus take to often cited K&B 45, turning a 12X6P (Top Flite?) propeller.

Considering we now have glow engines that rev up higher, lighter, better propellers, and the like. What would a modern equivalent now be? An imported 40 turning a 11 X 6p?



Wm.

[dgfisher2004]

I've wondered about the same thing. Alot of the older plans I've reviewed recommend very different setups than are recommended in more modern engines. I don't know how you could gauge the different degrees of seperation between the ages. The transition into more modern engines and propellers has always been a relatively gradual process.

The date of the recommended setup would be a huge factor in considering this. Also the varying stages of developement in glow engines and props would have to be chronographed to accurately place the out-dated setup in it's time period.

I think a general knowlegde of engines and props, built up with experience, would be the only sure way to convert out-dated setups to a more modern engine and prop. You know what they say, sometimes there's no substitute for experience.

[Ollie]

Thrust varies with prop type to type for one Dia. x pitch size. Thrust varies with air pressure, temp and humidty. Thrust varies with fuel nitro %. Thrust varies with tank pressure. Thrust with carb. adjustment. Thrust varies with tank position. Thrust varies with break-in history. Etc, etc, etc.

[vintage1]

I've wondered about the same thing. Alot of the older plans I've reviewed recommend very different setups than are recommended in more modern engines. I don't know how you could gauge the different degrees of seperation between the ages. The transition into more modern engines and propellers has always been a relatively gradual process.

The date of the recommended setup would be a huge factor in considering this. Also the varying stages of developement in glow engines and props would have to be chronographed to accurately place the out-dated setup in it's time period.

I think a general knowlegde of engines and props, built up with experience, would be the only sure way to convert out-dated setups to a more modern engine and prop. You know what they say, sometimes there's no substitute for experience.

If you want to duplicate a setup, you need an engine that will turn the same prop at the same RPM.

Electrifying vintage arircraft for old engines shows that they used large engines turning biggish props rather slowly.

You can fly on a geared electric at ridiculously low power levels: To do the same n an IC engine menas overpropping a big one to get the diameter up and the RPM down...very inefficient.

The only aircraft that seem to use IC engines at anything like their best power out pout are the very fast models using small props and extremely high RPM.

Although a good 40 2 stroke is capable of over a brake horsepower, my guess is that most setups barely achive half that, or a little more.

Thrust is always a bit of a red herring: But with engines turning similar props at similar RPM's ist a little bit meaningful and easy to measure. However it does not deserve the high reputaion as a yardstick of performance that it seems to have.

[JMP_blackfoot]

You have to take into account that the power to turn similar shaped props varies like rpm^3 and diameter^5.
To obtain similar thrust with say a 12x6 prop at 9000 rpm and a modern engine with 10 inch prop, calculate as follows :
required rpm with 10 inch prop = 9000*(12/10)^(5/3) = 9000*1.2^1.5 = 11830 rpm
Pitch can then be estimated thus :
6"*9000/11830 = 4.5"
Thus if your original older powerplant turned a 12x6 prop at 9000, you will get close enough equivalent power, flight speed and thrust with a modern engine turning a 10x4.5 prop at 11830 rpm. (That is if the fuselage has a reasonable cross section and streamlined shape)

In the same manner, an 11x5.25 prop turning at 10250 rpm is nearly equivalent to a 12x6 at 9000.

Another manner is to check the power curve of modern engines, and pick one which has the same power as the older engine at the required rpm for the original prop.
Loading a modern engine of the same capacity with the larger prop of the original engine often results in similar rpm, this being more or less true up to about 8000 rpm or so, the higher power of modern engines being largely due to their ability to maintain useful torque at much higher rpm than their predecessors. In the lower part of the rpm range however, the slope of the power/rpm curve is pretty similar for engines of similar capacity.

Hope it helps.

[coosbaylumber]

Blackfoot.....

You hit directly upon what I had assumed. It does not take same size cubic displacement with same diameter propeller anymore. I located one of those computer based thrust estimaters.

You insert propeller make, diameter, pitch and it will tell of the amount of thrust.

However, it does not tell of same for elderly brands of propellers, nor any of the electric types.

I was sort of thinking that a modern 35 or 40 could equal the thrust of an older 45. Plus save some weight.
Wm.

[JMP_blackfoot]

I am not sure that you would save weight in all cases.
The K&B Torpedo 45, for example is extremely light for its capacity (It is actually a bored out 35). On the other hand, a Merco 49 is heavy, its crankcase having been sized to house a planned 61.
Also, because the engines peaked at lower rpm, older props were not so wide and thick as nowadays.

[vintage1]

Blackfoot.....

You hit directly upon what I had assumed. It does not take same size cubic displacement with same diameter propeller anymore. I located one of those computer based thrust estimaters.

You insert propeller make, diameter, pitch and it will tell of the amount of thrust.

However, it does not tell of same for elderly brands of propellers, nor any of the electric types.

I was sort of thinking that a modern 35 or 40 could equal the thrust of an older 45. Plus save some weight.
Wm.

And thats the rub. IF you put a greabox on a mdern engine and ran it at say 5500 (prop) RPM swinging a big prop like the old sparkers and long stroke diesels did, you copuld fly on a smaller engine: That's how we get away with better performance when electrifying, on very little power.

But a racing 36 swinging a 9x4 at 18k is never quite the same as a big old 60 swinging a 12x10 at one third the RPM...no matter how much the static thrust and pitch speed remain similar.

What do you want? A vintage flying expeinec? Use the original engines ..and a valve single channel set!!or a model that flies OK but sounds like a hornet on amphetamine? Use a modern racer on a small prop...or the vintage experience that all the oldsters would have died for if it had been possible,? stick a geared electric and some LIPOS in it , modern radio gear and waft around quietly for almost ever...

[dgfisher2004]

Coverting an old setup to a more modern one, be it electric or glow, would be simple enough, but I was under the iimpression you were looking for auniveral "equation" or formula that would apply to all the vintage setups.

[JMP_blackfoot]

But a racing 36 swinging a 9x4 at 18k is never quite the same as a big old 60 swinging a 12x10 at one third the RPM...no matter how much the static thrust and pitch speed remain similar.
Vintage, I'm with you there. ;)

Basically, I would advise to use an engine of the same or a bit lower capacity, with the same prop size as the original. The now mandatory silencer and the trottle will keep the rpm in the correct range.

When I wrote the rules for the French chapter of SAM (4A - Association des Amateurs d'Aéromodèles Anciens - SAM chapter no. 70), I noticed, as explained above, that the bottom of the power curve is strangely similar for a given engine capacity, whatever generation it may be. For this reason, I specified that the prop should be a minimum diameter according to engine capacity. For example, a 2.5 cc engine must have a prop at least 250 mm (10") and a 10 cc. engine a prop of at least 350 mm. (14"). In this manner, we can use even a modern engine, but only in the lower rpm part of its power curve, where it does no better than the original vintage ones. This has successfully ensured that the whole character of the models are preserved.

I understand that the original question was aimed at more recent models, probably aerobatic, in which case replacing a K&B Torpedo 45 and a 12x6 prop with a modern 35 or 40 with an 11x6 prop would be an acceptable substitute.

Of course, it is quite possible to find the original engines on Ebay, which sounds like (and is, in my opinion) a good idea, but there are two snags which must be kept in mind :
- In case of a crash, engine spare parts will be difficult to find
- Because a silencer must be fitted, the power will be reduced.