Full Scale Flying speed vs scaled down RC flying Speed?
 

Ok, so I have been working on a power set up for my 160in C130, I have not weighed it yet, but I'm expecting 55-65 pounds.

First I know having 4 motors making x amount of thrust is more efficient than 1 motor making the same thrust as the 4 different motors have more disk area to grab, and they blow over more wing area.

I'm going with a scale size 16.5 x 12 pitch blades x 4 like scale. I know in full scale they are able to turn there pitch way up as they are not spinning massive rpms.

So using a SK 6374 192kv motor (huge) on 8s 5000 40c lipos, I was getting around 13.5 pounds of thrust on my make shift pulling testing rig. I measured the rpm at 5100-5200 rpm pulling a massive lol 45 amps at WOT and 20 amps at 1/2 stick.

At this rpm I was at 58-60mph prop speed, and a C130 is not the most aerodynamic(and I don't have retracts so wheels are always down-not far on a C130)

Full scale flying speed is 368 MPH and at 133 feet ws x 12in = 1596 round up to 1600 for easy math and my 160in C130 is 10th scale. So .10 of 368mph is 37MPH

There is no way this big plane is going to fly max speed of 37mph and still stay in the air! or I'm https://static.rcgroups.net/forums/i...s/confused.gif

My current prop speed on 8s is 59 mph, my boy suggested I move up to 9s vs 8s, but this will roughly give me 625 more rpm with a prop speed of 66mph vs current 58-60mph prop speed.

The extra thrust will not be a factor as I think I'm way over thrust as it is, currently with my 13.5 pounds x 4 = 54 pounds of thrust for a 55-65 pound C130 with a huge wing, Ill have more than enghof thrust as is, the extra lipo cell if all fallows the same lines will move me up to 15 pounds of thrust per motor with a total of 60 pounds, I'm not trying to hover here LOL https://static.rcgroups.net/forums/i...s/rolleyes.gif

What would you guys do?

Thanks

You didn't specify the prop pitch. at that weight, I'd expect about 65 mph flying speed and would shoot for 80 mph pitch speed, but by prop pitch and not by cell count.

Its a 16X12 4 blade prop, at current 8s its Pitch Speed is 59.5mph - 54 pounds of thrust, I have decide to go with the 9s set up, for 60 pounds of thrust and around 64.5 Pitch Speed, this will really help with my jato rocket take off in a 55-65 pound plane. Im still concerned about the low 64.5 pitch speed, I don't want to be flying full power just to keep her in the air.

Have you thought about going to a smaller prop? A 1/1 thrust ratio isn't really needed on a scale plane like that, so you could trade some diameter for some speed.

The correct dynamically scaled speed is F.S. Speed x the square root of the ratio of size. In this case roughly 116 mph.

Dynamic scale speed is useful to aeronautical designers but does not provide the correct visual flight appearance.

Yes it's true! Full size airplanes fly awfully slow compared to models, if you want true 'scale speed' then you need to build real light...think 'indoor'...
Evan.

To give an accurate visual flight appearance, you need to scale time as well. In other words, you can only observe it from film. Mathematically, models do not turn like full scale. For instance a standard rate turn takes two minutes. The bank angle depends on speed of the airplane. So if your model is banked at the same angle as a full size to give a prototypical appearance in the turn, it will happen in a much different length of time.

For the look of scale flight, you need to consider how many fuselage lengths the model moves per unit of time. If the full size airplane moves "x" fuselage lengths per second, then a scale model will give the same appearance if it moves the same number of fuselage lengths per second.

This can be easily observed by watching airliners land and takoff. A Boeing 747 looks like it's flying much more slowly than 737's or MD-80's even when it's actually flying much faster. That's because the large size of the airplane moves through fewer fuselage lengths in the same amount of time. Small bizjets appear to scream across the sky in comparison to the Jumbo jets, yet they are actually quite a bit slower.

Bax, you need to define "x" and "c"? What you meant to say (I think) is that the model should travel the same number of fuselage lengths per second as the full size does.

Example...the 747 is 250 feet long and at 500 mph moves about 3 lengths per second. A 1/20 scale model is 12.5 feet long and should also move 3 lengths per second. For the model this equates to about 25 mph for the model...very slowly indeed. You can see that a model of a 747 must be super light to fly "scale-like".

Another example...A P-51 is 32' long. At 400 mph the Mustang moves 18 fuselage lengths per second. A 1/5 scale Mustang is 6.4' long and should also move 18 lengths per second. For the model this equates to about 80 mph. You can see that a normal Mustang model can come much closer to "scale-like" flight.

Corrected my post by eliminating the typo and making it more clear.

I remember I was at the sport airport with a .40 size Decathlon and I was trying to make some simple scale like aerobatic patterns that I could train on, at the same time the local club J3 Cub was pulling gliders every now and then and I noticed the movements of that and tried to compare it to my model airplane. And I got to the conclusion that my model was never going to be able to fly slow enough, but I still think that when I look at a scale plane in the air I don't primarily look particularly at its airspeed. I look more at how it moves, and that have a lot to do with how the pilot of that model fly his plane.

I have to say you are all wrong.
And then again, nobody is wrong.
There is no right and wrong, merely different perceptions.

The question often arises as to whether a scale model is flying at a realistic speed. So what is "Scale Speed"? I suppose most people would argue that it is the scale proportion of the speed of the full size aircraft. We might expect to see a fifth scale model flying at one fifth the speed of the full size.

For a model aircraft to fly at so called Scale Speed, so that it covers its own length in the same time as the full size, the weight of the full size is divided by the scale factor to the 4[SUP]th[/SUP] power. That will give a model weight which will satisfy our speed requirement in straight level flight. It would mean for example that a fifth scale aircraft should weigh the full size weight divided by 5x5x5x5 (5[SUP]4[/SUP]) or 625. That means that a Hurricane whose loaded weight was 7600 lb, when reduced to fifth scale (20%), should weigh 7600/625 or 12.16 lb. I will let you be the judge of how practical it may be to build and fly an 8 foot span Hurricane that weighs just 12 pounds.
However, for realistic scale flight in which the radius of turn of the model is the correct scale fraction of the radius of turn of the full size (at the same bank angle and angle of attack) we can say that the ratio of the weights must be as the cube of the Scale Factor. For the one fifth scale example again that means dividing the weight by 5[SUP]3[/SUP] or 125 so for our 20% scale Hurricane, scaling the weight by the cube rule would give an eight foot span model with a weight of 60.8 lb. The wing loading is up to 96 oz/sq.ft. The result is rather a heavy, fast flying, model which is smooth and realistic in manoeuvres.
The conclusion we are faced with, if you have been following me, is that there is no such thing as scale speed. It is completely impossible to satisfy two conflicting requirements. We could go for a very light model which will fly straight and level at "scale speed", but whose manoeuvres will be much too tight and unrealistic and will look “floaty”. Or we can have a heavy model which will fly realistic manoeuvres, but which will land much too fast.
Naturally a compromise is needed somewhere between those weights. This compromise will be a personal preference, and nobody can say that you are not flying to scale speed if your model weight is within the weight limits set by the cube and fourth power laws above. That's where the art and skill of scale model flying comes in. It is in finding just the right compromise which lets your model fly manoeuvres sufficiently smoothly without looking too fast when landing or flying straight and level.
It is a well acknowledged fact that big scale models fly better and more realistically. And it is not just the size itself that matters, but the scale factor. That is because the bigger the scale factor, the wider the gulf between the requirement for scale speed in straight flight and scale manoeuvres.

The above is quoted from a series of 4 articles I wrote in R/C Model World, from June through to August 2009.

Thanks, Alasdair. Probably the best explanation I've read.

Another interesting concept of "scale flight" is "time". If you could scale time the same way you scale a model it would be possible to achieve perfect scale flight appearance....both with the proper speed in level flight and with the proper bank angle and turn radius.

A way to scale time is to film the model in flight and then show the video in slow motion. Take the example above of the heavy Hurricane model flying fast but with the correct turn radius. Showing that same flight in slow motion the bank angles and turns will look correct and the overall speed will also look correct. This is in fact how movie makers often film models to provide a true visual allusion...ie. The Battle of Britain: "The other need was for models in aerial sequences, and art director and model maker John Siddall was asked by the producer to create and head a team specifically for this because of his contacts in the modelling community. [SUP][Note 4][/SUP] A test flight was arranged at Lasham Airfield in the UK and a model was flown down the runway close behind a large American estate car with a cameraman in the rear.[SUP][citation needed][/SUP] This test proved successful, leading to many radio-controlled models being constructed in the band rehearsal room at Pinewood Studios. Over a period of two years, a total of 82 Spitfires, Hurricanes, Messerschmitts and He 111s were built.[SUP][19][/SUP] Radio-controlled Heinkel He 111 models were flown to depict bombers being destroyed over the English Channel. [SUP]"
[/SUP]
Here is an example: https://youtu.be/Q8bAMyKeB30

I remember reading of Dave Platt and a few other notable British modelers working on flying models for that shoot. That was just before I saw his SBD at the Olathe Nat's in 68.

At the time, they were really pushing the state of the art, today it would just be CGI.

But as I pointed out much earlier, you need dynamic scaling and time scaling with video to get the correct effect. A guy name Loran Tregallus did this for Beechcraft in Wichita about 50 years ago!

Getting back to Mike's C130....

Mike, you need two things to get up to speed and climb.

You need enough RPM and pitch to reach speeds the model will fly at comfortably. And you are right that on a large and fairly draggy model like a C130 that the model will not reach the actual pitch x RPM airspeed other than in a dive of some sort. So finding the speeds you expect to need to fly at for stall, cruise and max speed then proping and powering for a pitch x RPM that is about 20% higher at full throttle than your desired maximum level speed is good.

Your thrust rig being used to test for static thrust is misleading you. A 12 inch pitch prop is operating at a stalled condition when running static. And that means it sucks current like crazy. Far more than it will in the air at a reasonable speed. So during the takeoff run it will reach a speed where it stops being stalled. When that occurs the RPM will pick up and the current will actually drop. So for flying and climbing at a reasonable cruise speed in the climb you can expect more from each motor and prop than your testing indicates.

To lift off and actually climb you are way over powered for a scale like lift off and climb angle. You could achieve a scale lift off and climb angle with more like a thrust value that is down around 60% of the model weight. And even that would likely still be a higher thrust to weight ratio than the full size Herc when loaded. So even with your "lower power" setup you'll likely find yourself only using part throttle for a nice scale like takeoff. Full power takeoffs even with your lower cell count packs will resemble a hot fighter sort of takeoff. Or the takeoff of the real thing when unloaded and doing one of their impressive air show short liftoffs.

The only question now is if the 12 inch pitch and RPM that your pack voltage turns it at is a good match up for pitch speed to the speed the model will need. You want the prop to un-stall at a speed low enough that it will occur during the take off acceleration. But not TOO early or you might run out of top end pitch speed. I'm not familiar with the electric propulsion software out there. But one or more of these analysis and prediction packages might be able to tell you that sort of information. I do believe that most of them also will give you a good estimation on the stall speed. And given the cost and time in a project of this sort it might be worth looking into buying Motocalc or similar.

The big numbers like your Herc always look so intimidating to me. I don't work with that size model. My first thought was that this thing is way heavy. But then I stop and think about the large scale sailplanes that are up around that span which weigh in at around 20 to 25 lbs. And you'll have a lot more wing area because of the aspect ratio difference. Suddenly 55lbs doesn't seem so bad. You sure won't hit the "scale" stall speed by a long stretch. But I'm thinking that it might just look rather scale like anyway. Good luck with it.

To me, the prop seems to be a good choice. It's even "scale" in diameter and the pitch/diameter ratio makes it efficient. The motor is just a too low Kv type that would deliver far too much power at a cell count needed to reach 65 mph flight speed (80 mph pitch speed). Motocalc is not needed to see that. I'd rather choose a 380 Kv motor and 5 or 6 cells. Power/weight ratio could be as low as 0.45 and would still give take-offs like an unloaded original.

I have several 1/4 WW1, and friends that also fly them. They seem a little fast. But after I fly my 1/4 Neiuport 11 that is electric..with a big 22/12 prop...all those gas powered ones seem WAY to fast. My N11 is not super light compared to my gas powered WW1... but that bigger prop lets it fly slower with no feeling that I am to slow or about to stall. Possibly the better thrust over the tail helps in slow speed situations.

Either way I hope you find a good setup because I love to see airplanes flown in away that makes you think..

"Is that a model?"

Thanks guys you rock