SE5a - another approach - Page 3

Greyhead · 11-01-2008

Linney

I have found an article in a magazine about a 71" span electric powered Tiger Moth, not as “draggy” as an SE5a but it might be useful as a starting point, it has details of the motor / battery / prop. etc.

If you like send me a PM with your email address and I’ll scan it and send you a copy.

Grahame

LINNEY · 20-01-2008

Wonwinglo, you are absolutely correct when you talk of research being needed but researching electric motor makers, users etc is not going to give me the fundamental information I seek - although Greyhead's recent offer of Moth details may do. What I want to know is how to relate an aircraft's data - weight, wing loading, take-off air-speed required, drag effect (always an educated guess and I am not so educated) etc and from all that work out the size prop required running at what rpm in order to handle such an aircraft. The motive power is not important, white mice on a tread-mill would be fine if they could turn such a prop at the required speed - or a steam engine perhaps?
Anyway, let's see whay Greyhead comes up with.
Incidentally, pseudonym WONWINGLO. Any special significance that you care to share? When I was a kid the term "coming in one wing low" meant an approach with either the pilot or the aircraft in trouble and liable to crash - usually returning from combat. A shout of "One wing low chaps" meant all rush out and be prepared to help for whatever reason - certainly not neccessarily one wing low, this was a general term for trouble. How it came about I know not - but my Father certainly applied it to my models!
Many thanks for your interest.

LINNEY · 22-01-2008

I have now studied the article, discovered and sent to me by Greyhead for which many thanks, and it is very interesting. The author of the article was seeking information to help make his model electric and he quoted data comparing electric power sources for a 71" span Hawker Fury bi-plane weighing 9.5lbs - sensible because it is almost identical the to author's own model. The author also said that he knew that his aircraft would fly well with a 4-stroke 90 glo engine - so presumably would the Fury.

I then went back to various "expert" opinions. One says that the cubic inch capacity of an i/c engine multiplied by 1000 equals the watts required of an equivalent electric motor so a 90 engine can be replaced by a 900 watt motor - apparently. But another "expert" says that an electric motor should yield 100 watts of power per lb of weight so the Fury needs 950 watts. Very similar. But another "expert" says you only need that much power for aerobatics. For calm steady and impressively controlled demonstration flying you only need 80 watts per lb, i.e. 760 watts in this instance.

So what about the Fury comparisons? Well, the author quoted comparisons with 12 different motors but different props too! Why different props? Baffled me, the aircraft needs a certain diameter prop to "propel" it down the runway and with a pitch and prop speed (hence mph speed) sufficient to give it lift-off - if you want to actually fly.

Anyway, the 12 different comparisons were interesting. I will not quote them all, just the extremes. In all instances they were run at a (theoretical) airspeed of about 50 mph, calculated from pitch and engine rpm. For anyone not usually interested, speed in mph is very roughly prop pitch in inches multiplied by rpm divided by 1000, hence an 8" pitch at turning at 7,000 rpm would give an airspeed of 8x7 = 56mph. More accurately such quick figures are about 5% high, not really significant since all this is theoretical, actual flying air speed will be affected by "prop slippage", a function of drag and prop design.

So, to the author's figures. Well, one motor turned a 24x12 prop at
about 4,200rpm with a 2:1 gearing. Thus the engine was turning at about 8,400 rpmand apparently using 45 amps at 36 volts, some 1,600 watts! Bit high by "expert" suggestions. But at the other end another useful motor turned a 17x8 prop at about 6,200rpm with a 3.7:1 gearing and hence a motor speed of about 23,000rpm! And power? Apparently 36 volts consumed 38 amps, hence about 1,350 watts. Still higher than expert predictions - but so what? (no pun intended) Am I seriously being told that if I need a 24" prop to drag an aircraft down a runway I can alternatively do it with a 17" prop if I rotate it 50% faster? By that reasoning put 12" diameter props on a Lancaster bomber and run them at a few million rpm and the aircraft will move down the runway. Really?

And another Fury test showed a direct drive Outrunner motor (which I intend) turning a 20x11 prop at about 4,700rpm used 36volts at 40 amps = about 1,400 watts. Sounds interesting, but the Fury weighs only 9.5lbs against my (anticipated) 15lbs and unfortunately has no quoted figures for wing loading - well, none that I can find. And currently I am replacinga 60 4-stroke on my 71" 8lbs Cub with a 13x8 prop at 7,000rpm with a direct drive outrunner using 800 watts! Very close to the middle of the "expert" figures (by chance!). I wish I could read the current consumption at minimum actual flying speed but such kit seems a bit too expensive for me.
This brings me back to my constant point. An aircraft, model or otherwise, will need a certain size of prop to drag it along, depending upon its effective weight, and a certain airspeed to enable it to lift off, this latter determined by wing-loading and drag.

I repeat (boring, no doubt) if someone can tell me what size prop is needed by a "draggy" biplane weighing 15lbs and what airspeed is needed to enable lift-off for the same aircraft with possibly a 24oz/sqft wing loading then I can work out pitch/rpm and find a power supply to match.

Failing that I shall await with eager anticipation Greyhead's actual data when he starts flying his i/c SE5a. THAT is the real way to find out.

tigertc · 22-01-2008

"Failing that I shall await with eager anticipation Greyhead's actual data when he starts flying his i/c SE5a. THAT is the real way to find out."

^^This seems to be the most sensible a safest approach.

LINNEY · 29-01-2008

Greyhead, we are all waiting!

Greyhead · 29-01-2008

You’ll have to wait quite a while longer before I can supply any flying data for my SE5a, warmer weather is definitely required before I venture out to the flying field; I’m a fair weather flyer at the best of times, I want to enjoy my hobby not suffer for it. In addition for a maiden flight, which is basically to sort out any problems with the model, I want a nice steady breeze, I can’t see any reason to add to the stress by having to contend with anything other than ideal weather conditions!

johnsimmance · 08-09-2008

Quote:

Originally Posted by LINNEY

I was not too happy about starting another SE5a thread because Graham (Greyhead) is doing such a fine job but he and others said a different viewpoint would be welcome - so....
Actually, I was encouraged because apparently Graham and I come from two different directions. Clearly, he is a craftsman and a very committed one whilst I certainly am not, I am more of a technician and our differences in approach might, just might, be of interest. It is doubtful if there is a simple right-or-wrong, just differences to which there will be different reactions.

Firstly, what is "scratch build"? Some appear to think it is making a model from a kit as distinct from putting the finishing touches to an ARTF. Others say it is starting from a plain piece of paper, drawing up your own plans, buying all materials and getting on with it. I think slightly differently because this aircraft was "scratch built" by the Royal Aircraft Factory from plans supplied by the designer, H.P.Folland, in 1916 so my first job was to find plans. I settled on the 71" span John Simmance plans bought from the U.S. MODEL AIRPLANE NEWS and was provided with 6 sheets each about 5' x 3' !
Actually, 3 of these massive sheets give very excellent full-size plan and, where necessary, elevation details of all the wood and metal work required - and I discovered that there were to be nearly 400 wooden parts from over 120 different designs! Well, if I could look ahead five years I might manage, but even then not to the quality standard required so I went to Leon Cole of Belair Digital and asked if he could laser-cut them for me. No problem, and he has one big advantage, he can cut thick ply so there would be no need for laminating the most substantial parts. The metal parts I am prepared to take on.

The next thing was to study the plans - for very many hours, they are very complex - and work out a production sequence. There are of course no construction details, you work it out for yourself, and I would hate to find that I have built up a section but then need to get back inside it. Actually I always dry assemble everything, with Sellotape, until construction is well under way.

Then the next problem - no point in starting to build unless you know what engine you plan to fit because the engine housing will need to be modified according to that decision. O.K. so what engine? Well, as I have said elsewhere, what prop? Possibly 18lbs of aircraft (I tend to build overweight to benefit strength) with lots of drag - biplane with struts, fixed undercarriage, etc. And what air speed must be achieved to permit healthy take-off? No help from anyone, I cannot find anyone who works from the prop requirements backwards - which to me I am afraid is the only logical way to go. The best I could get was "use a 120 4-stroke and whatever prop goes with that". O.K., then the RCV 120SP seems possible, it apparently has the power to pull that weight and overcome that drag given a 20" prop and the big advantage is that being a radially-mounted "rotating cylinder" engine it fits totally in-cowl. But it rotates at half "normal" revs and has a recommended maximum of 5,000rpm. Now, RCV suggest a 12" pitch which at max. revs would give about 55 mph theoretically. I have no idea what it might be in practice or if that is adequate - does anyone please? The other point is that a 20" prop would have about 2" ground clearance in level flight and since this aircraft is said to have a tendency to tip on its nose (plenty of WW1 photos to prove it) it might be better to use, say, a 15x12 4-bladed prop. Such props were common in actual service. Any views please?

The next approach was to the actual plans. Firstly they are too massive for casual "whilst-working" reference so I had the local photo shop produce quarter-size reproductions for me and I can keep these by the work bench. The next, very common, situation is that the plans show the port side of the fuselage elevation and the port wings plan view and so of course it is necessary to make "mirror images". So I had the same photo shop print out the construction plans 1:1 on transparent material so that when building starboard I only have to turn the plans over. Easy. And of course I cover them with a self-adhesive clear film to which cyano and epoxy do not adhere.

And, for now, the final consideration - and some views would be appreciated. When making a scale model is one expected to reproduce an active service model, complete with wrinkled covering, distorted spars, patches, rust, oil, dirt etc or should one, as I believe, attempt to produce the designer's dream? I asked the Shuttleworth chaps and their view was certainly as mine. They considered that if you reproduce all the blemishes it will be considered to be just an excuse for poor workmanship. So I propose to try to build what Folland had in mind in 1916 and any blemishes will be my failings and not deliberate. But that is a long time into the future.

I wil add to this from time-to-time as matters progress but for now I await, in hope, for some helpful reactions please!

Hi - If you need any help or comments you can email me. When it comes to flying, the only thing you need to be aware of is that adverse aileron yaw (like in the original aircraft) is a problem, si either use aileron differential (non-scale) or be prepared to use rudder as well as aileron and elevator! johnsimmance.

vnkiwi · 08-09-2008

Will be following this thread, and Greyheads, with much interest. Linneys, because his approach to modelling is very similar to my own, the difference being that I also do my own drawings from my research. And Greyheads because there is a master who pays attention to the detail and I will try to emulate his level of skill within my own models. One day the pfaltz D3 at 1/10th scale. For now am into boats, namely the very early river boats from the colonial explotation in the period from 1850 to 1900. and as said before in this thread, so little information has survived, and we can only try and complete the jigsaw with the pieces which remain. I approach this challange from the technical engineering viewpoint, as that is what they where. Technical excersises in filling a need for a particular vessel or craft, be it water, air or land bourne. Thats my approach anyway. I do recognise and appreciate the artistic skills of all those fellow modellers who's approach is different to mine, and learn from them all every day. Excellent site this, with so much experience and talent. Thank you all.