aerodynamics for ultimate SPEED.

[vintage1]

..the subject came up as to how fast we could make a relatively inexpensive electric aeroplane go.

So I hacked around the net, and deduced from the stats that a 300mph+ spitfire took 320W/lb, but a 450mph+ Tempest took 1600W/lb. Well 320W per pound is on, but 1600W/lb is not. So, although 300mph looks doable, 400mph is probably not. Current speed record is around 220mph I think.

What I am after isn't the power train stuff..I think that is under control, I know how to get 300-400W into a prop at that sort of airspeed and get thrust out - its the airframe itself that is the problem.

Basically in the areas of drag, strength and stiffness, and stability - especially flutter.

Also landing and takeoff speed. Don't want to use an undercart, and experience shows that a hand launch at stall speeds over 20mph is a tricky business.

So, the issues devolve into what sort of airfoil works well at low altitude and 2-300mph, how to minimise drag, and especially how to do control surfaces that won't stall servos or add turbulence and flutter. Should the tail be all moving?

Won't bother with rudder tho.

Now this plane may never get built, but it seems like an interesting topic for discussion anyway. In these dark and dreary days of economic depression.

[HerkS]

This is an interesting subject.
Looks to me that to get 300 mph from a prop with 12 inch pitch you'd need to have it turning at about 30,000 rpm (at speed) You can use higher pitch, but the torque on launch would be difficult to counter. I had a small fast glow powered plane that had this issue. Had to be tossed just so, in a 60 degree right bank with full right aileron input - would still torque to about 45 degrees left before getting enough speed to make the ailerons effective. Or, if there was a practical variable pitch prop that would take the loads of such high RPM.

With electric power it can be done more simply. Use a very strong bungee to launch it power off to well above stall speed - then when the bungee drops away, start the motor. I have done this with several difficult to launch models -- very successful in my experience.

Will follow this thread to see what other input you receive.

[Brandano]

What are the rules? Takeoff from the ground or bungee is allowed? I suspect that a highly loaded delta with a thin profile might work out as a suitable tradeoff between low drag at high speed and a controllable high alpha landing, thinking in Concorde terms. Another option might be a variable geometry of some sort, or launching with a bungee and landing with a retrieve chute like many military UAV do. Another extreme option could be to take of and land as a tailsitter, and fly with almost no wing at all.

[GeeW]

A friend who's uprated Voodoo has been clocked at over 200 handlaunches no problem...with all that power on tap...just hold nose 60degrees up, open throttle and let go. Landings are a bit on the hot side but very dooable.
Much bigger problem is keeping track of the little blighter when at speed. If he's going for it then the pilot will have at least 2 spotters, with very good reason.
I think the idea of an all moving tailplane is NOT clever. Lots of glass and carbon and live wing skin hinging coupled with immaculate drives are the way.
For those not aquainted with the voodoo it looks like a speed 400 pylon racer model but with a 1KW motor on a 4s LiPo :D

Regards

Gordon

[vintage1]

There are no rules.

I suppose what the project is all about is taking a relatively small model and seeing how fast something that most people could afford - i.e. not much over 500W - could be made to go without getting stupidly difficult to fly or requiring the resources of a Formula One team to build.

Deltas in our experience don't fly fast. One of the embarrassments at last years show was having my 22" sharkfish (parallel chord wing, crappy towerpro outrunner, square box fuselage) overtaking deltas at similar power to weight levels and leaving them standing. The fastest plane at the moment is a simple copy of an Avro Tutor with a motor stick up its rear.

I think deltas work > mach 1, which isn't on frankly!

One of the major problems is actually the radius of turn needed to not stall the model.. and the problem of actually flying a small plane at 400-500 meters out, because that's the sort of turn diameter needed at 160mph or more.

I had a plane that I couldn't launch successfully until I put what was then insane power into it. That proved enough if launched at 45 degrees, to accelerate into control authority before it torque rolled into the ground.

I suppose we could make a twin with contra rotating props.. sort of mosquito. Or P38. Or bungee it, or put a dolly on.

30k RPM sounds about right too. for a mega 3 turn on 3 cells.

To be honest, the difficulties of flying a small model at these speeds are looking like 160mph or so is a practical limit for a sub 36" span model.

Even then you are pulling turns of 100 meter radius or bigger to avoid stalling the thing.

I am more interested in what makes a good wing section. Generally thinking slender and thin, and faintly elliptical. The compromise is that although thin may be best, its structurally weak, and this thing will need to be stressed to 10g+ to be able to turn in any reasonable space with any margin of safety.

[vintage1]

A friend who's uprated Voodoo has been clocked at over 200 handlaunches no problem...with all that power on tap...just hold nose 60degrees up, open throttle and let go. Landings are a bit on the hot side but very dooable.
Much bigger problem is keeping track of the little blighter when at speed. If he's going for it then the pilot will have at least 2 spotters, with very good reason.
I think the idea of an all moving tailplane is NOT clever. Lots of glass and carbon and live wing skin hinging coupled with immaculate drives are the way.
For those not aquainted with the voodoo it looks like a speed 400 pylon racer model but with a 1KW motor on a 4s LiPo :D

Regards

Gordon


Yep. That little voodoo is pretty much the sort of thing I had in mind. A bit smaller mind you.

what are they weighing in at with 1 kilowatt up the snoot..3lb? or less?

And how fast - 180mph?

And what props are they using?

[eflightray]

Hasn't a dynamic soarer, a glider, clocked over 300mph ?

[HugePanic]

two ideas:


two motors: one with a folding prop (100W) for takeoff & landing, one for speed....


use a flight stabilizer!! i am very impressed what the FMA-CoPilot can do. for landing, takeoff and speed i think this device is great. it simply holds the plane at it's attitude...

[GeeW]

Vin1
Weight is sub 3lb, small prop, kazillions of rpm. I'll ask him for more details and come back.
Huge Panic. FMA would be nice but I know that there is no room for even an extra servo cable down the fuselage, and I am not sure you could mount the sensor head! He really struggled to mount LEDs front and back to aid vision.

Gordon

[HerkS]

Size is a big issue. there are some forums on here with epowered F-16 models that appear to be going plus 200 mph. Very hard to keep track of such a model.

Half cuban eight is a better turn procedure than a banked circuit. Much easier to see and keeps the craft on-track better.

There is also a forum on here that allows speed measurement via doppler effect determined from audio recordings of speed runs. Works well once you master the techie bits.

[vintage1]

Ok.. well there's some more info coming in from the speed and high performance forum that is worth putting here

First of all, I cocked up on WWI warbird power to weight. The spitfire was abut 130W/lb and the Tempest about 160W/lb.

Secondly the hotline boyz reckon the bigger it is the faster it goes.

I think I see what is happening. Area scales as the square of the planes size, but weight scales as the cube. IF the dominant drag is skin friction and profile drag, then for a given power to weight top speed will scale directly with size. My guesstimates on L/D ratios and insane speed show that this must indeed be the case.

Not to mention why bumblebees can't crack the sound barrier in a dive ;)

What this means is that while a 40ft Tempest can do 400mph on 160W/lb, a 4 foot plane if similar shape will need 1600W/lb.

All this drag - skin, interference and profile would appear to be a function of velocity squared, so if we look at say 500W/lb, and our putative Tempest, the best top speed will be a function of its size.

What this means is that its hopeless to shoot for realistic speeds, but that wing drag and airfoil shape is perhaps far far less important that a sleek small body, proper fairings, and simply small wing AREA, that being the largest part of the skin exposed to skin friction. In fact we need wing area really only to allow some sane takeoff speeds. We don't want to emulate the full size tempest that took off at about 120mph..

So using unitary method maths, if a 400mph tempest at 40 ft span does it n 160W/lb it should be capable of 707 mph equipped with 500W/lb. haha. I love maths when its silly.

So a 1/20th scale model - 24" span. At 500W/lb. should do what? 707 divide by SQRT(20)? 158 mph.

Now that feels about right. I get the impression that is about where a small speed 400 size hotliner gives up.

When equipped with a sort of screaming mega 3 turn and a fat 30A or so battery pack. Motocalc is saying similar, though it reckoned 140mph was the brick wall.

Taking 500W/lb again. and looking for 200mph, gives us around a 40" span model. My guess is we are up to 2-3lb here..so 1kw-1.5kw is going to be needed. Ouch. No way is that going to be cheap. 8s 60A packs or something, and hefty motors.

Oh well. Looks like a sensible target is around 180mph on a 32" sort of plane. Around 1.5-2lb, and 750W-1kw. That is a sensible balance between performance and cost. A couple of good 3s 2100 packs running at 30A, is about 600W, and maybe we can squeeze 40A out of them. 800W.

mega 22/20/4 looks about right.

Except that motocalc reckons drag is way high for 180mph.
140 yes, 160 maybe, 180? no. Not without crucifying the wing area and having a plane that comes in at 40mph. And caning the packs and motor within an inch of their lives anyway.

And it looks like motor weight is itself getting significant, 80% plus efficiency ..needs to go to 90% to get the motor weight down a bit.

The good news is that even with ridiculous props, these things look fully vertical. Just point the nose up an let go.. it should settle into a 40mph climb soon enough. :D :D..that's above stall speed and should have control authority.

I think this goes about as far a it can for now. Looks like for sensible money 140mph-160mph is all we can expect sadly..for a sub 1Kw plane.

looks like a sort of 6x10 prop works best.

[Brandano]

The biggest issue with a delta is the oodles of drag in a high AOA configuration, and most people tend to overdo the turns just because they can. I agree that mostly elliptical wing will work better, the problem is how to get the thing to land in one bit. An elliptical wing with trimmed tips should be quick and benign enough in most flying attitudes. Wouldn't go for the all moving stab because it's quite vulnerable and doesn't give much of an advantage if it isn't coupled with a fly-by-wire system. Too easy to have the throw so large that the stab stalls on its own. What would you think about a flying wing with highly swept crescent shaped wings bending at the tips into vertical surfaces? The battery and motor(s?) could be housed in a cigar shaped fuselage, it could even be possible to set it up as a pusher-puller. If anything the number of fuselage-flying surface interfaces would be minimal. Another thing to consider is that at a small scale it might be better to trade off high AR and low induced drag for a bigger chord and spar section, and better Reynold numbers.
[edit:] I don't know if I am making much sense. Think in terms of something like this: http://www.luft46.com/bv/bvp215.html but without dihedral, with thevertical fins replaced by smoothly contoured wingtips and the elevons moved inboard of them. Less sweep and a a wing with elliptical lift distribution complete the picture

[GeeW]

Vin1
Something to consider. You will not be running the motor flat out for more than about 10 seconds before throttling back and lining up for another speed run, then after about two or three cycles you have to slow down enough for you to breathe now and again! At 180mph you are travelling 264' per sec, at that rate of progress 10 seconds is quite a long time.
Regards
Gordon

[biber]

Study Martin Hepperle's website at www.mh-aerotool.de (it's in english), there are ltos of pylon and speed airfoils.

Further do a web search for 'Speedcup', should bring lots of info on the German Speedcup.

biber

[Neil Stainton]

I agree with Gordon, you don't need such a large motor or high current pack, as power is applied in bursts. A Mega 16/25/2 on a 4S or 5S 1.3ah 40C pack would be quite fast.

Are you aware you will probably need a composite wing? The limitations on the pylon and DS models is flutter and wing folding. I think the reason why delta (ie low aspect ratio) models are popular is because of the ease of a manufacturer kitting them cheaply, not because of any aerodynamic advantage.

I made a solid balsa+carbon winged Speed 400 racer some years back, based on the Russian Stinger pylon racer. I was very fast, but the wing broke in a max G turn. I rebuilt it with a stronger wing and now fly it with a 30g high Kv outrunner and a 2S lipo. It flies well but the ailerons flutter in high speed runs.

Neil.

[vintage1]

Hi Neal!

yes, I assumed that a glassed foam core plus full depth spar was a minimum wing.

Possibly with balsa veneer over..that should be stiff enough flutter wise.

Don't fancy those ailerons you have there though. Short and stubby is my thinking,with servos and CF pushrods right up close. Possible also 'both up' as airbrakes...

I take your point about only running peak power for a few seconds. That relaxes cooling on the motor at least.

not sure about a 2 turn on high voltage either..too much RPM=tiddly prop.

Best RPM seems to be 20k-30k as far as I can see.

[HerkS]

Neil > Ailerons fluttering at high speed

I've had some success in eliminating this by anchoring the outboard end of the ailerons to the wing. It's amazing how much control you still have by just twisting the ailerons from the inboard end. Has eliminated the problem on two of my planes. This is especially helpful on very thin wings.

For really high speed flight the ailerons can be quite small and they don't have to extend to the outer part of the wing.

[biber]

The aileron tip anchoring has been done quite commonly among the F5D competitors some years ago.
Better still are shorter ailerons and making the light and stiff (as heavy and stiff won't get you anywhere).
Carbonfibre is king, you don't even ned much of it, as lighter controls are less prone to flutter anyway.

So, V1's approach on that is correct imo.

biber

[Neil Stainton]

Thanks for the ailerron suggestions, I will initially try fixing them at the tips, as that will just require a bit of tape.

Vintage, I am no speed expert but most very high speed fliers seem to be running square or oversquare 4.5" to 6" props at 35K to 50K rpm. That gives a high pitch speed while keeping the prop tips subsonic. Search the High Performance forum.

Neil.

[biber]

Yeah, what you want is at least a square prop, or oversquare, but these are rare.

RPMs are quite unimportant, you need to prop according to the motor you are using.
Low RPMs require bigger props which again require torque and that means a heavy motor.
I think that is one reason to go for high RPMs, but you can also do it very well with lowish RPMs.

biber

[vintage1]

yeah..it was the props that first got me thinking. Diameter is actually irrelevant. The prop tip speed is the pitch speed times pi times the diameter to pitch ratio.

i've run everything from sort of 4x7 to 16x28 through motocalc, and there isn't a huge difference..motocalc SEEMS to say that the high RPM stuff has a wider speed range but its less efficient at full power.

thanks for aileron tips as it were..

[reinking]

Balloon launch, parachute decent. Forget lift below max speed-25%.

Go for it. Have fun.

Reinking

[JetPlaneFlyer]

Sorry if I've missed something but why attach so much relevance to W/lb statistics?

When I last checked maximum speed occurs when thrust = drag.

Whereas power may be closely related to thrust potential, weight has very little to do with drag at high speed.. (ok it influences induced drag but that's a tiny proportion of overall drag at speed)

If you want maximum speed your efforts should surely be directed at designing the lowest drag airframe as possible rather than focussing solely on W/lb ?

[Joel K. Scholz]

Here is some video of a wing I built for speed. I believe it has the potential to go faster with a proper power source. Seems to hold speed in the turns pretty well.

http://www.youtube.com/watch?v=5Gm9SiI_kj4

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

[kral27]

..the subject came up as to how fast we could make a relatively inexpensive electric aeroplane go.

So I hacked around the net, and deduced from the stats that a 300mph+ spitfire took 320W/lb, but a 450mph+ Tempest took 1600W/lb. Well 320W per pound is on, but 1600W/lb is not. So, although 300mph looks doable, 400mph is probably not. Current speed record is around 220mph I think.

What I am after isn't the power train stuff..I think that is under control, I know how to get 300-400W into a prop at that sort of airspeed and get thrust out - its the airframe itself that is the problem.

Basically in the areas of drag, strength and stiffness, and stability - especially flutter.

Also landing and takeoff speed. Don't want to use an undercart, and experience shows that a hand launch at stall speeds over 20mph is a tricky business.

So, the issues devolve into what sort of airfoil works well at low altitude and 2-300mph, how to minimise drag, and especially how to do control surfaces that won't stall servos or add turbulence and flutter. Should the tail be all moving?

Won't bother with rudder tho.

Now this plane may never get built, but it seems like an interesting topic for discussion anyway. In these dark and dreary days of economic depression.
"...and experience shows that a hand launch at stall speeds over 20mph is a tricky business."

LOL...:D

[Brandano]

weight translates quite directly in drag, as was readily found out during WW2. More weight means a higher AOA to maintain level flight at any speed, and this turns into induced drag that is only minimal at low AOA. It can be a small amount at high speed, but the drag increase it exponential, therefore a little weight gain can result into a high drag increase, expecially if he wing is already highly loaded. Actually increasing the wing size might reduce drag, like in the TA152 developement of the FW190

[JetPlaneFlyer]

Actually increasing the wing size might reduce drag, like in the TA152 developement of the FW190
How does that reconcile with accepted and well proven practice of clipping wings on WWII fighters and on WWII fighter derived reno racers to increase straight line speed?

The TA152 is possibly a bad example to use because it was a very different aircraft to the standard FW190, including a far more powerful engine which worked much better at high altitude. I think that you may find that it was so fast mainly because it could operate efficiently at very high altitudes where thinner air reduces drag... Not a strategy that's likely to work on an RC model unless you live on a mountain ;)

I think you need to check your facts on the theory that drag increases exponential to weight. All other things being equal induced drag increases linear to weight but as induced drag is only a small proportion of overall drag at high speed then the overall effect of weight on drag (at high speed/low AoA) is very small.

Having said that in the 'real world' maximum speed in an RC model depends as much on how well the model can maintain it's speed through turns and how quick it accelerates as it does it's theoretical maximum straightline speed... So keeping the weight down is very much worthwhile, to say nothing of the benefits light weight brings to take off and landing practicality.

[Brandano]

Clipped wing racers are also usually lightened to the extreme, and their range is severely reduced. Even on the spitfire the clipped wings were introduced to get a better roll rate, when bigger and heavier engines were introduced the wing shape was revised to increase the wing area. I agree that induced drag is only a small portion of overall drag at high speed though. However if all it took to get better speed was to clip the wings, a plane with no wings would have the best possible LD ratio. Btw, i found an interesting page about the spitfire's wing structure: http://spitfiresite.com/reference/variants-technology/2008/04/spitfire-wings-02.htm

[biber]

I second JPF, the task the Ta 152 was made for was flying high altitudes, thus at high Cl and thus induced drag was a big concern.
For a given aircraft clipped wings will improve topspeed a bit, but you pay for it with higher stallspeed and poor performance at higher altitudes (you won't even reach the same altitudes as with unclipped wings).

biber

[JetPlaneFlyer]

Clipped wing racers are also usually lightened to the extreme, and their range is severely reduced. Even on the spitfire the clipped wings were introduced to get a better roll rate, when bigger and heavier engines were introduced the wing shape was revised to increase the wing area. I agree that induced drag is only a small portion of overall drag at high speed though. However if all it took to get better speed was to clip the wings, a plane with no wings would have the best possible LD ratio. Btw, i found an interesting page about the spitfire's wing structure: http://spitfiresite.com/reference/variants-technology/2008/04/spitfire-wings-02.htm

Actually the Spit's clipped wing improved roll rate and low altitutude top speed, at the expense of high altitude performance. Yes Reno racers 'can' clip their wings because they dont need to carry the load of the original fighter nor operate at high altitude, they can also tolerate higher landing speed, however the simple fact remains that clipping the wings DOES make them faster in a straight line.

You bring L/D into the argument.. i never mentioned L/D and it's not really relevant to top speed. At very high speed and low altitude producing enough lift is not a problem, drag is the enemy. Interesting comment about removing the wings altogether being the ultimate arrangement for top speed... I know you meant it tongue in cheek but if you look at real very high speed aircraft that are not limited by the practicalities of take off and landing requirements than you will find that the wings pretty much have been removed altogether as at very high speed the fuselege is more than capable of generating all the lift thats required. http://en.wikipedia.org/wiki/Boeing_X-43
I'm not suggesting Vintage removes his wings because his target speed is more modest so the wings still have a job to do but you can perhaps see the general trend?

All this is interesting but truth is that the wing size will be governed not by ultimate top speed considerations but by the need to keep landing and take off speed down to sensible proportions.

[JetPlaneFlyer]

I thought i'd put some numbers on how big the 'ideal' wing should be if ultimate straight line speed was the one and only concern...

Lets say our model weighs in at 2Kg and the target speed is 200mph..

Optimum L/D of the wing will occur at a Cl of around 0.4 (give or take). So by using the lift formula it's quite easy to figure out what wing area would be needed to get the wing operating at it's most effcient L/D when flying at 200mph.

I'll save repating all the math but the answer is a wing area of around 100 sq cm.. So exluding fuselage width that would be a wing of about 25cm (10") span and 4cm (1.5") chord :eek:

That would deliver a wing loading of 20g/cm^2 (4.6oz/sq in) and a stall speed of about 134mph... not very practical :(

Steve

[biber]

Just have a look at what already exists:
http://www.rc-network.de/forum/showthread.php?t=118568

Go from there, it is a status quo of a evolution going for some years now.
Throwing around with fantasy numbers and ideas does lead to often in an impractical result.
There's at least some reasonable point to start from.

biber

[vintage1]

All this is interesting but truth is that the wing size will be governed not by ultimate top speed considerations but by the need to keep landing and take off speed down to sensible proportions.

I believe this to be entirely the case.

I am using watt/lb because it represents an upper limit of what can reasonably be achieved with packs and motors and sensible airframes. i.e. 500W/lb seems an achievable target.

When you translate that into speed, what comes out of the calcs seems to be that L/D is almost irrelevant in high speed straight flight: What you are fighting is profile and skin drag.

Like I said a tempest can do 400mph on 167 W/lb, but at the expense of having a stall speed in excess of 100mph.

There is a direct relationship between top speed, watts per lb and wing loading.

When it comes to turning though, a high wing loading means a stall is reached at much lower G than a lower wing loading. Lower wing loading planes pull more g! When you think about it that is totally reasonable: the stubby winged heavy plane needs to run at a higher angle of attack to pull the same g.

I suspect there is a simple formula that says that e.g. '50W/lb will enable you to fly at 3 times stall, 150W/lb at 4 times stall, and 500W/lb at 6 times stall' or something.

In terms of a PRACTICAL small plane that CAN turn without risking a stall at something like 5-10g - necessary for keeping the thing flyable by reasonable pilots, we have to push the top-to-stall speed ratio out a long way.

Or go in for vertical takeoffs and parachute landings, and reduce our craft to a pair of contra rotating props on a virtually zero wing plane!

Bear with me on all this though, as the whole point of this thread was that I did not understand the issues, and that why its proving such a darned interesting one.

[biber]

Note, that if you have the speed, you have to build the model for more then a 10 g load factor, anyway.
Even if the pilot does not touch anything, a gust can easily force serious loads on the wing.
Considering 10 g as a safe load factor would be careless.

biber

[vintage1]

Just have a look at what already exists:
http://www.rc-network.de/forum/showthread.php?t=118568

Go from there, it is a status quo of a evolution going for some years now.
Throwing around with fantasy numbers and ideas does lead to often in an impractical result.
There's at least some reasonable point to start from.

biber

Ah, but you miss the point. Which is NOT to come up with fantasy numbers,but to come up with numbers that reflect what is out there and does work, understand why the numbers work, ad see if they imply any particular thing that could be improved. Reality is always the final arbiter of the imagination of a designer. :D

And I have to say, what is coming out of the numbers looks very very similar to several of those planes.

If you like a summary of 'data points' so far they would be..

1/. the frontal and wetted area of the model determines its high speed drag.
2/. That means wings should be thin and small. but..
3/. increasing wing loading results in a model that wont take off, land, or turn..
4/. to get over about 140mph requires over square pitch to diameter ratios to avoid transonic tips.
5/. the ratio of top speed to stall speed relates directly to its power to weight ratio. I have yet to calculate the likely formula.
6/. The ability of the plane to turn at high g also relates to this.
7/. Issues of turbulence can manifest themselves at high speed leading to flutter, or excessive drag, or fatigue failures or any or all of the three.

[vintage1]

Note, that if you have the speed, you have to build the model for more then a 10 g load factor, anyway.
Even if the pilot does not touch anything, a gust can easily force serious loads on the wing.
Considering 10 g as a safe load factor would be careless.

biber

I agree that 10g is a load factor that is inadequate, if for no other reason than it implies a lack of stiffness as well, that may cause flutter. How high a g loading it is possible to get, I am not yet clear on. I suspect -10 +25g is reasonable.

[biber]

Sounds sensible so far.

Next would be choosing a feasible motor/esc/batterie/ combo and designing prop and model around that.
No need to stop crunching numbers, however.

biber

[vintage1]

Yup. I spoke to Mike and he has many mega 16-15 range motors, so my starting point would be to design an airframe for one of those, and leave the possibility for a 16-25 sized motor too.

As long as there is room for a 500W pack, we should be OK My thinking there is to have that on edge under the motor, to provide some fuselage below the prop center line to protect it in what will be pretty high speed landings.

That isn't so good for frontal area, but by keeping the nose short, we can reduce the overall fuselage surface area.

I am shooting for about 32" span, as being a compromise between decent landing speeds, turning radius and visibility, and overall top speed.

Even so, it will land about 30mph plus, to be safe. I built a plane of similar size and wingloading before, given enough power (160W /lb of brushed motor!!) it would hand launch OK..45 degree javelin and it was beyond stall speed (about 25mph) with control authority in less time than it took to hit the ground :D

[Neil Stainton]

I think the F5D (pylon) and F5B guys are pulling arround 40G.

One should be able to estimate the DS models' G levels by timing the secs per revolution in one of the videos posted here (http://www.rcgroups.com/dynamic-soaring-126/), along with the speed gun obtained velocity.

Neil.

[RCAV8R13]

How about EDF?
I recommend keeping it light to reduce induced drag.

Kip

[biber]

EDF is good for the look of it and you stop breaking props.
In any other regard it is considerably inferior to well done prop drives.
Especially efficiency clearly rules out EDF.

biber

[vintage1]

I dint think EDF is as efficient until sort of > 200mph speeds. That's the project after next ;)

[banktoturn]

If I were building something to meet your stated goals, I would think along these lines:

1) Modest aspect ratio (say, ~2), simple linear taper with a moderate taper ratio (say ~0.7)
2) Inboard flaps coupled to elevator & deployable on a separate channel for landing. Inboard flaps, properly configured, can help tailor the spanwise lift distribution when coupled to elevator in high-CL conditions, which is the only time that induced drag becomes significant.
3) Cowled engine, no external linkages, accurate airfoil for a usable drag bucket.
4) Tiny wings (missiles are faster than planes).


banktoturn

[rebell]

V1, I had the same objectives as you and build this plane: http://www.rcgroups.com/forums/showthread.php?t=869105With a Mega Mega 16/15/5, a 3s 1500 LiPo and high pitch prop it reached 233km/h.

[vintage1]

Rebell, that is so close..but I think you are a shade overpitched..some thing like a 4x7 or 4x8 might be faster.

Are you in S Africa?

[rebell]

Since the time I used that prop a lot and only landed in tall grass. I once landed on mowed grass and the prop broke on the landing.

I made a folding prop from an APC 5.25x6.25. It launched much better but the top speed came down a lot. It needs more rpm to get to the higher speed.

[Ralph Walton]

Hey Vintage,
What about picking a scuccessfull proven airframe , such as the Opus MCT glider, which has been DSsed to around 325MPH and regularly flown to high 200's, and build something with similar characteristics?

Its all proven, flutter issues have been all but eliminated, you won't break one in the air, they handle and land like a pussucat, and theres a ton of handy info on building them for speed. Heck, maybee just stick a motor in one!

My 100oz one, carries energy like nothing else I've ever flown. There's somthing special about them.

If you REALLYwant to go fast, Joe manors 160 inch glider has been clocked at 371MPH so far. Hes using a laminar flow airfoil designed by Joe Wurtz. Its call a JM26 and coordinates are readily available.

[vintage1]

No challenege in it Ralph :D

The idea is to see what can be done to develop a cheap simple kit that goes like stink, and see if the maths can tweak it a bit faster than the rest.

I.e. draw on the experiences of others, add a little bit more in terms of analysis, and produce something a little bit better. Fast gliders are not the target: the target is a small engine powered plane. Fast gliders are all about minimal drag and strenth..the engine powered stuff is slightly more complex.

[rebell]

Ralph, I also understand that there is a lot of existing models on the marked for speed. That is not the point. I wanted to design and build something myself. That gives me more satisfaction than buying something else. Also, that is the way the hobby develops. If the person you buy from decided he will buy rather than design, where will you buy from?

One of my objectives was to keep the cost as low as possible, in my case it was around $10 US for the airframe. What is it? ₤7? Now, if Charles designs a kit that is cheap to buy, easy to build and fun to fly at high speed with cheap hardware installed in it, more people will be able to enjoy it. That is what he said in his opening line.

I used hardware I already had. Even the servos are cheap. It is 9-gram E-Sky servos, the same you get in the small Lama helicopter. Because of the wing and aileron design there is NO flutter at high speed. The wing is very strong. I was flying at high speed when I got a serious glitch. The plane pulled hard and fast upwards before I had control over it again. It did not get a crack or any damage what so ever. There is no carbon spar or any reinforcement in it, it is plain cheap painted balsa from tip to tip.

Another model to look at is the 750mm Slipso 400. Designed to be cheap for a speed 400. Different power systems are installed in it, from mild to wild. Some high speed claims are done.
http://www.rcgroups.com/forums/showthread.php?t=418625

[62pilot]

Has anyone considered variable pitch props? I know whats on the market is not made for flat out speed. But maybe modify a regular ep prop. That way once your up to flying speed and you want to turn on the power,mixing the two, you could also crank in some pitch. Might be something to consider. With the right motor,going from a 4 inch pitch to a 8 inch would be like turning on the afterburners.

[rebell]

Yes, I was thinking about it. The motor then needs to be with hollow shaft in order to control the prop. I am using a inrunner that has a solid shaft. It is only small 3D motor that is supplied with hollow shaft.

Any ideas?

[Brandano]

use a swashplate instead of a hollow shaft? You don't even need an uniball inside it, just a brass bushing. I wonder how much power it will steal from the motor due to the added friction, and if this would cause prop blade flutter due to the play in the linkages

[62pilot]

It would have to be slop free.

[rebell]

I was considering that. The main rotor swashplate is to long so that is out of the question. The tailrotor control links is pointing in the wrong direction if one wants to make it a folding prop. You will also need a long shaft on the motor for the slide to move over. I had it on the motor. I will have to make a new variable pith hub for this purpose myself.

[HugePanic]

i build sth like this about a year ago. but the goal was to control the vector of the thrust.
i used parts of a coax-helicopter. (i don't think these parts are good for more than 150-200W.)
here (http://www.rcgroups.com/forums/showatt.php?attachmentid=1608736) is a video.. i also should have some pictures showing the parts.........

[vintage1]

From a speed perspective, variable props. like undercarriages and wings, are just excess weight you need, to take off and land ;)

[Brandano]

Well, on full sized planes constant speed props act like the gears on a car and allow you to accelerate to speed better by giving you maximum thrust at all speeds. Perhaps on a bungee launched plane the weight and complexity gain offset any advantage, but with the proper governor circuit it might help a plane exit faster from a turn. Anyway helicopter components wouldn't work, it would have to be custom machined for the job.

[vintage1]

Yup. I should have said:
"From a speed perspective, variable props. like undercarriages and wings, are just excess weight you need, to take off and land and turn in a reasonable radius" :D :D

[Red_N_White]

Another thing to consider...
Vairable sweep wings.
Although it would add complexity and maybe slop in the wings, and could decrease wing strength, you could take off with more frontal wing area then sweep them back to reduce frontal area.

All this being said, I know the disadvantages of deltas have been mentioned and simple is often better..But food for thought anyway.

[biber]

Sweep does nothing to reduce drag at all.
Frontal area does not play the major roll in drag issues below mach 0.8 or so, wetted surface does.

Variable pitch would help a lot.
It enables to have a prop optimised for speed and limit the load on the esc. motor and battery during all other phases of flight.
Big problem with power trains optimised for speed is, that the equippement can't stand the high currents you get as long as it is not at speed.
To throttle it down often doesn't offer a solution to this, becaus the currents an esc will hold up to are much less at partial throttle than with full throttle.
Either way you can't have it at low speed for more than a really short time, otherwise you will fry some stuff.
The variable pitch does solve the problem efficiently and you will be able to use smaller esc, motor and battery.
If you are able to build a VP prop, it will certainly be worth the extra weight as can safe you weight in other parts of the gear train, but it's an extra technical effort to make.
And you don't want to have it breaking at a landing.

So, generally its a good thing, but not if you want to keep it simple.

biber

[eflightray]

I've seen models get blown over that were 'parked' behind a model when the motor is throttled up. Awful lot of wind coming off a prop. Also the faster a model goes the greater the drag on all that surface area.

Now here comes the 'stupid question' -

Shouldn't the prop therefore be at the back pushing? It reduces the 'drag' caused by air stream coming off the prop? :)

[rebell]

I also though a VP prop to be a good thing, but not any more. The extra space needed for an extra servo and links is not available in these small planes and the extra weight will reduce speed.

[JetPlaneFlyer]

Now here comes the 'stupid question' -

Shouldn't the prop therefore be at the back pushing? It reduces the 'drag' caused by air stream coming off the prop? :)

Not a stupid question at all.. Thare is still some excelleration of air in front of the prop, though your correct in the fact that the air in front of the prop is not moving as fast as the air to the rear, so the friction drag over the fuselage will be slightly reduced. However a pusher also has it's downside; the air approaching the prop is disturbed by the fuselage/wings etc so the prop is being 'fed' turbulent air and is therefore less efficient than a tractor prop.

Overall i'm not sure what layout is theoretically most efficient? It probably depends on numerous variables, but i suspect the difference in efficiency between pusher and tractor is very small.

[vintage1]

Judging by the noise, tractor is more efficient.

Wing wetted area is the most significant drag - fuselage is relatively small.

And lack of control authority in pitch is a real problem with hand launched pushers till they get up to speed. Seen many nasty flip up and stalls..and no way to pull them back until they are going fast enough..to explode on impact :D

[Brandano]

And lack of control authority in pitch is a real problem with hand launched pushers till they get up to speed.
There is no real reason why a pusher should lack pitch authority when compared with a conventional layout at any speed. However, they are obviously harder to hand launch, and I suspect that many problems are just due to the launcher letting go too early to get away from that prop

[rebell]

There is no real reason why a pusher should lack pitch authority when compared with a conventional layout at any speed.

I beg to differ. Just as you have control with a 3D plane in a hover or other slow moving manoeuvres because of the airflow from the prop over the control surfaces, the same does the steam of airflow of a puller prop give you some control over pitch and rudder. Most small hotliner models don’t have a rudder, but surely can benefit from it.