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Jun 19, 2012, 01:23 PM
Bye Bye VP Aug 2010 - Aug 2012
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Discussion
Aerodynamic Arguments.

When people think they know what they're talking about, but actually, they don't.

http://www.rcgroups.com/forums/showt...&pp=25&page=45

Quote:
 Originally Posted by Gerry__ Wind - steady movement of an air mass - has no affect on aerodynamics. Turn rate remains the same regardless of wind.
Quote:
 Originally Posted by wjs Tell that to my firebird statos or my champ when it can't turn in the wind. I guess they missed class that day :-)
http://www.rcgroups.com/forums/showt...0#post21912847

http://www.rcgroups.com/forums/showt...0#post19565450

http://www.rcgroups.com/forums/showt...0#post17413895

http://www.rcgroups.com/forums/showt...0#post21989740

Quote:
 Originally Posted by ggunners For an example, assume 10 mph airplane stall speed and 5 mph wind speed. If 1/2 throttle provides 10mph in calm air and we're heading into the 5mph wind, the airplane is performing with 15mph "air" speed. When you turn 90degrees, your speed becomes 10mph "air" speed as the wind speed effect is zero on the "air" speed. When you turn downwind, your "air" speed becomes 5mph, 10 mph from the 1/2 throttle and -5 from the wind. In this case, you would need to advance the throttle enough to yield an additional +5mph to net 10mph air speed. You are correct in that the airplane only cares about "air" speed. It does not know or care about wind speed or ground speed, only the combined amount of "air" going over the wing. -- ggunners
Last edited by Gerry__; Jul 07, 2013 at 02:22 AM.
 Jul 23, 2012, 04:13 PM Bye Bye VP Aug 2010 - Aug 2012 United Kingdom, London Joined Jan 2005 6,053 Posts Weathervaning Here's another fab myth:http://www.rcgroups.com/forums/showt...0#post17934233 Last edited by Gerry__; Oct 31, 2012 at 12:25 PM.
Jul 26, 2012, 01:43 AM
Bye Bye VP Aug 2010 - Aug 2012
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A member explains basic aerodynamics and how, in his mind anyway, wind influences the best climb rate performance of the MPX TwinStar II

Quote:
Originally Posted by FozzyTheBear
Quote:
 Originally Posted by Gerry__ Hi Fozzy Really interested in how you think wind affects the climb rate of the TwinStar II. Thanks in advance Gerry
Hi Gerry,
Sorry we can't talk about this on forum but it's a bit a bit of a sore point for a lot of people. FlysLikeABeagle was actually technically right about a lot of things, the same as Jurgen was right about a lot of things. They were also both wrong about a lot of things as well.

It was just that it was unfortunate that they were both obstinate people and it got way too personal. Really what happened there, just turned everybody off to listening to either of them for a while.

I recognise your questions, because they're ones that a few years ago I had as well. I don't know how experienced you are, so I apologise if any of what I say is teaching grandma to suck eggs.

I'll try and and explain the strange behaviour of the Twinstar... It's actually common to a lot of aircraft, but people just don't realise it. The twinstar especially in the stock set-up does have some peculiarities that are not common though. I'll try to get to those later.

Yes... the airspeed over the wing is (mostly) the same flying upwind or downwind. I agree with you. The rate of climb however, is not. It's a bit of a misnomer to talk about rate of climb though... I'll try to explain that.

If this were a real aircraft, then the capable rate of climb would be the same no matter which way you were headding. However it isn't the same for a model.

First of all lets get rid of this term, rate of climb...Rate of climb defined as increase in altitude over time.

What we should really be talking about is altitude increase relative to ground speed. Because we as a pilot are static on the ground and not travelling with the aircraft. So consequently you will gain more height going into wind up the field length.... than you will going downwind down the field length. Because the airspeed over the wing is the same even though the distance travelled over the ground is different. I.E. you appear to travel faster going downwind even though the airspeed over the wing is effectively the same.

OK so that's the common to all aircraft stuff..... and it's what you'd expect to happen with the twinstar..... but.....

The Twinstar especially in the stock set-up has some peculiarities and these are partly why people consider it to be under powered. The props don't produce huge amounts of static thrust. Nor because of their small size do they generate much prop torque. They produce enough to fly it, but given any sort of wind conditions, steady or turbulent it does struggle.

Going downwind... you'd think that the increased effective groundspeed would actually counter the tailwind and give you effectively the same airspeed over the wing. Unfortunately, in the stock set-up this doesn't happen.

This is because it's very easy to end up with a condition where the plane can outfly the thrust being produced. You then end up with a near to propstall condition. Which, because the plane is such a floater, you don't really notice. It does however, drop the effective airspeed over the wing, even though it's flying faster. Hence not only do you have reduced climb per foot of ground speed, you also end up with reduced actual rate of climb per second as a result of the reduced airflow over the wing.

So the answer is that with the Twinstar, it behaves much more like a glider when it's going downwind and much more like a powered plane going upwind.

That's reason 1.....
Now on to reason 2.... The twinstar has nearly no washout on the wing. so higher rates of climb produce more wing turbulence and more drag... this is more noticeable at lower effective airspeed where the wing is closer to a stall condition. But again this only becomes noticeable (or at least more noticeable) when going downwind when the effective airspeed actually drops as a result. Hence again a lower climb rate.

OK now reason 3.... The wing is not a simple semi symmetrical section. it actually has a sort of turnup to the rear edge along the full length... it's more like a glider wing in that respect, but it's even more like the wing of the WWII rocket fighter the ME163. Which as you probably know, began life as glider technology, but proved perfect for very high speed flight. The twinstar's wing is a real floater... you'll discover this the first landing you do. It's actually very difficult to stall it. Hence quite difficult to get it to land. It wants to fly even in conditions where the theory says that it shouldn't. This is part of its downfall in terms of climb rate going downwind. Simply because it will keep flying in a near stalled condition even when you don't want it to. But because of that the climb rate suffers. That's why people get the sensation that it's under powered. And trust me, you will feel that when you have it in the air.

So the answer is fly it stock.... and feel these things for yourself mate! Honestly, experience tells me what I feel with the aircraft in the air. Which is why I fly mine brushless. It makes a hell of a difference to the behaviour it exhibits and makes it more stable in the turns as well as evening out the odd behaviour.

Hope that kind of clarrifies things a bit.

The CG change helps a bit as well....

[
Quote:
 Originally Posted by Gerry__ Reading your part one and two, it's clear that you think flying down wind adds a negative component to flight performance. It doesn't.
The only thing I can say to this Gerry is fly the Twinstar when you've got it, with the stock set-up and experience it for yourself. Like I said to you earlier this plane has some peculiarities that only show up on this plane.

The supposed laws of aerodynamics are mostly a pile of horses*&t anyway. As far as I know, there are five different theories covering why an aircraft actually flies, and none of them is perfect. That's to say that every single basic flight theory has a big "BUT" attached to it.

They kind of work and they kind of fit with what happens in the real world, but they're not perfect and when you get an aircraft that defies some of the supposed rules, everybody jumps up and down and tells you it's impossible.

I'll just remind you that in the 1890's some very serious scientists were jumping up and down telling everybody that it was impossible for a heavier than air machine to lift the weight of a man into the sky...... QED.

Go fly it in some wind and experience for yourself the lack of climb going downwind and the much higher rate going upwind. I look forward to you Then coming back to me and going "Oh yes it does do that" and if it doesn't feel underpowered to you when you're going downwind then I'll eat my Twinstar along with my hat.

Quote:
 Originally Posted by Gerry__ Why the subject of the TSII's flight performance should suddenly be banned from the thread in question is beyond me.
Because breaking the laws of aerodynamics seems to upset people.... LOL It's also not a good idea to turn up an a thread and start your question by telling everybody that somebody is full of s*+t!

Just enjoy your twinstar mate. I know it's a bit frustrating but they don't want to talk about the strange behaviour it has because it upsets their model of reality.

Quote:
 Originally Posted by Gerry__ There is no lack of climb going downwind. If you are perpetuating this myth through the TSII thread, then you are badly misleading people You are (or were, with the stock motors) clearly running out of room at you flying field when trying to climb out with the wind. Take off and climb out into wind instead. This stuff is just so basic.
Gerry I'm a relatively experienced pilot who has been flying since the 1970's.... I don't need telling to take off into wind thank you.

I've shown you nothing but courtesy and politeness, I've taken time out to answer your questions based on my actual experience of this aircraft. You on the other hand have never flown this aircraft and clearly seem to think that you're right about something that you have no experience of.

I'm now not sure if you're actually asking genuine questions or just turning up in the thread to intentionally cause trouble and insult people..... why you would do that I have no idea. Perhaps you're bored, perhaps you just like disrupting things... I don't know... and quite frankly I don't care either.
Last edited by Gerry__; Jul 06, 2013 at 01:24 AM.
Jul 28, 2012, 10:23 AM
Bye Bye VP Aug 2010 - Aug 2012
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Joined Jan 2005
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Member Rockin Robbins does a nice job of summing up the luddite attitude of some people: http://www.rcgroups.com/forums/showp...1&postcount=37

Quote:
 Originally Posted by Rockin Robbins The worst part is discovering that you can't communicate with these people because they are strictly hung up on their from the ground perspective and have no tools whatever to visualize that the plane is doing something completely different from what they are seeing. They make the mistake of believing their eyes, which are only reporting the truth. It is their brain which is lying to them, not integrating what the plane is doing with what their eyes report. Without a basic appreciation of aerodynamics, it is impossible to integrate the two frames of reference, from the ground and from the air, to fly the plane properly. It may not be any easier for a full size pilot to learn to fly an RC plane, but full size pilots have a body of knowledge absolutely necessary to fly RC WELL.
Latest blog entry: Foamie Resurrection
Jul 30, 2012, 05:34 AM
Bye Bye VP Aug 2010 - Aug 2012
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Joined Jan 2005
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Flite Test Confusion

In this video offering from Flite Test, Josh and Josh grapple with some basic aerodynamic principles, the only problem is the wind is there to confuse them. The results are hilarious as Josh (the cleverer one) proves himself wrong without realising it.

Notice that he makes several references to a reduction in lift when flying downwind. In spite of the evidence before his very eyes, he remains unable to let go of this flawed thinking:

 Flite Test - Anemometer - REVIEW (8 min 48 sec)
Latest blog entry: Foamie Resurrection
 Aug 25, 2012, 04:33 AM AMA 994002 United States, RI, Westerly Joined May 2012 1,179 Posts That's why I come to these forums. While I may have somewhat of a grasp on flying a model, I don't know squat about the physics of it other than real basic stuff. Half the fun for me is picking through what's fact and what's perceived as fact. But I am learning...every time I take my model(s) up I learn something. Then I come here and find out WHY it did what it did, and for that what can I say, but Thanks! Latest blog entry: Stability issue
 Jul 07, 2013, 02:20 AM Bye Bye VP Aug 2010 - Aug 2012 United Kingdom, London Joined Jan 2005 6,053 Posts Beginners get all upset about basic aerodynamic truths: http://www.rcgroups.com/forums/showt...&pp=25&page=45 Latest blog entry: Foamie Resurrection
Nov 21, 2013, 03:20 PM
Wallop!
Bussum, Netherlands
Joined Oct 2009
901 Posts
Here's another mistake, at 10:40:

Quote:
 Originally Posted by glassdogangle At 10:40 That's incorrect, guys. Your control surfaces are not any more sensitive flying into the wind than flying downwind. Sheesh! All these new pilots out there getting incorrect information...
Martin
 Nov 22, 2013, 11:08 AM Bye Bye VP Aug 2010 - Aug 2012 United Kingdom, London Joined Jan 2005 6,053 Posts Yep, that's the sort of thinking that gets you stuck in the down wind turn trap. Latest blog entry: Foamie Resurrection