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Member postings for Richard Sharman

Here is a list of all the postings Richard Sharman has made in our forums. Click on a thread name to jump to the thread.

Thread: Beaulieu Flying
22/04/2014 11:21:06

New news: Beaulieu Permits (required for flying at Beaulieu Airfield, New Forest, Hampshire) are now available via the Southampton Model club's website at www.southamptonmac.org.uk. There is full information there on what the site is like, the rules and regulations for flying and availability. You can get a personal permit online there for £5. The Beaulieu Model Flying Committee (BMFA approved) manages the site under licence from the Forestry Commission.

Richard Sharman, chairman BMFC

Thread: Ducted fan theory and practice
21/08/2012 21:49:07

Now, here's different kettle of fish (which I don't think Mark has seen yet):

The e-flite Habu. This is a very sophisticated piece of kit (mostly of fibreglass, wood, etc No foam in sight). Notice the ultra smooth finish of the exterior (the duct interiors are similarly beautifully finished). The duct entry lips are nicely smoothed, the ducts are nice and straight and not obscured in any way. The ducting tapers nicely as per theory. The model has retracts, flaps, full functions etc.

Main dimensions are: span 1030mm, wing area 25.3 sq dm, AUW 3.55 Kg, wing loading 137 g/sq dm (=45 oz/sq ft)

Operating figures: 6s 4000mAh lipo at 70Amps gives around 1600 Watts with Delta-V 32 80mm fan and 2150Kv DF32 brushless motor.

How does it fly? Amazing! Good take-off, very fast in flight, awesome in the air, very precise control behaviour, but quite difficult to land, even with flaps. So, despite a wing loading which would kill some models, and power that is not out of this world, the result is very satisfying: near turbine performance at a fraction of the cost. I think EDF has finally arrived.

21/08/2012 21:16:38

Posted by Erfolg

With Richards background in Computational Fluid dynamics, I am a little surprised that he has not knocked up a programme which would permit the examination of various duct and fan arrangements. It certainly would be an interesting project, with his skills and toolbox.

Posted by Mark Powell 2 on 21/08/2012 17:23:12:

CFD/Richard/Knock up. Didn't know that. I was employed for many years at the same place as him, and he taught Computet Science at the local Tech, presumably as a 'sideline' to which I went.

Knock up a program? Maybe he wants to keep his work separate from his pleasure, or maybe he is holding back, just interjecting to put us straight now and again. (the Mark Powell 2 unauthorised biography) His planes fly well.

Yes, it's all true, I do have a Ph.d in CFD, and I did teach Comp.Sci as a sideline, and Mark was one of my students, and I do keep work separate from pleasure, and I am holding back. Tony K, John Olsen, Mark and Erfolg are doing a splendid job of keeping the discussion going, so please continue -- it's all very interesting, and just a little entertaining at times!

On a serious point, "knocking up" a serious program to make useful deductions about flows in ducts is just a little more work than it might seem, even if you have all the right toolkits available. And you still need serious hardware to do it properly. It's all been done in academia, high-tech start-ups, and the aerospace industry, so it's not worth even contemplating the effort here. Maybe someone can borrow time on a system somewhere ?

21/08/2012 20:58:11
Posted by Tony K on 20/08/2012 11:52:42:
Posted by Richard Sharman on 18/08/2012 12:45:27:

The question is: why does it fly so well ?

The basic parameters must be understood; wing loading, aspect ratio, thrust line, tail moment, control surface size, etc.

Are there other properties which should be considered, eg. polar moment of inertia, or is it just the inter-reaction of the basics?

I don't know what the real answer is, it just seems to be a happy co-incidence of power, weight, balance and control. I've known quite a few models that seemed similar on the face of it, but were disappointing. In this case the designer seems to have got it the relationships just about right.

Just in case anyone is thinking of rushing out and getting one, I would add one item of caution: the swing mechanism leaves a lot to be desired, and I did spend quite a bit of time improving it.

21/08/2012 20:51:09
Posted by Swissflyer on 21/08/2012 11:47:31:
Richard, please can you give me the wing area of your GR4 Tornado in sq dm, then we can do some calculations for it.

The wing dimensions of the Tornado are different according to the variable geometry wing setting:

Extended: span 810mm, area 9.925sq dm, wing loading 94g/sq dm (=30oz/sq ft)

Closed: span 620mm, area 8.835sq dm, wing loading 105g/sq dm (=35 oz/sq ft)

The lighter wing loading clearly helps on take-off and landing, the heavier wing loading corresponds to higher flight speeds with less responsive controls. What else can you tell from this?

R

21/08/2012 20:30:38
Posted by Simon Chaddock on 19/08/2012 16:27:53:

"Considering its only foam......

This was just a (semi-) humerous jibe at modern construction methods ! Of course foam is a great material, in the right place at the right time. But it has been experience of recent fixed wing planes at our club that the smaller, cheaper, foamier, ready-builts tend to be less satisfying, less robust, and more expendable than the larger, conventionally built ARTF's especially from the more up-market companies. Perhaps you get what you pay for ? So it was a (pleasant) surprise that the Tornado was successful when it could have been, well, disappointing.

18/08/2012 12:45:27

Andy,

Glad you like the Tornado. The figures I quoted are MY measurements on MY tornado just before takeoff. It should have said 930g (=approx 2 lbs) but I made a typo, sorry. Therefore Watts/lb in excess of 200 -- not bad ?

I am using an Overlander 2200mAh 60C 4s Lipo which is quite heavy for its size, but does withstand the 33 Amp current draw which is sustained. I have used the supplied servos, wheels, fan, motor, esc etc with a Spectrum 6200 Rx.

The question is: why does it fly so well ? Considering it's only foam, has standard components, poor aerofoil section, and lot's of other technical deficiencies, (presumably to keep the cost down) it shouldn't be as good as it is. But it takes off quickly, flies smoothly in up to 20mph winds, and lands predictably.

17/08/2012 21:56:02

It's all Richard Sharman's faultsmiley Seeing him flyinh his little ones... So, it's all MY fault !!??? Well, I'm delighted that people are still discussing this topic! Long may it continue.

The plane I was flying that Mark saw when we met on the flying field recently was this one:

which is the Max-Thrust GR4 Tornado (with working swing wing). I got this going for fun, and to try out the swing wing concept before developing a larger example. It flies extremely well considering it's only a lump of foam - very fast, takes off easily, not too hard to land, etc. The swung back wing does allow the model to speed up a little, but makes the controls rather less responsive. The swung forward wing is used for take-off and landing with some success.

On the other hand, there is a lot wrong with the model - the intake lip is rectangular, not smooth; the ducting generally is rough; the swing mechanism obscures the intake duct; the outlet duct bifurcates poorly, etc. The basic geometry of intake size to fan size to exhaust size is good though.

Basic figures for the model are: 450 Watts, all up weight 390g (=2 lbs) so watts/lb is good.

Richard

PS. "circulation theory" for lift on a wing in my book. But, a recent Channel 4 program on the Colditz glider escape story repeated the "bernoulli theory" with very elaborate CGI animation diagrams - what a pity!

Thread: The Hawker P1121 - again!
18/12/2011 12:43:19
Nice work Simon, looking forward to see the finished model !
 
On the power reduction question: yes, the longer duct will have an effect, but so will the degree to which the exit narrows. Both probably having an effect here. 12oz for a 16oz model should be fine, although more is better, obviously. You may be losing some power in those thin wires ? You could try a more powerful motor ? you could try a higher C-rated battery ?
Thread: Ducted fan theory and practice
28/11/2011 19:22:02
"high lift device" is just jargon for flaps, really. Here is the picture:
Note the wing camber and thickness, the cleanness of the wing, the smoothness of the aileron/flap, and the closeness of the hinge gap. This wing has much better lift/drag ratio and the result is that the take-off roll is much reduced, stall recovery is better, and landing speed reduced. The same ducted fan/motor/battery setup now seems more powerful but I don't have any proper flight performance measurements yet.
 
As we've all observed before, one of the key questions is "what fan/motor/battery" do I need for a specific model ?" Of course we are vitally interested in what makes a good assembly but it's a subject we don't have a lot of control over since few are prepared (or able) to make their own fans. Obviously, there should be an odd number of fan blades, blades as stiff as possible, fan/duct clearance small, duct true, convenient fixing points that don't distort the fan, good static deflectors, etc. Often hard to tell good from bad, though.
 
For testing and selection we are still in the dark ages: pick a fan, do a static test, if it pulls the right amps and there seems to be a reasonable draught at the back then put it in the plane and give it a go. But it's not good enough, really, we need to do better.
 
I have a number of different fans(wemotec, kyosho, e-flite, lander, etc) with a range of test results but they are not easily comparable (tests were different). I never heard anyone grumble about a wemotec with a Mega motor (although I do have grumbles about the some of the others). I haven't tested a Schubeler fan yet but much is claimed about them.
 

 
Thread: The Hawker P1121 - again!
28/11/2011 12:50:07
Arrrrghh...! I've had the same problem - you need a fairly substantial grade of motor wire to cope with a serious current flow, but the length needed causes a weight penalty.

One possible suggestion: place the ESC as close to the motor as you get. Not only is this better from an electrical/interference point of view, but it means you only have 2 wires along the fuselage to the Rx (the DC circuit) and not 3 wires (the AC motor circuit). This might save a third of the wire weight ? And you might even be able to put the ESC in the fan exhaust for cooling ?and dispense with the heatsink ?

Edited By Richard Sharman on 28/11/2011 12:51:17

Thread: Ducted fan theory and practice
27/11/2011 17:40:51
(I've been away, and busy rebuilding the Hawk with a new improved wing incorporating high lift devices which had its first flight yesterday, so I haven't followed this thread for a while. Anyway, here are some thoughts
 
"Drag" is a very complicated subject, but it's not true to say that most drag comes from the "frontal area". (subsonic) Drag comes from a variety of sources (friction, profile, interference, parasitic, induced, etc) according to plane geometry. But the largest component is usually lift-related drag from the wing. Changing a nose for an inlet won't get rid of drag. If you doubt any of this you need to read a textbook on aerodynamics. Mixing all this up with ducted fan performance is probably not very helpful.
 
"thrust and velocity" is another complicated subject, and for most fullsize aircraft propulsion systems thrust is a function of (airplane) velocity (and a lot of other things, like altitude, but that doesn't concern us here). In the case of a propeller or a turbofan, thrust will generally decrease with increasing speed, whereas with a turbojet thrust can increase with velocity. If you doubt this you need to read a textbook on propulsion. EDF is bit like propellers, and its a bit like a high by-pass ratio turbofan, that's why its interesting.
 
The "static vs dynamic" debate seems to have caused heat as well as light ! If you calculate thrust from merely converting power in a tube to mass flow then you are not modelling an aircraft in flight. You are probably modelling a vacuum cleaner. This may be similar to the static case, but it doesn't tell us much about flight. For this we need to have some component of the (mathematical) model which is related to the aircraft's lift/drag ratio. Scharnhorst does this (simply) by using the (steady) aircraft velocity. Where he is deficient is in the assumptions about (motor, fan, duct) efficiency, but otherwise it's helpful. Formulations which ignore flight forces are probably not going to be useful (if they were we'd all be using them). If you doubt any of this you probably need to read a text book on aircraft performance and design.
Thread: The Hawker P1121 - again!
27/11/2011 16:39:56
You're very brave to contemplate cutting the fuselage ! But what must be, must be ? The area rule should cause the fuselage to have 2 wide stations - I assume you mean the rearward one after the wing trailing edge? Of course, the purpose of the area rule is to maintain a constant total cross section, but you're not going transonic (i hope!).
 
I'm a bit worried about your ESC -- it's obviously ok at the moment, but could vibration in flight cause the contact again, with possible loss of control, if only for moment ? I think I'd be inclined to try a different type of ESC, or possibly glue the heatsink on rather than tie it on. Just a thought.

Edited By Richard Sharman on 27/11/2011 16:40:30

Thread: Ducted fan theory and practice
04/11/2011 18:17:16
Keith, on the data graph, I only posted it to show that it's hard to get the data we would like. Actually, I don't call an average height of 200ft "flying close to the ground" ! but let that pass. The exercise was an attempt to get some idea of flying speed in the straight and level case. In full-size practice this is easy to achieve, but for models it's rather harder. The plane was at full power all the time, but I don't know what the wattage was (no watts logging). It was doing a horizontal figure-of-8 pattern, and what we find is that it is climbing(and slowing down) and descending (and speeding up).
04/11/2011 17:59:04
Pete,
thanks for the photos -- I am currently putting bulkheads / spacers in to hold the inside duct wall in position. I'm a bit worried about adding extra weight, I've got enough of that already!
R
03/11/2011 22:12:16
Unfortunately, no measurement is simple, however it might seem.
 
1. Power - this requires a watt meter of some kind, the one I use on the bench gives instantaneous volts, amps, watts, but does not log values. It requires a visual inspection for a limited duration, with an estimation of an average - not always easy when the voltage is varying under load. In the air I have no watt meter, or even voltage for the power circuit. I have Rx volts logged in the EagleTree, but since that is regulated it is not very interesting.
 
2. Efflux velocity - On the bench I measure this with a pitot tube connected to the logger - I have instantaneous readout and logging. Yes, there is a clear velocity gradient across the duct, and some variation due to turbulence no doubt, but the readings are probably not far off truth. In the air we have no readings at all.
 
3. Static thrust. Open fan measurements are made by a device invented by Timbo, see
http://www.modelflying.co.uk/forums/postings.asp?th=11870&p=1 . (The picture of my equivalent device seems to have disappeared in the intervening years). Whole model thrust is tested by the "vertical model" method described in the "Fan Testing" thread. Timbo always stresses that this sort of testing is "relative" rather than "absolute" but I am less worried - it's probably good enough for our purposes. For dynamic thrust in the air we have no measurements at all.
 
Here is an example of why some values are hard to estimate:
Part of the log from a recent flight of the Hawk gives airspeed and altitude as follows:

The plane appeared from the ground to be flying pretty level and at a pretty constant speed (any reasonable observer would have said so). But according to the log both varied considerably (inversely, in this case - when the plane climbs it loses speed, and vice versa, of course). Do we believe the averages? what is the speed at straight and level flight ? It's certainly not straightforward.
02/11/2011 18:43:05
What sort of theory are we looking for ?
 
I thought I would just pen a few musings for the benefit of those with nothing to but read forums, so while I'm not flying (it's raining) and waiting for glue to set (on the Hawk replacement wing to new design), here goes.....
 
There are many properties that a good theory should have, but the primary ones I'm looking for in a "theory of ducted fan propulsion" are:
 
1. It should be based on a sound derivation from plausible assumptions. This means that we can not throw a dice, pluck a formula off the internet, whatever. These are not "plausible", even if they right. The assumptions have to be plausible with respect to what we know about physics, aerodynamics, fluid flow, and so on. Of course we may make erroneous assumptions, but these will be corrected eventually. When we derive something from the assumptions we have to do it by logic and algebra. Sometimes we can't derive anything -- it's too difficult, or too contentious, so we have to wait until it gets sorted out.
 
2. It should make it possible to calculate useful properties which relate to our task. This means that if the theory predicts how many cups of tea we will drink between flights, it may be very amusing, but it won't be very useful. It has to make it possible to calculate, say, the speed the plane should fly for a given number of watts input, or, say, the duration of the flight I will get for a particular plane with a given motor and fan. Sometimes we can't do a calculation because we don't know some important number, and so we have to make "guesses", "approximations", whatever. They may be wrong even though the basic theory is right.
 
3. It should make predictions which agree with observations. This means that if it predicts the flying speed of the plane for a given power input AND I actually get to fly the plane AND measure the speed it flies at, then there should be some measure of agreement between the two. We're not looking for perfect matches to 10 places of decimals. Even a rough order of magnitude would be useful, and agreement of +/-10% is probably fine for model flying. If we were designing the next Airbus I would want to do better than that.
 
Why is this useful ? Only because it gives us some criteria we can use when thinking about a proposed theory. There are two cases which are relevant here:
 
Case A. The theory meets criteria (1) and (2) but not (3). This is the case with a lot of what we have discussed in this thread. We have to try to understand what is wrong - the theory, or the data ?
 
Case B. The theory doesn't meet (1), or possibly (2), but it does meet (3). This is the realm of "the rule of thumb" -- it works, but no one knows why. The rule "use 150 watts per pound of model" falls into this category. It's useful, it works (at least sometimes) but you can't explain it. Sooner or later a model comes along for which it doesn't work, and we say "bother, it didn't work - it always worked in the past". So we use another "rule" and say "up the power a bit more". But will this work? More power means a bigger motor and battery, which means more weight, which means higher wing loading, which means we need a higher flying speed,.....
 
Working towards a theory is therefore a "good thing".

 
So, we need a theory !

Edited By Richard Sharman on 02/11/2011 20:26:40

01/11/2011 21:55:41
I take your point, Dizz, but it's a bit of a council of despair..? Admittedly we have some dodgy measurements, and some theories which don't quite add up, but that only means there's work to do. Even if we had the Farnborough wind tunnel, and all the equipment at Cranwell, Warton, whatever, we'd STILL need some theory !
 
I'll look forward to some data from the FD2 someday - your picture looks reminiscent of a H Hunter fuselage, interestingly. What factor will you allow for those triangular shaped inlet ducts? Not simply the gross intake area I should think ?
 
In dismantling the Hawk, I discovered that the inner face of the inlet duct (analogous to the inner flat bit before the ducts join in your picture) has become rather soft (it was quite stiff when built, but being hidden cannot normally be inspected). As a result it could either swell or collapse when in use -- could be a cause of low efficiency ? What's the cure?
Thread: The Hawker P1121 - again!
01/11/2011 21:34:09
Simon, great progress, looking good !
 
Question: will there be a gap between the elevon and the fuselage? or is there some other way that the movement of the elevon won't be restricted by contact with the (fatter) fuselage at the point of the spar ?
 
I only mention this because in the analagous position on my Hawk the all moving elevator was restricted in movement because of unplanned contact, actually with the pushrod. Fortunately it only restricted the extreme of down elevator(stick forward) so was not too onerous.
Thread: Ducted fan theory and practice
30/10/2011 16:17:21
A quick look at the test figures....
 
Yes, this is the problem -- the calculated values and the measured values don't seem to agree ?
 
I have implemented Tony K's method in an Xcel spreadsheet (like I did for the Scharnhorst theory) and so can do the calculations in full precision without any rounding. (By the way, when I did try to follow the rounding shown in the 28/10 posting I tripped over the fact that the area ratio is rounded (1.4888 -> 1.5) in the derivation, but used un-rounded in the calculation of Ve. There is some other rounding and truncation which I tried to emulate but couldn't quite get it right as I don't know the precision with which all the various calculations are made. Anyway, ignoring this issue as "noise" the answers come out close, but not exactly agreeing. Using the full value of density=1.2252 also effects the result slightly. Actually, this is lesson in the use of arithmetic which computer users are well aware of !).
 
For reference, I get vi=47.13 ve=70.16 mdot=0.2202 and T=5.07 for the example given.

So, in test1 using the measured values for input watts (368W) and duct geometry (as before), the spreadsheet version of the theory predicts vi=47.92 ve=71.34 mdot=0.2239kg/s and T=5.25N which is consistent with the reference example above.
 
BUT, while measured T in this case is 6.1N which is similar, but some way off, 5.25N, the measured ve is 47 which is a long way off predicted 71.34.
 
ON THE OTHER HAND, if we look (easy to do with the spreadsheet, iteratively) for the predicted power input which would produce the measured ve=47, and its corresponding predicted T=2.14N (see previous post) then we find that the input watts should only be 96W, not the 368W actually observed.
 
Something is wrong, and I don't know what it is. I suspect the measured values are inaccurate, but the discrepancy is rather large. Maybe there are other considerations ?
 
Richard
PS real testing has come to a halt as the Hawk was damaged on landing yesterday. After a blistering series of flights it happened that on one landing a wingtip was clipped causing the plane to cartwheel on the runway. I will be fully occupied repairing for a bit. For Hawk enthusiasts: this model has an wingtip airfoil section with very low thickness ratio, leading to a tendency to easily tip stall, but keeping the speed up leads to overfast landings - beware ! I have just been punished with an expensive lesson.
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