Rc planes not so scale?

[noobflyer92]

Take a full scale f-16. It weighs like 28k pounds and puts out 29k pounds trust. Scaled down to a 1/8 scale, it should have 3625 lbs of thrust and weigh somewhere around over 3000. But, a eight scale rc jet we would fly would weigh like thirty pounds, and output maybe 35 lbs trust, whether edf or turbine. Why are we so not to scale? Is another reason full scale planes stay airborne so much longer the extra weight from fuel? Why don't we fly planes scale like that? Too expensive? Too hard to find an engine with that much thrust output? Maybe an advantage is our lighter planes glide better, but lack flight time.

[bwalt822]

There are many factors at work here.

First and simplest to understand is that weight scales approximately proportionally to volume of the plane. Volume of the plane scales to the cube of it scale factor i.e. wingspan. So if you took your 30 pound 1/8 scale jet and made it 1/4 scale (making it twice as big) it could weigh approximately 2^3=8 times more or 240 pounds. Scale that 1/8 plane to full scale (8 times bigger) and you have 8^3=512* 30 = about 15000 pounds which is about half of the weight of the full scale thing.

But our planes do not carry proportionally as much fuel or avionics as a full scale plane so ours have more empty space which accounts for some of the difference.

Lift scales with the square of the scale factor since lift is dependent on area of the wing. Lift also is a function of velocity squared which is why bigger planes generally need to fly faster.

There are Also more complicated factors at work such as the lift not scaling exactly linearly due to differences in reynolds numbers and what not.

[Accu157]

Yup. You have to take into account the scaling properties. You have to take into account the number of physical dimensions you are affecting. Two dimensions is squared, 3 is cubed, and even then, some other details are not so obvious.

[JetPlaneFlyer]

Yep, what bwalt said. Even if you took a real F16 and put it under a 'Honey I shrunk the kids' machine and reduced it to 1/8 scale then it's weight would reduce by a factor of 8 cubed (512)... So if it was 28k lb to start with it would end up about 55lb. Because engine power is related to volume the thrust would (simplistically) also reduce by the same factor.

Unfortunately due to the fact that air can't be scaled down the shrunken F16 would not perform very well. The coefficient of lift possible for small scale models is much less than full size aircraft, so landing and take off speed for a model of that weight would be incredibly high and minimum turn radius very large. The engine would also have a hard time working with 'full size' air.

Steve

[vintage1]

In fact a 'true scale' plan does fly realitisically, if you slow time down by the
sq root of the scale factor.

I.e linear scales by X
Area scales by X^2
Weight and power scale by X^3 ( so power to weight is the same)

Stall speed, which is the sq root of wing loading, scales by X^0.5

However this lads to very high flying speeds... the 1/5th scale jet that might land full size at 180mph, would have a scale landing speed of 80mph.

But, viewed thro a slomo camera slowed down by 2.23:1 it would THEN look as if it were landing at 36mph, and as if it were flying at say 80-100mph, which would be scale speed for 400-500mph full size.

So if you want to get the bank angles right, and the turn radius to scale and the G loading the same, that's how you do it. For a film model anyway.

For a live audience the problem is well nigh impossible. Either the model flies well below aerodynamic scale speed, which makes it look OK on the straights, but it turns impossibly tight corners at far too little bank, and has no energy retention, so needs constant throttle attention to e,g. fly a scale loop, or it takes of and lands impssibly fast, and seems to approach at a totally non scale speed.
.
That is why the bigger the model the more realistically it flies. At quarter scale and above, the difference between linear scale speed (1/4) and aerodynamic scale speed (1/2) is not so great, and an acceptable compromises can be found.

Sadly, with large aircraft, this leads to impossibly large planes. Imagine packing a 1/4 scale 747 in your trunk...

[Brandano]

Sadly, with large aircraft, this leads to impossibly large planes. Imagine packing a 1/4 scale 747 in your trunk...

That's a bit of a moot point, really. You'd just fly in to the field on it.

[JetPlaneFlyer]

If we could scale the air down so that the spacing between the molecules was reduced in line with the model scale factor then all would be fine, even Re Number would be the same. What we need is a HUGE hyperbaric chamber operating at pressure equal to atmospheric x model linear scale factor and we all go fly our models inside it..... Easy peasy, problem fixed :D

[eflightray]

I much prefer to build light and scale'ish so that the model flies at a more scale looking speed.

Having seen some amazing looking scale models at the UK Nationals one year, I felt rather let down when most of them flew at totally un-scale like speeds.

To me 'scale speed' would be when the full size aircraft was circling you at distance, and your model profile matched its size and speed, perhaps not 'true' scale speed, but visually correct.

[JetPlaneFlyer]

To me 'scale speed' would be when the full size aircraft was circling you at distance, and your model profile matched its size and speed,

This is very difficult if not impossible to achieve in practice due to the effects described above. For instance a 1/4 scale Piper J3 Cub would have to weigh about 4lb to fly even close to 1/4 speed of the full size :rolleyes:
Even if you could build one that light it would not look 'right' in the air because it's lack of inertia would mean it would get bounced about by every little air movement and it's turn radius and bank angles would be all wrong.

Best just build as light as reasonably possible and accept that scale models dont fly exactly like their full size cousins. (unless you have the giant pressure chamber mentioned previously ;) )

Steve

[Accu157]

If we could scale the air down so that the spacing between the molecules was reduced in line with the model scale factor then all would be fine, even Re Number would be the same. What we need is a HUGE hyperbaric chamber operating at pressure equal to atmospheric x model linear scale factor and we all go fly our models inside it..... Easy peasy, problem fixed :D

Which would mean that there is absolutely no difference between a model and reality. What would that be called? A homogeneous scale model? :D :p ;) That would be like shifting the number scale on the size of the universe when it is still going to be be the same size and behavior relative to what it was before. I claim that a scale model of a 747 is now the size of the known universe. But, now the universe is awful big too and nothing changed. :D

[HX3D014]

There are many factors at work here.

First and simplest to understand is that weight scales approximately proportionally to volume of the plane. Volume of the plane scales to the cube of it scale factor i.e. wingspan. So if you took your 30 pound 1/8 scale jet and made it 1/4 scale (making it twice as big) it could weigh approximately 2^3=8 times more or 240 pounds. Scale that 1/8 plane to full scale (8 times bigger) and you have 8^3=512* 30 = about 15000 pounds which is about half of the weight of the full scale thing.

But our planes do not carry proportionally as much fuel or avionics as a full scale plane so ours have more empty space which accounts for some of the difference.

Lift scales with the square of the scale factor since lift is dependent on area of the wing. Lift also is a function of velocity squared which is why bigger planes generally need to fly faster.

There are Also more complicated factors at work such as the lift not scaling exactly linearly due to differences in reynolds numbers and what not.
I do not agree with this part.

I would say 120lbs

If it was 240lbs then you would need to increase the speed.
IE if the 30lb 1/8th plane took off at 30kts then the 1/4 240lb plane would need to get up to 42.5kts for take off. (give or take a bit for Re #'s)

The only part of the lift formula you changed was S, surface area is scaled at a 2D rate, so that is squared.

You increase the size by two in all directions so its volume increase by a factor of 8, but the surface area only increased by factor of 4.

So when doubling the scale size to twice as big in each dimension you only need to double the weight twice.

Bryce.

[JetPlaneFlyer]

Bryce,
Isnt that why large planes fly faster than small ones?
Volume does scale to the cube of the linear scale factor, that's a basic fact (width x length x height). So if you scale a model up, using same construction but with each part scaled up in proportion in all dimensions, then it's weight will increase by the cube.

Obviously if you change construction or materials then it the increase mey not be exactly to cube, could be a bit more, could be a bit less, but it will almost certainly be more than the square in every case.

If you could scale aircraft up by the square of the scale factor then a full size aircraft would fly at the same speed as a 13" span peanut scale model ;) .. in fact due to Re full size would fly slower :rolleyes:

Steve

[HX3D014]

Bryce,
Isnt that why large planes fly faster than small ones?
Volume does scale to the cube of the linear scale factor, that's a basic fact (width x length x height). So if you scale a model up, using same construction but with each part scaled up in proportion in all dimensions, then it's weight will increase by the cube.

Obviously if you change construction or materials then it the increase mey not be exactly to cube, could be a bit more, could be a bit less, but it will almost certainly be more than the square in every case.

If you could scale aircraft up by the square of the scale factor then a full size aircraft would fly at the same speed as a 13" span peanut scale model ;) .. in fact due to Re full size would fly slower :rolleyes:

Steve
yes, that is true.

but you did not mention that the weight of the 1/8th scaled up to 1/4 would need to increase V to maintain lift.

so to keep the speed as it was, then you would need to do the Area only.

then if you want to do the speed. then do that at a ^2 rate separately.

remember that lift is working on the Area of air mass that is affected by it.

if you scaled the area up to 1/4 (From 1/8th) then the area of air that the wing will work on is 4 times more.

So now you can quite simply have 4 time more mass to that aircraft and it will still fly at the same speed.
IE
30lb x 4 = 120
Now. if you want to air speed to be to the same scale up effect. then you can add more weight again
IE
that 30lb 1/8th aircraft was taking off at 30kts (For example)
then the 120lb 1/4rth aircraft is now able to take off at 30kts
if you want that 1/4th aircraft to take off at a scale speed of the 1/8th at 30kt then the new scale speed would be 60kts
so the 1/4th scale aircraft can now have more weight
the resultant increase is 120lbs doubled twice = 480lbs.

what answer do you get if we took the "1/8th scale aircraft that can take off at 30kts and weighs 30lbs" and then scaled it up to 1/2nd in dimensions and speed
IE
the wing span on the 1/8th was 4ft but on the 1/2 scale aircraft it is now 16ft
the Take off speed of the 1/8ths is 30kts but now we want it to take off only at 120kts (but that seems to fast <for safety or ??> so we compromise and set it to 100kts is the point where it will be able to take off)
What would be the max weight in this case ?

Bryce

[HX3D014]

Bryce,
If you could scale aircraft up by the square of the scale factor then a full size aircraft would fly at the same speed as a 13" span peanut scale model ;) .. in fact due to Re full size would fly slower :rolleyes:

Steve
listen carefully :rolleyes:

If you increase the dimension scale of a 747 to 2/1 then;

the area on this new 747 is now 4 times more so the weight could be doubled and doubled again to become 1440 odd tons at the same speed IE 140kts odd

using your math
the aircraft is now 8time more volume so now you can double the weight and then double the weight and then double the weight again and still be able to take off at 140kts.

Now can you see why it is area (^2) and not volume (^3)?

PS
Using my math (which is not that good but just a ball park # and then needs work to account for Re #'s)
that 747 weighing 360'000kg that is now twice as long and twice as high and twice as wide would be able to fly at half the speed,
IE
if the normal scale 747 weighing 360t needs 140kts then this one needs only half that. even though its area has increased by a factor of 4. not anywhere near a " 13" span peanut scale model ;) "

Bryce

[JetPlaneFlyer]

yes, that is true.

but you did not mention that the weight of the 1/8th scaled up to 1/4 would need to increase V to maintain lift.


Bryce,

Errr.. yes i did ;) .. My opening sentence was "Isn’t that why large planes fly faster than small ones?"



Your turn to listen... :rolleyes:

Your logic appears to be based on the assumption that all planes regardless of size must, for some as yet unexplained reason, fly at the same speed. This in a fundamentally incorrect assumption. Large planes (all other things being equal) do in fact fly faster than small ones. That's why a peanut scale Piper Cub can fly at walking pace but the full size one stalls at about 40mph.

Due to the (simple) facts that volume, and therefore weight, increases as the cube of scale factor and area only increases to the square then as a plane gets larger it's wing loading increases and it must fly faster. How can you could argue this point??

And yes I do fully appreciate that aircraft are not solid lumps of 'volume' but weight still does tent to scale with volume because the structural components that make up the plane are three dimensional and generally tend to increase in all three dimensions when scaled up.

Steve

[JetPlaneFlyer]

Bryce,
Lets test our two theories with a real world example.. Take for instance the Peanut scale J3 Cub I talked about earlier in comparison to the full size version..

A reasonably well built Peanut scale J3 Cub would weigh about 16g (0.016Kg) give or take. The scale factor is 1:32


According to your theory weight increases by the square of the scale factor..

So 0.016 x 32^2 = 16.3Kg -Your math predicts that the full size J3 Cub will weigh 16.3Kg :rolleyes:



Using my 'theory' weight increases by the cube of the scale factor..

So 0.016 x 32^3 = 532Kg -'My' math predicts 532Kg for the Cub



The actual max take off weight of a full size J3 Cub is 550Kg I'd say that's pretty conclusive. ;)

Steve

[nmasters]

Didn't we have this conversation a few days ago? (http://www.rcgroups.com/forums/showthread.php?t=1014998) An airplane doesn't get it's lift just from the air that's in contact with the surface of the wing. The passage of the airplane disturbs a volume of air and the air in that volume has mass. When you scale the airplane you also scale that box of air and the mass in that box scales as the cube of the linear dimensions just like other massive objects.

--Norm

[fnev]

M = 1/2ρSV^2Cl

M = mass

ρ = air density

S = wing area

V = air speed

Cl = coefficient of lift

Note: Cl is function of Re. To have the EXACT same lift curve you have to fly at the exact same Re. Re is proportional to the reference dimension (in this case the mean chord of the wing) AND the air speed.

Do the math…

[nmasters]

To have the EXACT same lift curve you have to fly at the exact same Re. Re is proportional to the reference dimension (in this case the mean chord of the wing) AND the air speed.

Do the math…
Exact? :rolleyes: Well if you consider hugely different Re then, yeah, the polars won't be similar but I have seen many wind tunnel results showing that the origin and slope of the lift curve are essentially identical below the cl where separation starts. The main things that mess this symmetrical scaling up for modelers are laminar separation bubbles and they can be controlled with turbulators. Stay abouve the critical Reynolds number of the airfoil section you are using and you'll get predictable results.

[HX3D014]

Bryce,
Lets test our two theories with a real world example.. Take for instance the Peanut scale J3 Cub I talked about earlier in comparison to the full size version..

A reasonably well built Peanut scale J3 Cub would weigh about 16g (0.016Kg) give or take. The scale factor is 1:32


According to your theory weight increases by the square of the scale factor..

So 0.016 x 32^2 = 16.3Kg -Your math predicts that the full size J3 Cub will weigh 16.3Kg :confused:



Using my 'theory' weight increases by the cube of the scale factor..

So 0.016 x 32^3 = 532Kg -'My' math predicts 532Kg for the Cub



The actual max take off weight of a full size J3 Cub is 550Kg I'd say that's pretty conclusive. ;)

Steve
this show you were not listening,

the cl value takes care of the 3rd dimension
I already stated that the Re # will change as you change the scale.

Bryce,

Errr.. yes i did .. My opening sentence was "Isn’t that why large planes fly faster than small ones?"
My boo. it was Bwalt that did not mention the speed change value.

that aircraft example you gave;
So 0.016 x 32^3 = 532Kg -'My' math predicts 532Kg for the Cub
now you have not mentioned the speed change value.

I have to go to a riding course now.

see if you can work out the scaled example I asked of you before, here it is again;
what answer do you get if we took the "1/8th scale aircraft that can take off at 30kts and weighs 30lbs" and then scaled it up to 1/2nd in dimensions and speed
IE
the wing span on the 1/8th was 4ft but on the 1/2 scale aircraft it is now 16ft
the Take off speed of the 1/8ths is 30kts but now we want it to take off only at 120kts (but that seems to fast <for safety or ??> so we compromise and set it to 100kts is the point where it will be able to take off)
What would be the max weight in this case ?
back in 10-12hrs

Bryce

[JetPlaneFlyer]

the cl value takes care of the 3rd dimension
I already stated that the Re # will change as you change the scale.

I'm missing your point here..
All I'm saying is that when you scale a 3 dimensional solid object the weight changes by the cube of the linear scale factor; simple as that. This applies to aircraft just the same as any other 3D object (or a structure made of multiple 3D objects), as proved by the Cub example. Cl does not come into it, it's just elementary geometry.


now you have not mentioned the speed change value.


Because speed was not what I was talking about.. I was just talking about geometry, volume and weight. Unless you approach light speed then speed has no impact on an object's size and weight ;)
However now you come to mention it... On the basis that weight increases by cube (as it does) and wing area by square then stall speed will increase by the root of the scale factor (exc. Re effects). So if the peanut Cub had a stall speed of about 7mph then the full size would be 7 x 32^0.5 = 40mph.. Whadya know! That's exactly what the stall speed of the real one is :eek: Spooky how come all this stuff keeps working out when I’ve got my principals so wrong? I must just be lucky:rolleyes:


see if you can work out the scaled example I asked of you before, here it is again;


Excluding Re your answer is 5333lb (worked out from the principal that stall speed is proportional to root of wing loading) .. But this has nothing to do with predicting the likely weight of a model when you scale it up ;)
In fact I'd have predicted a weight of 1920lb (30lb x 4^3) and a take off speed of 60kts (30kts x 4^0.5) for the 1/2 scale version. But you were the one who made the rules up on this one.. maybe you build heavy ;)

Steve

[HX3D014]

I’m missing your point here..
Steve
I think so :)

Give me an example to work out and see if I build heavy.

Excluding Re your answer is 5333lb (worked out from the principal that stall speed is proportional to root of wing loading) .. But this has nothing to do with predicting the likely weight of a model when you scale it up
In fact I'd have predicted a weight of 1920lb (30lb x 4^3) and a take off speed of 60kts (30kts x 4^0.5) for the 1/2 scale version. But you were the one who made the rules up on this one.. maybe you build heavy
Nah.... I was so tired last night I kind of forgot what aircraft example I was wanting to do. I was working on the 747 one myself that’s why I chose 100kts.

here is my working for the 1/8 scale converted to 1/2scale (Scaling up)
flying at 60kts (per your liking)

1/8 scale peanut flyer
weight : 30lb
Take off speed : 30kts

1/2 scale Peanut flyer
Weight :to be worked out
Take off speed must be 60kts

So Here we go;

Cl (we should change due to Re #'s but I do not know how)
1/2roh (dose not change)
V^2 is changed (set to 60^2) which is just a change of a factor of x 4
S is changed by a factor of x 16

So The weight is just 30lb x 4 for speed = 120lb
and now for the 16 times the area = 1920lbs

Well what do you know.

My first post to BWalt was correct, he dose need to increase the speed to 42 if he was to have a weight of 240.

NE way.

Give another example to work out. see if I can match yours (Again).

But you were the one who made the rules up on this one.. maybe you build heavy
not making the rules. just pointing out the scale errors from RC to Real.

I have discussed this previously.
on other forums
http://www.knifeedge.com/forums/showthread.php?t=20743&highlight=weight+wheels

Bryce.

[HX3D014]

This is very difficult if not impossible to achieve in practice due to the effects described above. For instance a 1/4 scale Piper J3 Cub would have to weigh about 4lb to fly even close to 1/4 speed of the full size :rolleyes:
Even if you could build one that light it would not look 'right' in the air because it's lack of inertia would mean it would get bounced about by every little air movement and it's turn radius and bank angles would be all wrong.

Best just build as light as reasonably possible and accept that scale models dont fly exactly like their full size cousins. (unless you have the giant pressure chamber mentioned previously ;) )

Steve
before I made that post to BWalt,

i checked your answer and I came to some thing similar. 4.7 if I remember.

But when you answered , I forgot it was Bwalt i was making that post to.

My apologise Steve.

Been a big day for me (3days).
Bryce

[HX3D014]

In fact a 'true scale' plan does fly realitisically, if you slow time down by the
sq root of the scale factor.

I.e linear scales by X
Area scales by X^2
Weight and power scale by X^3 ( so power to weight is the same)

Stall speed, which is the sq root of wing loading, scales by X^0.5

However this lads to very high flying speeds... the 1/5th scale jet that might land full size at 180mph, would have a scale landing speed of 80mph.

But, viewed thro a slomo camera slowed down by 2.23:1 it would THEN look as if it were landing at 36mph, and as if it were flying at say 80-100mph, which would be scale speed for 400-500mph full size.

So if you want to get the bank angles right, and the turn radius to scale and the G loading the same, that's how you do it. For a film model anyway.

For a live audience the problem is well nigh impossible. Either the model flies well below aerodynamic scale speed, which makes it look OK on the straights, but it turns impossibly tight corners at far too little bank, and has no energy retention, so needs constant throttle attention to e,g. fly a scale loop, or it takes of and lands impssibly fast, and seems to approach at a totally non scale speed.
.
That is why the bigger the model the more realistically it flies. At quarter scale and above, the difference between linear scale speed (1/4) and aerodynamic scale speed (1/2) is not so great, and an acceptable compromises can be found.

Sadly, with large aircraft, this leads to impossibly large planes. Imagine packing a 1/4 scale 747 in your trunk...
BWalt Steve and vintage 1

I want to study this formula some more.

I understand all you are saying and can see that it seems to come to the same answer as my way. but addresses each through their individual components. IE Weight Lift Thrust Velocity.

I came up with my formula
based off of the lift formula.

when I did that back then, I was working on getting a RealFlight G3.5 Aircraft to fly scale. I did get a close weight. but was getting perplexed when I was contemplating slowing the sim down to compensate for the fact that it would fall out of the sky at an un scale rate, Among other things.

Here is the air craft I was working on (http://www.knifeedge.com/forums/downloads.php?do=file&id=3768).

In G3.5 you can slow the sim speed down to 1/4 (seems to be nearly nowhere near the amount needed per Vintages post, but is as good as we will get.

the aircraft was about 1/7.35thth scale.

it proved to complex at the time so I just edited it for an RC one.

Bryce

[HX3D014]

I was working on getting a RealFlight G3.5 Aircraft to fly scale. I did get a close weight. but was getting perplexed when I was contemplating slowing the sim down to compensate for the fact that it would fall out of the sky at an un scale rate, Among other things.

Here is the air craft I was working on (http://www.knifeedge.com/forums/downloads.php?do=file&id=3768).

In G3.5 you can slow the sim speed down to 1/4 (seems to be nearly nowhere near the amount needed per Vintages post, but is as good as we will get.

the aircraft was about 1/7.35thth scale.

Bryce
wait.

If RealFlight has an ability to slow down to 25%
then that would mean we could slow the sim speed down enough for any aircraft from 1/1 to 1/8th
Am I getting that part right ? :confused:

Hmmm

SU-30MKI
weight loaded 54,895lbs
So the volume_weight scale down would be
7.35^3 = 397
so
weight = 54,895/397 = 138.27lbs

is that correct ? :confused:

and the speed would be reduced by a factor of (7.35^0.5 = 2.71) so if the real bird lands at 140kts then the scale one would land at
140/2.71 = 51.66kts

slowed down in the sim to (as a percentage) 1:2.71 we get 36.9%

Am I getting it or am I way off ? :confused:

NE way. still the end of a long day, and I have to get up early tomorrow morning. (3:15am) So i should leave it there before I cook something ;)

Bryce. :confused:

[vintage1]

No: aerodynamic speed varies as the square ROOT of the scale factor.. So you need to slow a 1/4 scale model by half. A 4:1 reduction in time suits a 1/16th scale model.

[HX3D014]

No: aerodynamic speed varies as the square ROOT of the scale factor.. So you need to slow a 1/4 scale model by half. A 4:1 reduction in time suits a 1/16th scale model.
Ahhh.

Of cause.
The square root of 16 is 4 so a 4:1 reduction in time is good for 1/16 scale.

Thanks again
Bryce.

Will try it on the sim later this week.

I still havent goten around to that :o

Do you have G3 or G4 or ?

Bryce.

[jbarradale]

Okay, here is another slant.

There are a very large number of things that are in full scale aircarft that do not exist at all in a whatever scale RC aircraft.

1. Pilot. A pilot in a full scale aircraft, 200 lbs or so, not including all of his gear. His gear would include a sidearm, ammunition, a radio, an emergency light and transponder / locator becon, a first aid kit, an inflatable raft, rations, water, and other personal items.

2. Radios/Electronics. The radios in military aircraft are very extensive. In the helicopters I flew (OH58 = Bell Jet Ranger), there were AM, FM, VHF, UHF radios. There was also an encryption system that was very heavy in the rear. That little piece of balsa with a decal on it? That represents a massive amount of gauges and also the sensors that connect to them. Hardly scale.

3. Armament. If you have ever spent any time in the miltary you might have lugged around a small box of ammo. Miltary aircraft will have many of these. 1 20mm canon makes for a lot of weight and the ammo is not light either. Missles are also heavy. Now add the electronics to not just fire them, but to aim them. We are talking computers, miles of wire, cameras, wire guided missles requires lots of hardware other than just the wire.

4. Ejection/seat. The ejection seat on jet aircraft is a great deal of weight also. There are rocket motors to propel the pilot out of the aircraft.

5. Armor. No pilot in his right mind would go anywhere in a plane with armor, self-sealing fuel tanks. The A-10 is well known for the titanium bathtub around the pilot. Even the skimpy little helicopter I flew had armor plates.

6. All of the above needs to have power. Even when the engines are not running in case of a flame out, you have to have a LOT of power. We are NOT talking about a 11.1 volt 3 cell LiPo either.

7. Now connect every bit of all of this with miles of wire.

8. Add fuel. If you know anything about size of motors, you will find that larger more powerful motors burn more fuel, kind of obvious. Like comparing a V8, or V12 truck with a 4 cylinder VW. Lower weight and a different horsepower to weight ratio. Acceleration is a key factor in military aircraft. You just have to have the power of afterburn. THATS a LOT of fuel. So, you also have to have fuel pumps to move the fuel around the aircraft, otherwise the right hand tank is empty and the left one is full and you lean hard to the left and fly in a circle.

Tons of other equipment not mentioned above. Lights, hydraulic pumps, ....

In the RC scale aircraft, it is all air.

And it is different for every aircraft, so no formula will work for everything. Not all jets have 20mm canon, some have 30mm, some just missles. WWII era aircraft have two to eight 50 cal. or 30 cal. machine guns and maybe some 20mm and/or 30mm canon.

You just cannot process and condense real life into concrete formulas.

But, it was probably mentioned before, if the aircraft is 1/8 size, then the airfield you lift off from is 1/8 size and the speed should be 1/8 size.

The problem arises also from the flying perspective. If the aircraft went at scale speed, the pilot on the ground would probably not have the time to correct any mistake because the view of the aircraft might hide the actual actions of the aircraft. In other words if it starts to tip in one direction, you do not have an artifical horizon indicator on your transmitter and so the aircraft will continue to tilt a bit more until you finally notice it, and may be a little too late at that point. To make them more flyable, slowing them down is a good thing.

120 mph stall speed and/or takeoff speed for a jet, 1/4 scale, 30 mph. That is a pretty fast speed to be ladning and taking off for a model. Most models I have seen land and takeoff in the 20 - 30 mph speed, and even slower.

Do we really want to try and control a model jet traveling at 300 mph? How would you even see what it looked like when it passed by? Would balsa stand up to that speed? :eek:

[Thomas B]

It is difficult and challenging, but possible to fly R/C models in excess of 300 miles per hour.

Here is a video of the fastest known r/c D/S soarer (dynamic soaring) taken recently as it set the unofficial D/S speed record of 392 mph......it is really a impressive flight.



http://www.youtube.com/watch?v=WaQB16ZaNI4

[JetPlaneFlyer]

Very impressive..
Of course those speeds were recorded on the downwind leg off the flight so the speed measured is not airspeed. You would have to deduct windspeed from the recorded figure and it looks like the wind was blowing VERY strong that day.. But still highly impressive and well over 300mph without a doubt...

Steve

[bwalt822]

I though the idea of DSing was that every leg is a downwind leg but dont tell funfly.

[Brandano]

Steve, I think that the radar gun is set up to read the moment when the glider moves towards it, so the speed would be measured when the glider is going upwind, climbing back up the slope. Dynamic soaring doesn't use the upwind portion of the cliff (like slope soaring does), but the rotor on the downwind side of the same. Here's a pic I found that explains the concept:
http://www.tmfc.org.uk/ds/ds.jpg

[JetPlaneFlyer]

Dynamic soaring doesn't use the upwind portion of the cliff (like slope soaring does), but the rotor on the downwind side of the same. Here's a pic I found that explains the concept

Ahhh.. I just assumed he was using normal slope lift.. I’ve learned something new today then... thanks!

And just to think when i flew slope years ago that i used to avoid that downwind area like the plague for fear of being sucked down in the sinking air :o

Steve

[bwalt822]

Steve, I think that the radar gun is set up to read the moment when the glider moves towards it, so the speed would be measured when the glider is going upwind, climbing back up the slope. Dynamic soaring doesn't use the upwind portion of the cliff (like slope soaring does), but the rotor on the downwind side of the same. Here's a pic I found that explains the concept:
http://www.tmfc.org.uk/ds/ds.jpg


I thought you get a big vortex of air behind the ridge so that the air close to the surface of the downwind side is actually going upslope. Then at a higher altitude the air is traveling downslope.

[Thomas B]

I thought you get a big vortex of air behind the ridge so that the air close to the surface of the downwind side is actually going upslope. Then at a higher altitude the air is traveling downslope.

That is the way I understand it. The glider is taking energy from both places by diving in the sink and climbing in the lift, and with the proper ship, the speeds are far, far beyond the wind speed.

[nmasters]

There's a neet little sim on the ESA web site called "algebratross" (http://esoaring.com/). It's in the right hand column about two thirds of the way down the page. The birds do low energy DS down wind of ocean swells and ships. I seem to recall that prof Drela did a simulation more recently but I don't know off hand where it is.

--Norm

[HX3D014]

I thought you get a big vortex of air behind the ridge so that the air close to the surface of the downwind side is actually going upslope. Then at a higher altitude the air is traveling downslope.
thats what I thought. (I have never done it or seen it done <Bar that video> ;) ) hehe.