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Author Topic: Dewoitine D.500 or D.510 plans?  (Read 2614 times)
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Pete Fardell
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« Reply #50 on: August 10, 2019, 11:07:08 AM »

I'd cut a test one out of 1/16 sheet with a linear 15% enlargement. The scale one may be ok with a lot of decalage (4-5 degrees) and a forward CG as the tail moment looks pretty good, but it does look a bit marginal
Cheers, Graham. I’m inclined to just make a bigger one and be done with.

What is the stab to wing area ratio?
As it stands, it’s about  1:7.5 by my calculation.
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Pete Fardell
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« Reply #51 on: August 10, 2019, 04:29:31 PM »

New tailplane made. Same chord, 15% wider in span as Graham suggested. I decided I want it to maximise its flying off the board chances and hopefully avoid a lengthy trimming fiasco, even if it takes a static marks hit.
Pics for comparison...
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Squirrelnet
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« Reply #52 on: August 10, 2019, 04:42:52 PM »

That looks good to my eye, I'm afraid I have no science to back that up.

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Pete Fardell
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« Reply #53 on: August 10, 2019, 04:57:38 PM »

Cheers Chris. Me neither, but I feel happier with it now!
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billdennis747
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« Reply #54 on: August 10, 2019, 05:01:28 PM »

Me neither, but I feel happier with it now!
That's all that matters. But I've only ever built one model that had an untrimmably small tailplane - a Halberstadt at about 5%.
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Pete Fardell
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« Reply #55 on: August 10, 2019, 05:09:31 PM »

Well I’ve still got the old tail, so if I find the trimming tediously over-easy perhaps I’ll swap it back!
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Graham Banham
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« Reply #56 on: August 10, 2019, 07:05:12 PM »

Looking good Pete: when i suggested +15% linear, i meant in all directions rather than just span to preserve the scale outline whilst bumping up the area as much as possible, but give it a whirl. Looking forward to seeing it in action!
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Pete Fardell
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« Reply #57 on: August 10, 2019, 07:49:17 PM »

Thanks Graham- I did wonder if that’s what you meant. The advantage of keeping the chordwise dimensions the same though is that it doesn’t mess with the proportions where it meets the fin, rudder and fuselage and so less fudging is required. (Anyway, I’m not making a third one yet!)
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DavidJP
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« Reply #58 on: August 11, 2019, 04:17:58 AM »

I rely on instinct too having no scientific sense.  I do think the way you have increased the area retains the character of the tail.  To increase the area all round might have made it look rather chubby and out of “scale”.   Still  envious of your stamina in building.
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Pete Fardell
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« Reply #59 on: August 13, 2019, 04:53:31 PM »

Thanks, but I think ‘indecisiveness’ is a better word than stamina for my building progress. I’m still half inclined to use the original tail and hope for the best.
Anyway, that decision can wait. I’ve sanded a noseblock more or less to shape and tissue covered the fuselage sides before gluing the wing into place. The plan now is to lock it in forever by building up the underside of the fus. with the big underslung radiator tunnel and a few stringers etc.
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Yak 52
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« Reply #60 on: August 14, 2019, 04:01:49 AM »

Hi Pete,

Ten minutes with Hepcat's excel calculator and your 3 view from the wikiwand page gave me a scale tail volume of 0.43. That's in the "small but should be doable" area. Less than 0.4 would be a concern. Your 15% span increase bumps that up to 0.49 - ie fairly safe. For comparison I would put 0.5-0.6 on a sport scale model.

The good news is you have a nice long tail moment in pitch (2.86 times the mean chord) so the model should be dynamically stable even with the small tail.

The tail moment for the fin is on the smaller side at roughly 37% of the span (less than 30% baaad, over 50% good  Smiley ) So coupled with that large fin area you might have spiral stability sensitivity with the scale dihedral.

Jon
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billdennis747
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« Reply #61 on: August 14, 2019, 04:29:02 AM »

How were this parameters arrived at, out of interest? How was it originally decided that being outside these figures would be bad news? It reminds me in a way of the Moulton 'Flying Scale Models' book - excellent in many ways but with some very conservative recommendations for dihedral, tail areas etc.
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Pete Fardell
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« Reply #62 on: August 14, 2019, 05:38:35 AM »

Thanks Jon; I was rather hoping you’d crunch some numbers for me. Much appreciated!
I started out with the Fillon plan and little regard for scale fidelity but then abandoned Fillon and got a bit carried away, so am still not really sure how accurate to make this one. I suppose I’m still aiming for an easy flyer which embodies the character of the original to a notch or two above the dreaded term, ‘sport scale’! Dihedral is certainly not scale though. In line with the Fillon peanut plan I’ve got about 8 degrees (measured on the underside) as opposed to only around 3 on the scale drawing. This gets the wing tips comfortably above the thrust line. To my eye it doesn’t look too horrible; the wings are long, so the slope still appears gentle.

(How’s the ankle by the way?)
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Yak 52
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« Reply #63 on: August 14, 2019, 06:48:13 AM »

How were this parameters arrived at, out of interest?

Analysis of a hell of a lot of models, mine as well as other people's and time spent comparing real world performance (my own experience as well as reported problems or successes of others) to the physical parameters. All made infinitely quicker by Hepcat's excellent calculator. As well as a lot of time learning about aerodynamics and attempting to understand the complex interactions and derivatives that result as you change aspects of a design.


How was it originally decided that being outside these figures would be bad news?

There's no arbitrary line where good becomes bad. It's a spectrum from playing it safe and having confidence to pushing the limits and accepting something may be difficult to trim. The numbers allied to experience give you an indication of where you sit on that spectrum. Conservative or not is up to you: understanding where you are on that spectrum is the goal.

It just helps me to have the concrete measurement of a parameter, then go fly it and see how it compares and develop an understanding of what the numbers mean in practice. It's a method that allows you to learn an awful lot from each model.



Dihedral is certainly not scale though. In line with the Fillon peanut plan I’ve got about 8 degrees (measured on the underside) as opposed to only around 3 on the scale drawing. This gets the wing tips comfortably above the thrust line.

Ugggh Lips sealed Grin that's one rule of thumb I really hate as the dihedral has virtually nothing to do with the thrustline  Cool It's the large fin area and shortish tail that's more of an issue, a floppy rudder might be advantageous. As well as the biggest prop it can handle. 8 degrees should be safe enough (this is just TLAR as the analysis of fin and spiral stability is rather more involved.)


(How’s the ankle by the way?)

Good thank you  Smiley I'm hobbling about and hope to be able to fly (retrieve) again soon!
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billdennis747
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« Reply #64 on: August 14, 2019, 07:24:39 AM »

Thanks Yak. Is all this analysis mainly on duration models for performance, or scale models for flyability? As I said earlier, of hundreds of scale models, the only untrimmable one because of small tail areas was one with about 5%
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Pete Fardell
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« Reply #65 on: August 14, 2019, 07:26:12 AM »

Dihedral is certainly not scale though. In line with the Fillon peanut plan I’ve got about 8 degrees (measured on the underside) as opposed to only around 3 on the scale drawing. This gets the wing tips comfortably above the thrust line.
Ugggh Lips sealed Grin that's one rule of thumb I really hate as the dihedral has virtually nothing to do with the thrustline  Cool It's the large fin area and shortish tail that's more of an issue, a floppy rudder might be advantageous. As well as the biggest prop it can handle. 8 degrees should be safe enough (this is just TLAR as the analysis of fin and spiral stability is rather more involved.)
Sorry! In my defence, it’s a ‘rule’ you hear a lot and is quite attractive to people with lazy brains like mine. I think I imagine it a bit like the difference between rowing a boat sitting down and  trying to do it standing up. I now accept that that is a very flawed analogy though and will cross if off my rules of thumb list pronto!
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Yak 52
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« Reply #66 on: August 14, 2019, 09:37:57 AM »

Is all this analysis mainly on duration models for performance, or scale models for flyability?

A bit of everything. Tail volume is a measurement* of a physical parameter just like wingspan or wingloading. It's understanding what it means to your model and how you want to fly it that's important. And also being aware of other contributory factors such as for example a big prop reducing tail effectiveness, or a T tail being more efficient etc etc.

So:
an RC glider would have about 0.4 for nice precise control with lowest drag.
an RC trainer would be about 0.6 for easy flying
a rubber duration model could have as much as 1.4 to control pitching moments from undercambered wings and a big torquey power burst.
a CLG needs at least 1.0 for stability with very low decalage at both high and low speed.

For scale flyability I'd be looking for 0.5 to feel comfy, 0.4 as a sensible minimum, 0.3 possible but tricky, 0.2 get your coat.

This is not set in stone however and I'm always interested in people's experience with unusual models to add to the data set  Smiley

Squirrelnet's Wilga was an interesting example - the tail didn't look big but when compared to the high aspect wing it had a whopping 0.8 or something. Meaning that it was stable with the CG further back than expected from a TLAR approach.

I think people get the idea that the aerodynamic approach I take is a naysaying one - in fact it's quite the opposite: I got into this stuff to learn what is possible and to understand where the difficulties might be in order to make my models more scale not less. My Dart Kitten had a perfectly adequate scale tail volume and flew fine (it had vicious tip stall but that's another issue!) Conventional wisdom would have had me enlarging it unnecessarily. Hopefully Pete can get his scale tailed D500 going too Smiley That Halberstadt D.1 tail is teeny though! It would be interesting to work out the actual volume...


Pete, it's ok - it's not laziness Smiley and I wouldn't bother running the numbers if I thought you didn't appreciate it. 'As high as the canopy rail' is another one. But it would be easier and just as accurate to say 'give it a healthy wadge of dihedral'. It's perfectly legit as a visual guide but aerodynamics it ain't! Smiley



(* There are several inaccurate methods of calculating it out there but Hepcat's calculator is by far the best and simple to use.)
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billdennis747
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« Reply #67 on: August 14, 2019, 10:01:30 AM »

Why do Ivan Taylor's jets need such huge 'decalage'? Look at his Lightning in curent Aeromodeller?
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Pete Fardell
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« Reply #68 on: August 14, 2019, 10:03:33 AM »

I wouldn't bother running the numbers if I thought you didn't appreciate it.
Thanks again, Jon; I certainly do appreciate it! Although I don't have a very high understanding of the numbers, the fact that I'm building something that you're saying should be theoretically reasonably easy to trim makes a world of difference to my motivation.
Also, your constant efforts do have a drip feed effect. I often calculate wing loading these days (you have to for the comp forms anyway) but now I might give the tail volume formula a go too at some point. Before you know it I might even be considering... dare I say it... a torque meter. (Not today though!  Cheesy)
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Yak 52
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« Reply #69 on: August 14, 2019, 10:13:28 AM »

Whoa steady on old chap!
If you are not careful I'll be filling your inbox with excel files...
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Indoorflyer
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« Reply #70 on: August 14, 2019, 11:10:33 AM »

One could argue that *all* of these methods are "inaccurate", but certain assumptions made in the various methods/formulae just provide a consistent approach to determining relative numbers. Wing area "inside" the fuselage typically is included in many calculations, even tho' fuselage width can vary considerably. Even Aspect Ratio calcs include fuse width. Whether to use LE to LE distance as tail moment arm,  or other station to station measurements comes out in the wash, since they end up being relatively insignificant differences in ratios.  On our little airplanes it could be as little as the difference between the centerline of a spar and a face of the spar.  On a model with a  4"chord wing, "calculating" a center of gravity as 32.5% or 33% ends up being a difference of only .020"!
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Yak 52
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« Reply #71 on: August 14, 2019, 05:56:42 PM »

IndoorFlyer, I agree in principle but there is a fine point to be made: none are accurate in the sense of precision so all methods result in approximation (which is fine if you are consistent) but some of the methods espoused are methodically inaccurate -ie measuring the wrong thing in the first place. These methodical errors can accumulate large differences in results when applied to unconventional models.

... Whether to use LE to LE distance as tail moment arm,  or other station to station measurements comes out in the wash, since they end up being relatively insignificant differences in ratios...

This is a good example. The 'accurate' method is to use CG to tailplane aerodynamic centre (25% of the mean aerodynamic chord of the tailplane.) This is physically the tail moment arm, the aerodynamic force of the tail acting at it's aero centre, levering on the centre of rotation (CG) of the model in flight. Using the LE to LE method is a rough guess at the same but it doesn't measure the actual mechanism. And so it makes for some pretty inaccurate results on a large volume rubber duration model where the CG is back at 80% wing chord.

Does it all matter? Probably not. But Hepcat's calculator is methodically accurate even though it incorporates some approximation, for example using a two panel wing. You just punch in the wing and tail dimensions fortunately so the days of me working out tail aero centres are over. What matters most is that 1. You understand whats going on. 2. You measure the right thing (at least approximately  Grin)

Sorry for the thread drift. Boring myself now  Roll Eyes
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DHnut
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« Reply #72 on: August 14, 2019, 06:18:21 PM »

Jon,
      Well done for putting some consistancy on the method of assessing tail volume and hence the need for any change in area. The Aeromodeller Moth Minor has both increased fin area and dihedral that when returned to scale seem to work fine because of the long tail moment. We did the CG calculation using the McComb's equation on Stan's Vildebeast and came out with a 38% position for similar reasons to the Wilga. A little calculation can save a lot of grief. Jon can you send me the calculation spreadsheet by PM.
Ricky
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Pete Fardell
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« Reply #73 on: August 17, 2019, 06:30:08 PM »

A bit more progress... Made the radiator and glued it on, and then built up the underside of the rear fuselage with stringers. Before that I’d also fitted the forwardmost uc legs. (There are two other uc legs to go on each side to make tripods, but only these front legs will actually take any real strain on mine.)
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DavidJP
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« Reply #74 on: August 18, 2019, 05:14:04 AM »

Hmmm...... that looks good! 
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