8 hours ago, jcomm said:

What always "killed" my interest in scale RC is Reynolds fault 😞

Re=V⋅c / v  (in one of it's forms, c==wing chord, V== airspeed, v== kinematic viscosity)

added by wing loading factors... They ruin the realism of most scale RC airplanes who have to fly at speeds way out of sync with reality, given the scale...

These HUGE models which cost more than some real aircraft, can reduce the effect a little bit, and so do "indoor" models...

What scale do you think that A350 is and what is its crusing speed.?

9 hours ago, alanw2005 said:

What scale do you think that A350 is and what is its crusing speed.?

Ramy RC’s giant Airbus A350 model is built at approximately 1:19 scale, with a wingspan and length of about 3.5 m each. Typical flight speed for this model I guess should be in the range of 70–120 km/h, given the 120mm EDF thrust class apparently used for the "turbines"... (*)

(*) No, I'm not that clever 😁... I asked Copilot 🤣

 

Edited January 30Jan 30 by jcomm

18 minutes ago, jcomm said:

Ramy RC’s giant Airbus A350 model is built at approximately 1:19 scale, with a wingspan and length of about 3.5 m each. Typical flight speed for this model I guess should be in the range of 70–120 km/h, given the 120mm EDF thrust class apparently used for the "turbines"... (*)

(*) No, I'm not that clever 😁... I asked Copilot 🤣

 

So assuming a cruise speed of 90 km/h that's a scale speed of 923 knots or Mach 1.38. If these calcs are right, I then see the ridiculousness. The issue is the plane is too light and the energy capacitiy of the lithium ion batteries allows for some very powerful EDFs. I fly remote control planes but find flight simming more convenient for scratching an aviation itch, and you can sim anytime and in any weather.

Do you fly RC planes? What types if so?

1 hour ago, alanw2005 said:

So assuming a cruise speed of 90 km/h that's a scale speed of 923 knots or Mach 1.38. If these calcs are right, I then see the ridiculousness. The issue is the plane is too light and the energy capacitiy of the lithium ion batteries allows for some very powerful EDFs. I fly remote control planes but find flight simming more convenient for scratching an aviation itch, and you can sim anytime and in any weather.

Do you fly RC planes? What types if so?

I did fly, around 34 years ago 😁... RC gliders only... Graupner and Multiplex mostly. My last radio was the mighty Graupner MC-18.

I did consider helis too, more recently... Maybe one of these days I make my mind and get an electric glider, a scale one because those stunts I see don't aren't my cup of tea...

Edited January 30Jan 30 by jcomm

1 hour ago, jcomm said:

I did fly, around 34 years ago 😁... RC gliders only... Graupner and Multiplex mostly. My last radio was the mighty Graupner MC-18.

I did consider helis too, more recently... Maybe one of these days I make my mind and get an electric glider, a scale one because those stunts I see don't aren't my cup of tea...

I had 2 RC gliders. I with flaps and a folding prop. Very relaxing catching thermals on a hot summer day. A Graupner MC-18 is quite the iconic radio from a company rich in history.

7 hours ago, alanw2005 said:

The issue is the plane is too light and the energy capacity of the lithium ion batteries allows for some very powerful EDFs.

It's the opposite, a heavier aircraft needs to fly faster for the same wing area.

For that RC A350 to fly at the same proportional scaled speed of the real A350, it should be much lighter than it is.

Edited January 30Jan 30 by Murmur

4 hours ago, Murmur said:

It's the opposite, a heavier aircraft needs to fly faster for the same wing area.

For that RC A350 to fly at the same proportional scaled speed of the real A350, it should be much lighter than it is.

Yes, makes perfect sense!!! 👍I wonder what the stall speed is of the RC A350. Any guesses???
Maximum landing weight of an A350 is 236 tons. So if the RC A350 is 1/19th scale its landing weight should be 12 tons. I bet the RC A350 weighs 50 kilos? So the out of scale speed is due to the power to weight ratio being too high at the top end??. But if the landing speed of an A350 is 145 knots then the RC scaled landing speed would be 7.5 knots, which is too slow to fly. So as you say, it must be much lighter.

If that RC A350 has a Vr speed of say 25 knots, the scaled Vr speed would be 460 knots (totally crazy).

As @jcomm says, this is all messed up.

Any aeronautical engineers or physicists in the house? I am way out of my element on this.

Edited January 30Jan 30 by alanw2005

1 hour ago, alanw2005 said:

Maximum landing weight of an A350 is 236 tons. So if the RC A350 is 1/19th scale its landing weight should be 12 tons.

Weight does not scale linearly with dimensions. it scales with volume =³. So scale of 1/19th would actually put the RC-model at:

19³⁼ 6859.

236.000 kg/6859 = approx. 35kg

(don't worry, DaVinci made the similar assumption when he designed his Ornithopter)
 

1 hour ago, alanw2005 said:

If that RC A350 has a Vr speed of say 25 knots, the scaled Vr speed would be 460 knots (totally crazy).

Regarding the speed, RC scale models all battle physics: adverse aspects of low reynolds number, earlier separation of boundary layer and what not. It's a long list.

So RC planes will - in reality - need to fly much faster than a theoretically applied "down scaled" airspeed.
Perhaps not twice as fast, but a factor of 1,5 is not uncommon. 
bottom line, It's not possible to compare a real A350 and it's performance envelope, try to scale it towards a small RC-model, fraction of size and weight. 

(former RC-flyer myself)

Stall onset gets mitigated a bit by the increased relative surface roughness (unless it's super polished). They certainly aren't using the same airfoils though, at least not in the A350 case. But on the other side of the coin, compressibility effects are nowhere to be seen!

Also need to keep the a/c and heaters turned off in those big indoor facilities. The one thing that really can't scale properly without specialized equipment (if they are left turned on) is the clear air turbulence. That beastly ~1kg of inertia is going to need a pretty tame turbulence penetration speed.

1 hour ago, SAS443 said:

Weight does not scale linearly with dimensions. it scales with volume =³. So scale of 1/19th would actually put the RC-model at:

19³⁼ 6859.

236.000 kg/6859 = approx. 35kg

(don't worry, DaVinci made the similar assumption when he designed his Ornithopter)
 

Regarding the speed, RC scale models all battle physics: adverse aspects of low reynolds number, earlier separation of boundary layer and what not. It's a long list.

So RC planes will - in reality - need to fly much faster than a theoretically applied "down scaled" airspeed.
Perhaps not twice as fast, but a factor of 1,5 is not uncommon. 
bottom line, It's not possible to compare a real A350 and it's performance envelope, try to scale it towards a small RC-model, fraction of size and weight. 

(former RC-flyer myself)

👏👏👏👏👏

Nice to hear from all the big brains 🧠🧠!! Very interesting stuff. We would be living like cavemen without scientists and engineers!

Edited January 30Jan 30 by alanw2005

3 hours ago, alanw2005 said:

Maximum landing weight of an A350 is 236 tons. So if the RC A350 is 1/19th scale its landing weight should be 12 tons. I bet the RC A350 weighs 50 kilos? So the out of scale speed is due to the power to weight ratio being too high at the top end??. But if the landing speed of an A350 is 145 knots then the RC scaled landing speed would be 7.5 knots, which is too slow to fly. So as you say, it must be much lighter.

The calculations are different. The "geometrically similar" speed scales linearly with the scale, hence the dynamic pressure scales with the square of the scale (1/19 scale = 1/19 speed = 1/361 dynamic pressure). The wing area also scales with the square of the scale (1/19 scale = 1/361 wing area).

So for the same Cl, the weight scales with the fourth power of the scale (1/19 scale = approximately 1/130000 weight).

236 tons / 130.000 = approximately 2 kg. Whereas that A350 weights around 20 kg, i.e. 10 times more. It will hence have characteristic speeds that are  around sqr(10) = approximately 3 times higher: it will appear to fly at about 3x the speed compared to a real A350.

In other words, set the video speed to around 0.3 and it will appear to fly like a real A350. 🙂

Edited January 30Jan 30 by Murmur

 

2 hours ago, Murmur said:

The calculations are different. The "geometrically similar" speed scales linearly with the scale, hence the dynamic pressure scales with the square of the scale (1/19 scale = 1/19 speed = 1/361 dynamic pressure). The wing area also scales with the square of the scale (1/19 scale = 1/361 wing area).

So for the same Cl, the weight scales with the fourth power of the scale (1/19 scale = approximately 1/130000 weight).

236 tons / 130.000 = approximately 2 kg. Whereas that A350 weights around 20 kg, i.e. 10 times more. It will hence have characteristic speeds that are  around sqr(10) = approximately 3 times higher: it will appear to fly at about 3x the speed compared to a real A350.

In other words, set the video speed to around 0.3 and it will appear to fly like a real A350. 🙂

Cool stuff!! What is your day job?

2 hours ago, FPVSteve said:

 

Dude!! That is way cool!! Love how this thread has pivoted.