Scale Models flying with Scale Propeller?

[Joshua Harel 2]

Hello All

I just ran my Saito 220 on the bench with a 24-8 Zinger propeller on 5% nitro Omega fuel. Since I did not know what to expect, I ran it slightly rich and at full throttle recorded 4500 rpm without ever over heating.
My question is strictly to those who fly scale and are not afraid of going against commonly accepted (right or wrong) conventions:
Is anyone flying such a prop (or similar) for the sake of having a scale model with a scale prop and almost scale sound??

[Ady Hayward]

Hi Joshua,

It is largely a case of if the scale prop is suitable for your motor then it can propel the aircraft safely with little problem. The issues are if you over prop or under prop the motor then you are operating inefficiently and may not generate sufficient airspeed to fly safely, or operate in a regime that may damage your motor causing in flight engine failure and the issues that can bring.

Adrian

[Daithi O Buitigh]

It's the same with wing sections in a way - the air molecules don't scale down and remain the same size as those encountered by full size aircraft, so allowances need to be made.

From the very start of scale competitions, a scale prop earned points for static judging, but the rules always allowed its replacement by an 'efficient' one for the model during flying

[Biggles' Elder Brother - Moderator]

An interesting first post to introduce yourself to the community on here Joshua - proscribing who exactly is being addressed and presumably who is to reply as well?

Definitely different,...

BEB

[Martin Harris - Moderator]

My 1/4 scale Cub flies on an 18 x 6 (ASP 160 twin) and the full size uses a 72 inch prop. The prop was chosen for noise test compliance and the diameter is a happy coincidence!

However, the max RPM is probably 3 times that of the full size...

[Joshua Harel 2]

BEB and Adrian

I apologize for using such a topic as my first post, I agree that it is unusual. To make up for this, let me please introduce myself and explain the reason for my post.

I am 59 years old and have been modelling since the age of 12. My main interest is scale and, I always felt that true scale should include a scale propeller if and where practical. I am about to start building a 1/3.7 scale model of a Fairchild 24 Argus and it so happens that a scale propeller for this model would be 23-1/4 inches in diameter. I thought to myself: the Saito factory recommends a max. prop of 20X8 but question is: why? is that based on practical research derived from Dyno Testing or an arbitrary selection dictated by "Common Practice"? I suspect that the answer to my question is the latter and hence my experiment. Anyone who has flown a model this size (117" wing span, 20 - 25 Lbs) would most often use a 2 stroke motor of around 50 cc or larger spinning at around 7,000 rpm or more with a 18 - 20 inch propeller - either blasting away at unrealistic speeds or throttle back to 1/3 the takeoff power for a realistic flight. Question than is: why not do it with a realistic scale propeller from the get go?

I fully realize that this propoisition will not apply to a 1/4 scale model of say a Spitfire or Hurricane.

[Danny Fenton]

Mmmm interesting suggestion. I do like to fly scale with a scale sized prop, but I have the luxury of the torque provided by electric motors. I think to get the sort of torque you would need from a 2 or 4 stroke IC you would need gearing, and this indeed is what some do.

To me there is nothing worse than a toothpick pulling a beautiful scale model along.

Probably the finest scale model I have had the pleasure of seeing, Phil Clark/Ali Spit MkI Electric 1/4 scale with scale prop.....

Cheers

Danny

[Joshua Harel 2]

Danny

I agree that electric power can solve the problem of adequate torque but to me it lacks the smell and sound of a classic IC 4 stroke mill. I know that one can add a sound system but when you are talking 1/4 scale models - electrics start getting quite expensive and even cumbersome in terms of huge battery packs, charging at the field etc.

[Pete B]

Check your PM's, please, Pat....

Pete

[Concorde Speedbird]

Lots of the WW1 guys use gear drive reductions, here are some example of some others too. Personally I do not think it is necessary since when it is spinning you can't see it anyway.

CS

[Biggles' Elder Brother - Moderator]

Hi Joshua, thanks for the background - nice to have you with us.

yes I too am uncomfortable with the sight of a scale model with an obviously too small a diameter prop. I agree with the comment that absolute scale props are not always possible - or at least efficient - because of the flow characteristics. And of course IC suffers from the torque issue.

Like Danny I largely use electric power these days so I can, and do, usually go for "diameter" over "pitch" as the way to use the power and torque and, while my props may not strictly speaking be scale, they at least look a believable diameter!

BEB

[Frank Skilbeck]

Yet the one IC engine, the RCV SP series with the 2:1 built in reduction gearing, didn't seem find a great deal of favour with the scale guys. Maybe because finding commercial props of the correct size wasn't that easy for one reason, and due to the engine configuration cooling could be an issue.

Not scale but I'm just building an electric tug and will use a 20 x 10 prop on an 8s battery (235kv motor) over 7kg of static thrust at 5600 rpm and on a 10s it would provide 10kg of thrust at 6,600 rpm. Your 24 x 8 at 4,500 rpm has a static thrust of around 7.2kg with a theoretical speed of 55 km/hr, and is absorbing approx 1.3HP whereas on the benchmark prop 18 x 8 at 9200 rpm thats over 3.5 HP and approx 12kg of static thrust, so by running the engine well below its best power rpm you are only using 40% of the power available.

[cymaz]

Doing some quick sums, a full size Stampe could have a prop of 1.9m. Or 6 1/4 ft. Quarter scale Stampe would give you 19" prop size. My motor will drive a 20" prop so I will be keen to try that to see if it works ok.

[Joshua Harel 2]

Since I do not fly a glider tug I could care less about squeezing every last ounce of thrust out of the engine. I much prefer to operate at the point of max. Torque rather than Max. HP and if that come with realistic scale speed, and, less stress on the engine - I see it as a win-win situation. On top of that, I am much more comfortable with low noise at 4500 or less rpm than a screaming engine at 9200 rpm.

[Concorde Speedbird]

Here is a Hurricane build with a scale sized prop if you are interested.

CS

[Frank Skilbeck]

Posted by Joshua Harel 2 on 24/04/2014 01:10:32:

Since I do not fly a glider tug I could care less about squeezing every last ounce of thrust out of the engine. I much prefer to operate at the point of max. Torque rather than Max. HP and if that come with realistic scale speed, and, less stress on the engine - I see it as a win-win situation. On top of that, I am much more comfortable with low noise at 4500 or less rpm than a screaming engine at 9200 rpm.

Joshua, yes I wasn't saying your approach wasn't OK, just using this as an analogy of why Saito recommend a prop to get the best out of their engine. Of course a single cylinder engine at 9,200 rpm has the same number of exhaust pulses per second as a four cylinder engine running at 2,300 rpm.

[Simon Chaddock]

Joshua

I would agree entirely about scale props and as Danny points out this is easier to do efficiently with electric motors.

There is a downside of course that a scale prop in both diameter and number of blades has scale torque as well and for many high powered single engine types engine torque is a significant issue. A factor made worse by the the non scale aerodynamic requirements that mean models have to have a higher power to weigh ratio as well.

Then of course there is the issue of flying at a true scale speed.........

[Danny Fenton]

mmm scale speed, now that is a rock best left muddy side down

There are some scale prop manufacturers out their, Wayne Fussel runs Solo-Props for one and Christian Ramoser's VarioProps

Cheers

Danny

[Solly]

Frank, I also cannot understand why the RCV SP engines are not more popular. I have a number of these and find them very good. The oldest one, a 90 SP, has now done well over 12 hours of running with a fifteen and a half inch four bladed prop and pulls a ten and a half pound model around very nicely. The cooling question is easily solved by using a simple balsa sheet scoop inside the cowl to deflect air through the cooling fins.

[ceejay]

I can only say from experience with petrol engines in that they can be tuned to run on a scale or near scale prop, what will suffer is throttle response and revs, fuel consumption and thrust delivered, but if you are flying a scale model you probably dont need 3D aero type throtle response, you can in all probability fit a bigger tank, and as long as the thrust delivered is sufficient the revs are irrelevant so no problem, to see this theory in action see Ian redshaws HP42, you tube it, flying on 4 18x6 4 blade scale props on 4 Kalt (old zenoahs) 22,s

here,s one

**LINK**

cj

[Joshua Harel 2]

Ceejay

Thank you very much for the encouraging comments and proof. It just adds credence to my claim that model engine manufacturers are providing much useless power claims that don't amount to a hill of beans when an engine's performance envelope is concerned.

[reg shaw]

Joshua, do you have a model in mind? It really depends on your prototype as to how easy it will be to swing a scale prop, especially using IC for motivation. Generally, the larger the model the easier it will be to achieve, but speaking generally, a WWII aeroplane will be hardest to get the prop size to work, due to the huge props most WWII warbirds swing. Like Ceejay has said, petrol two strokes can be tuned to swing large props, silly large if they are geared. You used to be able to buy gear drives for glow motors, Avicraft was one maker, plus Webra and OS made geared 61 size two strokes that would almost swing scale sizes. The downside for tuning ungeared two strokes to swing large props is the slow throttle response, to the end that if the throttle is whacked open the engine will quit. This isn't really a problem though as you'd not be whacking the throttle open on your scale model anyway. I use the servo slow on my Tx to limit the speed so no matter how fast you are with the stick, the motors don't get flooded and stay completely reliable.

Ian.

[Joshua Harel 2]

Hello Ian

I plan on powering a Mick Reeves 1/4 scale Sopwith Camel. I have been talking to Jon at Laser Engines and (although Jon is understandably skeptical) I have a feeling that the 180 singly cylinder may swing a 25-1/2" scale propeller potentially around 4000 rpm with very lovely sound. I plan on buying one and bench testing soon.

Joshua

[cymaz]

Posted by cymaz on 23/04/2014 16:30:20:

Doing some quick sums, a full size Stampe could have a prop of 1.9m. Or 6 1/4 ft. Quarter scale Stampe would give you 19" prop size. My motor will drive a 20" prop so I will be keen to try that to see if it works ok.

Now use a 20x8 JE wood prop

[ZK BAP]

Hi guys, just noted post/question. I have a 1/4 scale Auster Aop9 with a DLE35ra. What would be a good scale size prop? Keeping in mind power is no problem, but want torque to maybe tow gliders etc?

Cheers

ZK BAP