What determines a planes speed?

[Erfolg]

If you are really interested in speed, you just need to look at models which only do the speed thing. There is nearly always a fly in the ointment though. That is usually a set of rules that speed freaks are required to comply with, that is to be compliant with class rules.

Possibly, one class that points the way is CL speed. Again there is an issue in the comparison, that is the control wires, which have lead to configurations that minimise drag due to the wires, a high aspect wing, to enclose some of the wires, and an outer wing just big enough to support the outer body component.

Yet looking at the body, it is very small, big enough to contain both the control horn and tank. Then just enough of a shape to contain, the engine and cylinder head in a favourable shape. In essence the smallest cross sectional area to contain either the Lipo, motor or RC equipment, whatever is the most restrictive.

In case of the wing, as BEB has indicated, no need for a good CL value, more emphasis on CD. Which indicates a low camber thin wing, and thin. Although contrary to intuition, a sharp Le, generally does not cut the mustard. It i better to have a good radius, which is tolerant to the AoA, not suddenly stalling.

With respect to plan form, if the model is required to operate mostly at low AoA, a pretty much parallel wing is as good as a tapered wing. Tip losses are not the major drag source. that is coming from the profile and losses at the wing to body junction. The trick is balancing the wing area needed at speed, with that at the opposite end of the speed envelope

As with most things when you want to go fast, weight is not generally helpful. Light is better.

What is possibly the maker and breaker of the whole set up is the power train. It has been stated, as much power, with as few losses, through an efficient propeller.

Sounds easy. Yet as with control line speed, models of this type, are like having a Tiger by the tail. That is why many speed orientated classes, have rules, to both keep the model safe and controllable, then a nod at affordability.

[Ben B]

I ran a 40" flying wing [Windrider slipstream] on a 2w25 on 4s at 80A with a 6*5.5 prop and it was stupidly quick. If you are looking at high kv inrunners do have a look at HET motors. They're great- and take serious power. I've got a 2w25, a 2w18 (in a YT Dago Red pylon racer) and (my most recent purhase), a 1w40 [3300kv, 2000W maximum] which I'm still looking at airframes for.

[Erfolg]

I have just seen the small prop. Small props are generally not as efficient as large diameter props. Again as any engineer will regurgitate,

P = 2piNT

Which indicates revs are useful as well as torque.

So it is back to the propeller, balancing prop efficiency, with toque and revs.

[001]

If you are really into speed and nothing else, take a look at the art and practice of dynamic soaring, you might be amazed, 400 mph., the right hill and no powerplant.

I think the record is now 446mph.

Edited By Chris P. Bacon on 15/12/2012 22:04:16

[Tony H]

Thanks Chris P. Bacon, I have seen video's of the Soarers you mention and they are impressive but not really what I am trying to achieve. It's not all about speed, just a plane that will fly very fast on a still day under its own power at a regular flying field (not from a hill), and it has to handle well too.

I'd probably be happy at around 150mph, it's a toss up weather to go electric or glow. It's not going to be a big plane.

I have a .15 size Sig Wonder which has been clocked at 106mph on gps and that is a fairly chunky little plane with quite a thick wing profile, I could go down the route of making a quicker version of the wonder, with slim profile wing and a smaller more streamlined fuselage.

[Martin Harris - Moderator]

The "fastest" model I've had was actually one of my slower models in terms of outright mph maybe pushing 60 - a little foamy Me163 of about 18" span. My 1/4 scale Cub is probably faster but any observer would say the 163 was ballistic and the Cub is a plodder.

So if you want an incredibly fast and exhilarating model, then keep it relatively small. After all, watching a full size airliner flying at @200 mph a few miles out from an airport on the ILS doesn't give any impression of speed whatsoever...

[Erfolg]

I have been thinking a little about the concept of just a fast model.

Although not expressly stated previously, as with all designs, it is about a package.

I have an interest in general history, in some contexts, designs can often be viewed in a historical context. If you take an example of the importance of packages, just look historically at both Ferraris and Honda in the 70's, both produced very powerful engines, onto which everything else was just built. Then think about the McClaren MP4, with a Tag engine. The engine being allocated a volume within a package, where the concept was optimisation of all the elements of the package. I think John Barnard was the project leader (designer). The packaged concept was a world beater for its time.

Likewise, go back to aircraft in the 1930'3, most knowledgeable people predicted that in line liquid cooled engines would produce faster aircraft than radial air cooled engines. The reality is, there was little to choose between each configuration.

So again we are back to there s more than one way to skin a cat. Though in a competitive world, the skinned cats all look very similar

I then also think about the role physics, and what is important. It is here that things can become difficult. I remember one of my first lectures in "Fluidics". On the lecturers bench were two lab type glass tall vessels. Two ball bearings, one smooth, one chemically etched. Which will fall the fastest through the gell in the vessels, that was the question posed. The lovely smooth, polished ball bearing, was the general opinion. The actual answer was the etched surface ball bearing. Why, it was the boundary layer having better flow characteristics. This was an introduction to the fact, that in fluidics all is not as it may be assumed. Being full of areas of surprise, difficulty in calculation and prediction.

Going back to our fast plane, it does seem logically that the motor is very important. It determines the front of the model, in shape and cross section. It does seem that an inrunner would produce the slimmest nose, but potentially a poor propeller size. Perhaps a gearbox would help. But then there is a power loss. Outrunners are normally off larger diameter for a given input wattage. This would produce a bulkier front end. Yet probably a more efficient package (without gearbox) when combined with a propeller.

It is easy to overlook the importance of the battery. Yet the capacity of the battery, ampage, is crucial. So how long is the flight going to be? Then you will need to consider the C rating etc. The weight and volume /envelope of the battery is important.

Your ESC will be at its most efficient when running unthrottled. So that is how it should be selected.

Having assessed all the weights of all the equipment, you now are ready to decide on the area of the wing, having decided on what you think the wing loading can be at a max.

Of vital importance is getting the AoA of the wing relative to the body as wanted, not flying nose down or up.

It seems that a circular torpedo shaped body will produce the minimum wetted area.

Now you have to consider access to equipment for maintenance and operation. Poorly executed access, can be a real drag producer. Again we are in the area of the ball bearings, how do you do this, with the minimum additional drag.

How big is tailplane, moment arm, will there be a fin/rudder.

It is time to reconsider all aspects of the model again in our design exercise.

In my mind, it will be pretty ugly, just like a Lazybones, CL speed, without an IC engine from the 60's.

The one thing I have confidence in, is that for a minimum aeroplane, you need to design from the motor/engine backwards.

[Peter Beeney]

In just simple practical terms, I’ve have thought that the main element that controls the planes maximum speed is the speed and pitch of the propeller. Leaving aside all the aerodynamics for a moment, and talking in approximate figures, if we consider the required speed of 150 mph, and our motor turned at 10,000 rpm we would need a pitch of some 16 inches. If we allowed a realistic 20% rev increase for slip this now becomes a 19 inch pitch; or we could leave the pitch alone at say 8 inches and adjust the revs, now we would be talking 20,000 rpm. Or 24,000 with 20% added for slip. Would it be possible to achieve this with an i/c engine? Certainly we need the maximum power from our lump, and this is not always realised at it’s maximum revolutions anyway. It might be easy to double the pitch of the propeller, but to maintain the same revs at that pitch would most likely take more than double the power, perhaps we’re even looking to square it. This might be fairly difficult, to say the least! Having said all that, Weston for one seem to produce some pretty impressive pulling power, maybe they can turn a 9.5 inch pitch prop at 20k, tuned pipes and all that jazz.

It might be possible to get closer to it with a small electric model, though, powered by an inrunner turning a 4.5 by 4.5 (say) at around 45,000 rpm. But this is now a small model consuming power at a high rate, up to 1kW/lb., and it’s also relatively very noisy, but because of the high power consumption the duration of flight is by necessity fairly short. My colleague is rather inclined to frequently fly this style of projectile, I don’t know exactly what the speed is, but one description might be called rapid!

That only leaves the gas turbine, this spins fast enough to reach the high forward speeds too, but now the performance is related to the cost. There are some very fast gas turbine powered models I’ve seen at shows but they certainly don’t appear to be the sort of thing you just sling in the boot for the odd half hour session. Now not beyond the reach of a dedicated modeller probably, but it must still be a bit of a specialist area; I’d consider it needs the right site to fly them for a start.

Going back now to the aerodynamics, if we improve this we just reduce the slip factor, to try and get the lowest possible. I’ll take a complete guess here, because I’ve never seen any information on this, but does it ever go below 10%, even using the most efficient of airframes? Certainly it’s never going to reach zero %.

So all in all, I’d say that 150 mph is perhaps not that easy to reach; particularly with a glow. My Irvine 53 turns an 11 by 8 at 11,000 rpm, that’s 83 mph on paper. But it’s a standard large airframe, so I’m sure that in reality it’s actually much closer to the 66 mph figure with 20% slip deducted.

Good Luck, it sounds as though there is some scope for a spot of design in there somewhere.

PB

[Tony H]

Thanks guys, all very good information. You are right about it being tricky to achieve 150mph+ with a IC but Weston models have achieved it with their expensive tuned pipe engines and deltas.

That said I think I will keep mine simple and on a budget, I think I will go for a high Kv outrunner something in the region of 2200kv and plan to use a 5x5 prop on a 3s lipo giving me around 345w, if I build a small slim plane weighing no more than 1.75lbs then I would have 197w/lb which should make it very swift.

I would probably use my 2200mah 3s lipos which will hande the 32amps fairly well and give me roughly a 4min flight (hopefully)

[Biggles' Elder Brother - Moderator]

Putting your kV values in would give you (theoretically) 24420rpm. At 5" pitch, and no slip, that would give you an absolute theoretical maximum speed of around 115mph - at 20% slip more like 92mph.

You could really do with a bit more pitch and umph if you want to be reasonably confident of getting over the magic 100mph.

BEB

[Simon Chaddock]

"A slim plane with a 2200mAh 3s weighing no more than 1.75lbs" ?

Remember weight needs lift and that creates drag.

Given that a 2200mAh 3s only weighs 6oz I would have thought you could get nearer one pound all up.

If you could it would also raise the power loading to 345W/lb as well.

[bouncebounce crunch]

I have found that trees, fence posts and the groud have had a significant bearing on the speed of some of my models over the years, and they seem to fly a lot faster when you have lost radio contact or are out of control

[Former Member]

[This posting has been removed]

[Martyn Johnston]

You ever seen the tiny little electric 'Hoellein Voodoo'? Or here.

I want one.

They are extremely pretty, and stunningly fast.

[Steve Hargreaves - Moderator]

Someone turned up with one of these at our patch Martyn.....I think it was on 4 or even 5S I can't quite remember but it was insanely fast.......I just couldn't see it half the time.....great fun to watch though even if I was looking in the wrong direction quite often......

I suspect I would need a lie down after flying one of those.....assuming I didn't crash it at the first turn.....

[Erfolg]

Personally I do think that the idea of watts per pounds will provide the speed wanted.

I am pleased that the motor and propeller are being seriously considered as a package. The Kv will provide an indication of the revs that could be achieved. The pitch will also indicate the speed possible at any given revs. The big issue is the motor capable of providing the torque, to make it all possible. A small diameter prop, may provide the revs, not necessarily the thrust, likewise a large prop, may not achieve the revs, due to lack of torque.

The Voodoo does look conceptually pretty much as I envisaged, other than it is prettier, possibly it is the crescent wing.

I would envisage that the Voodoo has another feature that I would have guessed at, that is a moulded body, and probably the wing also. I think a moulded body is almost a defacto necessary feature, providing light weight, stiff/strong and probably with good damping feature.

I am not necessarily convinced that the scimitar wing is required. I believe for a non moulded wing, that a rectilinear wing would be as good, at speed. I would think that the wing would need to be so thin, that a solid wing, glass covered would do the trick. As for section, i would think that a symmetrical wing would be as good as any.

I am impressed with the Voodoo, in that both the power train, and lipo are packaged into a very compact, fuz.

There is a Stryker in our club, which can be fast, but it certainly is not very, very fast. Certainly nothing like as fast as F3b's doing speed runs. The big difference is that they glide well, in addition to just going fast. Going back to our school days, they are converting PE for KE (I am reminded of springs and Newtonian ball), rather than a motor propeller combination. Different game, different solution.

[Biggles' Elder Brother - Moderator]

Posted by Andrew Ray on 17/12/2012 05:20:00:

Posted by Biggles' Elder Brother - Moderator on 17/12/2012 00:00:41:

Putting your kV values in would give you (theoretically) 24420rpm. At 5" pitch, and no slip, that would give you an absolute theoretical maximum speed of around 115mph - at 20% slip more like 92mph.

You could really do with a bit more pitch and umph if you want to be reasonably confident of getting over the magic 100mph.

BEB

Have I not read somewhere that a stock F27Q Stryker is pushing close to 100mph? Certainly 90mph.

Andrew

Hi Andrew,

looking up the spec the Stryker has a 6x6 prop, 3s battery and a 2200kV motor. So thats a theoretical maximum speed (ie if the motor reaches peak revs and there is no slip) of 139mph - allowing say 20% prop slip it would be 111mph.

The question then becomes is the drag low enough to limit the prop slip to 20% - if so the Stryker does indeed do over 100mph.

BEB

[Steve Hargreaves - Moderator]

I do think we need to be careful quoting theoretical figures & converting these to top speed.....the 24420rpm quoted above comes from a 2200kv motor with 11.1 volts applied.....

Two points.....a 3S Lipo under load won't show 11.1 volts....more likely 10 to 10.5 so thats the best part of 2000 rpm gone.....second the motor will not turn at its rated rpm as defined by the kv figure....never can, never will. It won't reach that figure even with no prop on it....why? Because there needs to be some slip to allow the motor to generate the torque required to overcome the various losses associated by running the motor (friction, air resistance etc) As the load increases (ie with a bigger prop) then the slip must also increase to allow more torque to be generated to turn the prop.

I don't know what the rpm reduction will be under these circumstances...I must try & find out....but I would think they will be significant.....

There is also the difference between theoretical & actual pitch speed as mentioned above....is 20% slip realistic? What is it based on? I really don't know so it would be interesting to find out

[Erfolg]

I think Steve is right on the button.

I think from previous reading that it has been suggested the best difference between calculated pitch is circa 15% less, at best. The rest of his contentions also seem to be reasonable.

Just from observation and what people with stop watches tell me, that F3b models are diving and doing speed runs at about 100mph, which seem to be about 14s or less (so I am told). These models look to be travelling a lot faster than the fastest electrics I have seen.

I am not sure it matters though, in that you cannot design a record breaking electric or IC model on the board. You can design a model with all the right characteristics.

To get that speed, or near to that speed as calculated, an awful lot of development work will be required. I totally agree that the motor and propeller are important. The real pain in the butt, is that everything else, has to be optimised to. Which takes an awful lot of development work and measurement.

If you just want to go fast, either buy a Voodoo or build something similar in concept. It will be fast, though you will be able to get it go faster, than as built, with more work..

Is it just straight line speed that is sought, or must it handle like a pylon racer?

[Peter Beeney]

I think if I were looking for 150 mph on a 5 inch pitch propeller I’d have to get seriously optimistic about the power supply. Here I’d require 31, 680 revs per minute for success, and if the prop were attached to anything at all that would cause any drag, such as a motor and a model, I would have to find a few more more, up to another 6,000 more. I’d also bear in mind these would have to be real revolutions, as clocked by a tacho, not as a kV projected figure. I’ve never really understood this anyway.

I didn’t know about the Weston i/c result, presumably timed with a watch or radar gun, but I know a long time ago they were quoting 1.64 kW output for one of their 46 engines, I suspect it may have been re-worked and piped. This, of course, is the mechanical power at the prop shaft, whereas the power of an electric set up is the battery exit power, to get 1.64 kW at an electric motor shaft, even if I only optimistically estimated 10% overall losses, then I’d need to see 1.8 kW at the battery exit.

Steve, I’m not totally convinced about your explanation of the electric motor, don’t know how that works, but the 20% estimate for slip or drag dates right back to the beginning of time when this very subject was being discussed, as it is now, but it is a variable, upwards from a total 100% slip when your model is in it’s restraint. 20% may be around your average model, I would think. At the same time a figure of 10 - 20% figure was decided on for the prop unloading in the air.

It seems to me that to get a true picture the speed has to be verified; I’m not sure how accurately I could estimate the speed of any model, really, just by watching it. I’ve always felt that in general they fly relatively slowly, from standing still heading into a wind, up to 60 - 70 mph. After that, I think the power has to be turned on a little bit more, or it’s a little more specialised model.

I think that I still think 150 mph will not be flown that easily, but I’m sure it’s not impossible……

PB

Edited By Peter Beeney on 17/12/2012 19:03:26

[Simon Chaddock]

I suspect the 20% prop unloading is also a guess.

The revs will rise with speed - it must. The problem is that the motor must be at is most powerful at the maximumat speed. This can mean for electric the motor will be over proped whan static and cannot be run at full power.

Exactly the same happens with IC but here engine cannot pull the revs so produces less power anyway almost to the point where take off becomes marginal hence varaible pitch props on full size that allow the engine to run a max power rpm at take off.

I think this engine speeded up a good bit more than 20% when it got going!

 

Edited By Simon Chaddock on 17/12/2012 19:35:29

[Erfolg]

The assumption that motors unload, that is the watts drawn reduces, is only just true. That is in some circumstances at least.

One of the club members has a Bubble Dancer, which uses an inrunner and gearbox with large folder.

It also had at one time some telemetry, one parameter was watts drawn.

He reported in passing that the watt reduction was only a few watts, nothing like the 10%, I had for one had imagined.

[Simon Chaddock]

On a slow flying plane the prop speed up and watt reduction will indeed be very small.

It is a speed related phenomenon hence in full size fixed pitch props were adequate up to 250mph although even then take off performance did suffer a bit.

In the video those planes were achieving 300kph (186 mph) and pull over 30,000rpm when at speed and have the equivalent of 700W/lb.

Speed needs power - lots of it!

 

Edited By Simon Chaddock on 17/12/2012 20:40:59

[Martin Harris - Moderator]

Posted by Simon Chaddock on 17/12/2012 19:34:06:

I suspect the 20% prop unloading is also a guess.

~

I think this engine speeded up a good bit more than 20% when it got going!

There's a bit more than just unloading going on there Simon - the tuned pipe achieving resonance has an awful lot more to do with the rpm increase!

[Biggles' Elder Brother - Moderator]

Steve, I agree with you completely on the matter of theoretical figures - one must indeed be very circumspect. But they do have their uses in that they define the absolute upper limit of what that set up could achieve in an ideal world. The plane might well not reach that - but one thing is for sure, it definately won't exceed it!

For example, if the target is, say, 120mph, then there is no point in even trying a 2200kV motor running a 5x5 from a 3s battery as even under the most perfect, ideal, theoretical conditions, it can't reach that speed!

Further, if we add in a slip estimate then things get even more interesting.

The allowance for the slip could crudely lumps together all the shortcomings of the system such as;

1. Voltage sag under load
2. The motor's inability to reach its full kV under load
3. the powertrain's inability to overcome the drag.

In purely aerodynamic terms how realistic is the 20% estimate for slip? Well I think its a bit better than just a guess. Prop's are not 100% efficient and there is always some slip - the lower limit on slip being somewhere around 10%. If the slip got to around 40% the blades would be likely to be at least partially stalled. Now that could well be the case at low speed for this set up - which is why I suggested a catapult launch. A bearly tolerable slip at max speed would be 30% - more than this and the model is just hopelessly over-proped. So the mid-way 20% figure is not so wild a guess.

So a good estimate for what you are likley to get overall, all being well, is probably a combined effective "slip" figure of around 30% - with 20% or so of that being down to aerodynamic effects and the additional 10% due to power train limitations such as voltage sag and motor effects.

BEB