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Author Topic: Why won't this thing glide?  (Read 3605 times)
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Crabby
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« Reply #25 on: August 01, 2017, 08:37:46 AM »

My advice, having built two of these is get back to the plan, build the stab like Headley shows it, and jack the wings leading edge up per plan. Stab at zero, wing looks like 3, maybe more. I am programmed to take orders. If Headley said to tape my mom to the wing, she'd be in trouble. It also occurred to me that there is an advantage to the rubber band mount of the wing, the incidence can be played with, I shimmed mine from time to time (under power). I used one loop of 1/4" tan.
« Last Edit: August 01, 2017, 09:40:14 AM by Crabby » Logged

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Hepcat
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« Reply #26 on: August 01, 2017, 11:31:59 AM »

It is interesting that such a simple query can bring forth so much useful and interesting discussion.  I still think the tailplane and the airflow around it are important aspects so I will add the following to be picked over.
A wing produces lift by directing air downwards, seen in the well known downwash behind a wing.  From the earliest days designers knew that a nice smooth outline to the wing allowed the downwash to flow away smoothly whereas things like ailerons and bits cut away to let the pilot get in and out caused vortices and did not give a smooth downwash.  This aeroplane must have had a fat pilot for the cut away is large and apparenty is immediately followed by two deep cockpits so I think the tailplane is flying in 'dirty' air, not in smooth air inclined slightly downwards.  Switching on the engine should cause the faster-moving slipstream to blend with the downwash air to the advantage of the tailplane.  Note: if there is downwash it is like having negative on the tail.

I think it is necessary to have a better picture of the tailplane section to understand what is happening.  If the section is typical of scale models, a flat bottom, a cambered top surface and the leading edge radius probably tangential to the bottom surface then the tailplane will have several degrees of positive incidence tending to make the model dive, and to dive even more if the speed increases because the camber will increase the 'nose down' pitching moment.
HepcatJohn.
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Crabby
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« Reply #27 on: August 01, 2017, 04:58:12 PM »

Ok I may as well release my confession right here. I am a prisoner to the plans and a strict adherer to the recipes of those who drew the plan. I am incapable of flying a blackboard therefore mathematical equations are nonsense to me. If Aunt Ruth gives me the recipe for her mincemeat pie I follow it to the "T" if it comes out tasting like "S" well she left something out! I built this FW 47d because I thought I could compete with David Aronstein at ETSU. He beat me of course he is a doctor of aerodynamics! I am a plumber of aerodynamics. My FW 47D glided like a dandelion seed! Why? I can only guess that Jack Headley was honest and forthcoming with his ingredients! He already built a wheel that rolled so why change anything? This is my confession. I have nothing to add to a great design, except strict adherence to the plan! One fly away, and one kept under close surveillance so it wouldn't depart! I am gonna rebuild the wing soon and let it go where it must!I am no Hepcat, so I have to follow recipes as written!
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« Reply #28 on: August 01, 2017, 08:58:14 PM »

It may be an idea bill to try some glides without the propeller (replaced with ballast) to try and identify the glide problem more accurately and separating the glide from any propeller or thrust effects.

Jon's (YAK52) points re high drag centre sound possible to me and adjusting for a good glide - by moving the Cg back first as you mentioned, should help to identify this.

If a CG position in the range Jon(YAK 52) mentions doesn't work with then it seems you may have problems with the tail as mentioned by hepcat John and myself or the wing (too thick) as Jon has also covered.

The propeller will change the settings slightly when refitted(depending on it's size) but this approach should get you close.

Good luck with it and we are all waiting for your results.

John
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Prosper
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« Reply #29 on: August 02, 2017, 02:48:33 AM »

The thing is, Crabby, the air doesn't identify aircraft as well as we do. If there's a Jack Blenkinsop design of a Mustang with a thin flat-bottomed wing and a flat tail enlarged 10% all round, and a Bill Bloggins design Mustang with a scale-ish wing section and an aerofoil-section tail enlarged 10% in area, the air  treats them as very different flying objects. Let alone whether their propellers differ, their weight distributions differ, and other variables. Then there's size -  ZK-AUD says that Headley's model is 52" span - the air sees that as a completely different flying contraption to Bill's 28" version, even if we 'umans choose to label them both Fw 47Ds. So following the recipe exactly is a wise thing, assuming a trusted designer, but the recipe may well not be transferable.

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Yak 52
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« Reply #30 on: August 02, 2017, 10:51:39 AM »

I like the baking analogy Crabby  Cheesy

Definitely stick to the recipe when you have a tasty result Grin

But I suppose if you want to scale up from a cup cake to wedding catering (or vice versa) you may have to tweak the proportions and oven times a little  Wink

Either way the trick is to steal the recipe when you enjoy the grub  Cool
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Crabby
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« Reply #31 on: August 02, 2017, 03:22:12 PM »

This thread has me worried sick. Tossing and turning! I am about to book a flight!
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« Reply #32 on: August 02, 2017, 06:28:35 PM »

Hey- not until you find out what recipe the aircraft builders followed  Cheesy

John
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Hepcat
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« Reply #33 on: August 03, 2017, 06:35:06 PM »

Bill,
I was browzing Darrol Stinton's mighty tome earlier today when my eye fell upon something that was nothing to do with my search but was so related to this thread that I thought I must quote it for everyone to enjoy.  It is very near the start of the book, in the children's section you might say, where he is explaining that the air circulates around a wing section, flowing backwards on the top surface and forwards on the bottom. He then goes on to say, and I quote:"A torque has been applied to the air in establishing the pressure distribution associated with circulation and, as Newton tells us that every action has an equal and opposite reaction, the air attempts to force the aerofoil surface to rotate in theopposite direction, nose-down. That reaction is called a pitching momentwhich is always present in some degree. It has to be resisted (balanced) by an equal and opposite nose-up torque from the stabilizer for trimmed straight and level flight (as anyone who has ever built a model aeroplane knows)".
hepcat John.
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« Reply #34 on: August 03, 2017, 07:35:09 PM »

Typical of Stinton's clear and accurate explanations - thanks for that John.

John
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billdennis747
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« Reply #35 on: September 27, 2017, 02:32:26 AM »

OK, I'm back on it. I brutally bent in some more wing incidence so the bottom is at 2 degrees, and hacked out the tail and put it back at -1.5 degrees and it glides.
Under power it goes very nicely in a straight line but any turn under half a mile diameter eventually winds in, which is tricky as it is meant for indoors. I shall fiddle about with ailerons.
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« Reply #36 on: March 11, 2018, 02:53:18 PM »

enjoying the  topic but seem not to understand what is meant by renoylds number, parasol e.t.c.
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J.I.Joshua
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« Reply #37 on: March 11, 2018, 03:14:52 PM »

Hello Joe
I don't know about Reynolds numbers except that they are big, but a parasol wing is one mounted on sticks above the fuselage.
Just to wrap it up; while it would not turn outdoors, it miraculously detected the walls at Nijmegen and stayed away from them until the end. I haven't touched it since but will have another go outdoors.
The solution was more decalage than I have ever seen.
https://www.youtube.com/watch?v=j-_7T2MyiKo
https://www.youtube.com/watch?v=5BSTu3ikFVM
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