RF7 Aerodynamic basics

M

Moto-Guzzi

Active member
Having a blast tinkering with this program (I wont call it a “game”!)
Got it for my kid for Christmas, he has yet to get his hands on the controller! LOL!
So if anyone wants to discuss cause/effect properties, I have a few items I’m digesting…

1) I understand the graphic scale is just a visual overlay of the physical model….increase it 200% and you have a big picture on top of a small plane…that will of course appear to fly “unnaturally” because it has the same loop and turn radius and stall speed of that little airplane …I get that…

2) I also assume the mass/weight of electronics servo and powerplant components would stay the same regardless of any visual or physics scale changes you’d make….they “are what they are” regardless of how big or small the plane is they go in…makes sense so far…

3) But what Airframe Properties change when you adjust the Physics Scale? Is it JUST the physical dimensions (span, chord, fuse cross section) or does the corresponding WEIGHT of each airframe part also change? Meaning, when you “rescale physics to 150%” does it automatically increase in the MASS/WEIGHT of each airframe component by the same ratio? Or do you end up with a larger airframe, but with reduced wing-loading (tends to “float”) because the AREA was increased but the WEIGHT was not?

I think that is the case, (up size physics reduces wing-loading) but I haven’t yet checked any before/after component weights when re-scaling the physics to confirm my suspicion.

4) Lastly, do all the other “effects” like drag, prop wash and whatever else is “built-in” RF7 as an aerodynamic variable also “move” proportionally when you adjust the Physics? Like, does the effect of GRAVITY change? (I think that’s absurd but worth getting a definitive “no” from the experts)

Thanks in advance, sorry to take to such lengths in text, but thats how I was taught to read-n-write in the days before electricity!
 
Have you read Doug's tutorial on physics for RealFlight? It explains just about everything you need to know about physics for RealFlight.

Click here
 
More generalized

Thanks, I have read that before, its a bit too detailed on "tactics" (particularly the electronics and servo selections) and not enough on "general strategy" for me to use just yet....

Right now I'm just trying to mentally digest what happens when you make a particular setting change (rescale physics) to an existing aircraft....

Basically, I am at the "cave painting" level of RF7, the tutorial is the "rosetta stone"...
 
Let's see....

Re: Question 1.

Yes, you end up with an unnaturally large visual mesh on the physics of the original model.

Re: Question 2.

Yes again, since changing the visual scale does not change the physics at all.

Re: Question 3

Yes it is just the physical dimensions that change, nothing else.

Realflight does not know the intended wing loading or weights of the new target components. It is up to you to adjust those things separately.... as are prop size, engine weight and power, fuel container weight, fuel container size, wheel weights, etc. etc. etc.

Physics scaling is merely an "aid" to help you along making a larger plane out of an existing one.

Re: Question 4

Eh, I have to split this hair as it is an important one....

As in real life drag, prop wash etc. do NOT "move" proportionately when you scale things up.

They stay the same... in other words, drag itself is NEVER "proportional". It is for all intents and purposes a physical constant ( Yeah I know about temperature variations and elevation changes for those into the physics... I'm simplifying for the sake of instruction... )

Instead the amount of oblique surface exposed to the airflow, and wing shapes, surface smoothness, etc.. affect the amount of drag the aircraft itself experiences... but drag itself is not something that "scales".... that would be like saying that gravity scales as you increase size... it doesn't, gravity is fortunately for us a constant. The increase in mass is what makes something heavier.

So as you increase the wing size, the effect of drag on the plane does indeed increase. But that is because this effect is calculated based upon the parameters of the "new" model larger model as you would expect..

The effect of gravity does not change because you have not changed the weights of the components to reflect a real world plane of the new size. If you scale the physics up, you end up with a very large plane with a small engine, little fuel, tiny prop, and almost no weight.... so everything is still out of balance.


To fix everything you have to have a better understanding of what is going on... and I point you back to Doug's Tutorial which CSGILL75 mentioned.

Novices tend to try adding weight willy-nilly, or just telling the sim via the overrides that the plane weights XXX pounds.

That doesn't work, those variables are really there only for fine tuning. The end result is that over 85% of the uploaded variants are just junk.

People who know what they are doing spend a LOT of time tweaking things and will even compare the second and third order characteristics of the real planes versus how they behaving in the sim, adjusting the latter as necessary.

Once you know how, the sim can be a good predictive tool too... but as with any simulation, the simulation must be "tuned" by comparing it against real world behaviour.

That creates a chicken & egg problem... to accurately predict how a model will behave, you need an accurate model that reflects real world capabilities and characteristics.... & a direct comparison to validate the simulation.
 
Last edited:
Thanks!

I did scale up a plane to 150% just to see what happened....big light and totally unmanageable just like I thought...Took the major components weights (wing fuse tail rudder) and went up 50%...I know its not a direct ratio between size and weight increases, but...

Ended up with wing loading "kinda sorta" in line with an existing good flyer of that size...cool...

CG was "flyable" and Stall Speed increased "about right" so it settles down to land more realistically for how big it is...nice, predictable cause and effect occurances, nice...

But it had an electric motor, and is now pretty underpowered (I only did takeoffs and landings to check balance).

The editor for that plane lists 200 or more electrics but no gas to choose from....(that was a suprise, I thought it pulled them from whats already on my system)

I get cc's, 2-stokes and horsepower, Mah amps and ohms not so much!!! I dont know whats "more powerful" from the list and process of elimination sounds like "not much fun" LOL!

And what the heck is an "Air Pump"?...My compressor in the shop USES power, not produce it...lots to learn but this is great, I'm loving it...

I guess I can steal a gas motor I'm familiar with from my other files?...but thats a "PC skills" issue, not a "plane design"

Gettin there!
 
Each aircraft has a root level property called Power Plant Type. That property determines what motor options will be available to you.

It sounds like you started out modifying an electric plane, so it is still an electric plane. You can change that property and select Internal Combustion Engine to instead see all the gas and glow engine options (at which point electric motors and turbines will be hidden).
 
https://forums.realflight.com/showthread.php?t=29291
https://forums.realflight.com/showthread.php?t=29293
https://forums.realflight.com/showthread.php?t=27627

Happy reading :) Its good stuff but RF is a great base for prototyping. It does have limitations. A perfect example is my L-4 Cub. I've everything scaled and the right airfoils and all. My actual plane tip stalls far more severely than what RF portrays. So, I had to change the airfoil at the tips to better simulate the tip stall tendency.
 
Switch from electric to gas engine

Ryan Douglas said:
It sounds like you started out modifying an electric plane, so it is still an electric plane. You can change that property and select Internal Combustion Engine to instead
Click to expand...


It was indeed electric to start with...that gas engine sound profile it came with lulled me into complacency, I never realized it was electric until I said "now just swap motors and I'm done " DOH! LOL!!

I will need to make that change to internal combustion in the planes file/folder somehow? I dont see an option like that when RF is open and the plane is in edit mode....I'll keep pecking around and see ..

And wilsonman, thanks I've read those before and have them earmarked for further study as I get better at this...good stuff from everyone, I'm really happy with the RF experience...Just have to start managing my information (overload!) better to use everything thats here...what a fabulous product this is, I'm still stunned by the "volume of value" you get from this, its really unbelievable :)
 
You must log in or register to reply here.
Back
Top