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Author Topic: A Question of Incidence  (Read 406 times)
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Rhys
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« on: May 03, 2020, 05:21:33 PM »

Cheers all Grin

While reading through a few of the many posts here, I see mention of attention to wing and decalage incidence. Plus 1°, 2°, 3° or minus those amounts. I'm relatively new to freeflight but am becoming ever keen to realise the importance of proper construction, airfoil, balance, et al to achieve hands off flight. How do we account for the advantage of such minute changes compared to the original craft and miniature replica? The air molecules are the same size but the surface greatly diminished. Can they react the same? Is there a compensating formula for this? Perhaps this is a situ relegated more to Reynolds numbers than angle of entry.

Just curious.

Rhys
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piecost
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« Reply #1 on: May 03, 2020, 07:04:09 PM »

Rhys,

You hit the nail on the head with mentioning Reynold's number. This is the best means of identifying the impact of scale on aerodyamic performance. It is not a straightforward subject. If you are interested then do a search for xfoil on this and other forums. This tool enables the analysis of aerofoil sections. You will find that poeple always run a number of Reynold's numbers as the impact is large. On scale models very thick wings tend not to work well on smaller models and so the aerofoil may be reduced in thickess from the original.

Also, if you scaled down a full sized plane to model scale to be a free flight scale model you might find that it would lack the level of static stability to fly without a pilot. The centre of gravity might be too far aft. The tailplane might need enlarging; this and a more forwrd CG would increase static stability to be large enough for stable free flight. The wing dihedral might need increasing or the fin might need changing in size. If we move the CG forward then the angle between the wing and tail will need increasing relative to the full sized plane.

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Rhys
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« Reply #2 on: May 03, 2020, 07:28:17 PM »

piecost-

Thanks immensely for your response. Also very appreciative of pointing me toward xfoil. I used to dabble a bit with Reynolds numbers when scratch building r/c but it seemed to burden me a bit. Time to look into it again. So is incidence merely to get the aircraft heading into the right direction or is it that instrumental in airflow with our small copies?

Thanks again!
Rhys
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charlieman
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« Reply #3 on: May 04, 2020, 03:32:31 PM »

I see tend to perceive things a bit differently. Grin It's ok to disagree with my understanding etc. Just saying, up front
 
A purposeful shape, passing thru the atmosphere in stable flight, can be described as a mechanical and aerodynamic assemblage, said total to be operating in "balance" at a given airspeed. The size of the balance matters not, as long as the basic proportions, moments and relative forces remain ""in scale.  I think scale incidences provide the ideal place to begin trimming one's scale model of whatever.  After all , a model P-51, in order t fly successfully as a FF entity, HAS to balance within the same range as full size article, pilot or no.

So, gotta ask: do the effects of Reynold's numbers, working on  scale sized surfaces automatically adversely affect/upset the balance tremendously, as the size of the model is reduced?

We tend to think /speak in specifics "IF we do this , we get that". However, we actually apply them in generalities, and those  allow for much latitude, successes, and enjoyment.


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piecost
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« Reply #4 on: May 04, 2020, 06:28:45 PM »

I believe that the fine balance will be lost as different characteristics vary as we reduce the Reynold's number.

Another thought experiment; what if we took a full scale P51 and fixed the stick and rudder bar and flew it as a free flight model. Could we trim it to fly in stable circles? It is unlikely as the stability of this fighter will be insufficent for free flight. However, a BE2c WW1 biplane was known for its extreme stability and I think would work as a full sized free flight model.
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Rhys
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« Reply #5 on: May 05, 2020, 12:08:36 AM »

Good point!

Rhys
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dputt7
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« Reply #6 on: May 05, 2020, 02:52:04 AM »



Another thought experiment; what if we took a full scale P51 and fixed the stick and rudder bar and flew it as a free flight model. Could we trim it to fly in stable circles? It is unlikely as the stability of this fighter will be insufficent for free flight. However, a BE2c WW1 biplane was known for its extreme stability and I think would work as a full sized free flight model.
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dputt7
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« Reply #7 on: May 05, 2020, 03:02:31 AM »

I believe that the fine balance will be lost as different characteristics vary as we reduce the Reynold's number.

Another thought experiment; what if we took a full scale P51 and fixed the stick and rudder bar and flew it as a free flight model. Could we trim it to fly in stable circles? It is unlikely as the stability of this fighter will be insufficent for free flight. However, a BE2c WW1 biplane was known for its extreme stability and I think would work as a full sized free flight model.
        There are numerous accounts of WW1 recce aircraft landing with a dead crew.

My poor brain is tiered but I also remember an aircraft taking off with no pilot after failing to safely "chock" the wheels while hand starting and it circled Port Phillip bay. The RAAF then scrambled 2 fighters who kept an eye on it until it ran out of fuel. 
     I'm sure some one will know all the details, Paul ?
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RolandD6
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« Reply #8 on: May 05, 2020, 04:03:39 AM »

I believe that the fine balance will be lost as different characteristics vary as we reduce the Reynold's number.

Another thought experiment; what if we took a full scale P51 and fixed the stick and rudder bar and flew it as a free flight model. Could we trim it to fly in stable circles? It is unlikely as the stability of this fighter will be insufficent for free flight. However, a BE2c WW1 biplane was known for its extreme stability and I think would work as a full sized free flight model.
        There are numerous accounts of WW1 recce aircraft landing with a dead crew.

My poor brain is tiered but I also remember an aircraft taking off with no pilot after failing to safely "chock" the wheels while hand starting and it circled Port Phillip bay. The RAAF then scrambled 2 fighters who kept an eye on it until it ran out of fuel. 
     I'm sure some one will know all the details, Paul ?

No Dave, I do not know anything about that incident. It is probably documented somewhere in the RAAF Museum library but I have not been there for about 10 years. Gave up the volunteer work shortly before that. Too far for me to drive to now.

I have read about an RE8 flying with a dead crew until it ran out of fuel. Don’t remember if it landed or crashed.

Paul
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dputt7
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« Reply #9 on: May 05, 2020, 07:03:42 AM »

   Ok, so most of my facts were wrong.   I rang my brother, who being older and wiser related the facts to me.  After this I was able to find a description of the event that tied in with Trev's version.
 
On the morning of August 30, 1955 Anthony Thrower, the pilot of a rented Auster J/4 Archer light aircraft was starting his plane by swinging the propellor at Sydney's Bankstown airport.
 
Suddenly the aircraft coughed to life and began to roll, gaining speed as it took off down the runway with no one at the controls.
 
Soon the pilotless Auster was climbing high over Sydney's north western suburbs, tailed by an RAN A/5G which took off after it.

Enter the defenders of Australia.
 
First an old Wirraway fighter-trainer was scrambled from RAAF Richmond , west of Sydney, approaching the rogue aircraft at 10,000 feet only to find his guns had frozen and jammed.
 
Scratch one RAAF. Next a Gloster Meteor F.8 jet fighter turned up from RAAF Williamtown, flown by Squadron Leader M. Holdsworth, moustache bristling, but guns also jammed. Holdsworth diverted to Sydney airport for fuel. Scatach two RAAF.
 
A second Gloster Meteor, flown by Squadron Leader J.H. Flemming arrived, and was just about to line up on the runaway when two RAN 805 Squadron Sea Furies swept through into the attack.
 
Fury VW645 flown by Lt John. Bluett and WZ650 flown by Lt Peter McNay - both Royal Navy officers on exchange duty with the RAN - had been engaged in gunnery practice over Nowra - when they picked up the radio chatter of the pilotless aircaft over Sydney.
 
The rest is history. Both putting a machine gun burst into the Auster the Navy pilots sent it splashing into the sea three miles off Broken Bay at 11.43 a.m. that day.
 
It had flown itself over the suburbs and shoreline for two hours.
 
Deeply embarrassed, the RAAF later explained that its fighter squadrons at RAAF Williamtown, were stood down when the incident happened.
 
 Sorry to divert you thread, back to you.
 
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piecost
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« Reply #10 on: May 05, 2020, 12:02:10 PM »

Great storey. Thanks for telling it.
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TimWescott
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« Reply #11 on: May 05, 2020, 12:07:14 PM »

Hey Rhys:

I'm not sure what all you picked up, but if you can get your hands on a copy of "Circular Airflow" by Frank Ziac, do so.  It's from the 1930's, but the air still moves the same way now that it did back then.

In short, yes, part of the reason for needing to adjust tail size on our little toy planes is because they're little.

But another part is that a typical full-scale airplane is designed to be very stable in pitch, and it's designed to have someone in the cockpit fiddling with the trim at need.  Many full-scale airplanes are stable for hands-off flight (like that Auster, which is basically a Taylorcraft with a strong British accent), most of the ones that aren't are dead stable in pitch but slowly diverge in roll and yaw (Google "spiral instability").

That nifty pitch stability comes at a cost, though -- it happens by trimming the surfaces for a loop, then putting the CG forward enough so that at sane flight speeds the loop doesn't happen.  If you give the airplane a lot more power than it's trimmed for -- a loop happens.

So for free flight, where you want strong climbs and gentle glides, you need to step away from correct practice for big airplanes.  You need to put the CG back, so you can trim for less "loop".  Doing this with "normal" sized tail feathers make the trim setting super critical -- so you need to make the tail feathers bigger, and if you can, you make the wing-tail distance larger.  At some point, you hit a happy medium where the tail incidence is still critical, but not quite so bad as if you just made a perfect scale model of that Auster.

This is why super-duper competition free flight airplanes look so "weird" -- the CG is way far back, the tail is huge, and the fuselage is long and skinny.  It's most pronounced when there's a screaming-fast engine on the thing, less so when there's rubber power involved, and even less for scale or some pleasant Sunday morning sport model where you can adjust the engine for a long, mild run.
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charlieman
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« Reply #12 on: May 05, 2020, 01:05:46 PM »

Good stories! and sort of proves the point. FF Full Scale is not unknown, and clearly not " impossible", IF the conditions are right. Sort of like flying FF models isn't it?  Shocked

BTW, when working at Dawn Patrol Aviation (1990-2009), I remember being rather shocked to find that most full scale conventional aircraft have their balance point located closer to 25% of M.A.C.  I believe same holds true for multi winged aircraft, but with a bit more math locating that pesky M.A.C., if manufacturer's balance data not available.

Quite a few full scale have noticeable decalage, Usually seen in a negative stab angle. Others may surprise , such as P-51, which remained constant +2 deg at stab, for all variants until mid production P-51D and late production P-51C, which saw that lowered to +1/2 deg.  Interestingly, with approx. 2.5 deg incidence at wing, results in  avery reasonable 2 deg decalage. I'm of the opinion that would be would be a very good place to start trimming a model p-51.  Earlier P-51B's and C's, which were also retro fitted with the infamous Dorsal Fin Fillet mod, also had their stab incidence lowered to +1/2 deg. From my experiences, related below, I suspect even early Mustang decalage is workable in model form.

How many designers/modelers have decreased scale Fokker Dr1, D VI, D VII or  D VIII stab incidences needlessly? Typically the common wisdom says these simply "won't work", and at a minimum, these "must be brought to Zero, or even slightly negative" (see latest Fokker plans in Builder's gallery). I've experimented with small balsa gliders and stable flight is very practical with thses scale settings and surface areas. There were no surprizes with  trimming and or balance, etc. I needlessly worried that the glider might "tuck under" during higher speed launches. I hope to be trying them (with scale airfoils) on a IC powered Fokker model, soon.

I like frsank Ziac's knowledge, He knew what was what and still is going on. However, I don't think he was overly dogmatic about what one HAS to do.  For me, it's the wiggle room that'smost interesting about SCALE FF matters and options.
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