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Twin Wing Centre of Cravity and Centre of Pressure

8 years 2 months ago #1535 by AlanThomas
If I wanted to go bush I would be considering a gyrocopter. Ever since the Q2s came out I have wanted one. I like everything about them. However in New Zealand we have a genuine problem in as much as all of the airports suitable for private users have short runways that are mostly just over 1000 feet. I trust that you can now see why I am seeking more confidence by achieving a more reliable slower speed.

Lex I quite understand your viewpoint which is a viewpoint I would probably share if I lived in the "land of the free and home of the brave"but because I live in "God's own" I need to look towards "tall tales and true from the legendary past" and the word of the original test pilot who I'm sure was flying the "sh1t" out of the Q2. Yes I'm confident that levitating at 39 kn is reasonable given the ground effect but to achieve anything below 50 kn and certainly down to 42 kn is miraculous. I have seen videos of them doing it just above 45 kn and even then only briefly.

Most microlights (UltraLite) in New Zealand would have a manufacturing cost of around $200,000 or more. This is really the only section where you are allowed to do your own maintenance which goes a long way towards keeping the cost of running manageable. We don't seem to have very many of those old-fashioned "powered kites" these days. They don't seem to go very far and are kept in restricted areas. Our microlights of the more regular one or two seater type of aircraft, must achieve 45 kn in landing configuration, can weigh up to 600 kg and you can fly as fast as you dam well like "faster than a speeding bullet is ok" so long as you don't use rocket power or turbojet et cetera.

Lex we certainly have different environments and I would have to say that we do not have the luxury of runways that are longer than you can actually see. To us a short runway would be something less than 500 feet which a top dressing plane would operate from. New Zealand pioneered top dressing and even if you have zero interest in getting in and down some of these difficult to land places I'm sure we could give you something to think about. My mate has just won the river running competition (bush plane) and he can land and take off on one of your runways sideways without needing to turn around. Another mate, an old guy, (older than me) who is overseeing my flight training and reclassification was also probably a bit of a barnstormer want the same time doing certifications for the CAA.

Over these coming years we are going to see a new generation of the Q series that other electric and with wings that sprouts stuff to maintain the high-end speed while landing at even less than 40 kn at an operating cost of about a dollar an hour. Yes engineering has many things to contribute that was not available 30 years ago while still maintaining the owner operator cost effectiveness while achieving fantastic performance. Even though the rule of thumb is fast planes don't fly slow we must be mindful of jetliners that do fly fast and by necessity must be able to fly slow enough to land which is why we see them sprout all kinds of engineering magic. To achieve these changes by necessity the centre of pressure has been pushed to the forefront of the thinking is the impact on a twin wing plane is more noticeable with any changes made.

Personally I think the Q2 is a perfect baseline to advance experimental aviation. Having the capacity for slowness, while maintaining quickness, if nothing else is a fantastic safety feature when you must put your aircraft down in an untraditional location such as a highway.

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8 years 2 months ago #1538 by SteveQ
Hi Lex, Hi Alan,
I designed a few canards in school and am finally building my Q-2. I am building it as a tail dragger, with most of the recommended modifications. I like the T-tail more than reflexors, but again this is the first one I'm building on my own. That being said, I don't expect it to perform in any way other than advertised.

Getting back to Alan's original post. The center of pressure in a conventional aircraft is typically near the center of lift. Having one main wing, that puts it close to the center of lift of only one wing, with the horizontal stabilizer compensating for the difference. The center of lift can be changed with flap settings or angle of incidence.

The same variables apply to a tandem wing, they just act on two different surfaces. Rather than a stabilizer to compensate, you vary the lift on the canard using the elevators. They do act as a big set of flaps. As a matter of design, if the tandem wings are close together, with no other control surface to compensate, the center of gravity falls within some very narrow limits. You have to treat each wing as a single entity, then average the two to get the centers for the whole airframe. In the Q-2 (and all Quickie types) if it's constructed properly and you and your passenger are sitting in the right place, the designed control surfaces will compensate for the Center of Gravity within those limits.

As for changes in design or handling methods... I live in Arizona near a huge desert. Payson's airport is on top of a Mesa. I'd love to take off from a 5000 foot runway 5127 feet above sea level at 105 degrees F, but with nothing but forest at both ends, I'm not going to risk it. I intend to get to know the limits of my aircraft in a step by step systematic way. The Q-2 is neither a short field bird nor a Mach 3 jet. If you want to explore the low end of the performance envelope, do so at a high enough altitude to have room for recovery.

A former Navy pilot in the hangar next door looked at the Q-2 wings this morning and commented on how tiny they are compared with a Cesna 150. Hmmm... High wing loading... Those wings are in their element at 120 Knots indicated, not 42. Any changes you make to get you down that slow will certainly come with a price tag elsewhere in the performance envelope.

As I build my Q-bird, I'm also working on flight proficiency. You have a huge advantage living near sea level. I learned to fly at the Canterbury Aero Club, and am more familiar with the South Island. If you want to get into fields like West Melton near Christchurch with 1800 feet of grass, rent a Cherokee or a C-150.

There's my two cents.

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8 years 2 months ago #1539 by AlanThomas
Thanks Lex for your contributions into this interesting subject addressing the expiration of the design in the lower speed range. obviously a very good pilot seeks to fly in such a way as to not go close to the extremes either too slow or too fast. However having said that I'm trying to explore technology of the absolute limits of what the design will achieve.

Take it from the top of the design I think it is fair to say that the canard is the area of most interest when reaching the lower speed in as much as there is a predictability of the stall speed in relation to the angle of attack. The original design was to achieve a fallback stick approach which has the effect of flaps (collaborators?)giving maximum lift of the canard and let the speed reduce until a landing is achieved.

This parameter has of course been changed with the introduction of the T-Tail which effectively has the capacity of changing the angle of attack. The introduction of a reflex arrangement for the main wing provides the effect of flaps (flapper-ailerons?)both of which provide additional lift which introduces new numbers into the calculations concerning the weight of the plane which in turn has an effect on the slowest possible speed prior to anything stalling.

Of Course these various gizmos not only add to the pressure but provide the ability to manipulate the centre of pressure and provide a greater degree of control over and above any conventional plane. as we are the Q2 type enthusiasts it follows that we want to explore these "advantages" to the fullest degree.

I am however having difficulty in establishing any reliable design information addressing these issues of centre of gravity and the centre of wing pressure which is a prerequisite to expanding upon the original design even further than the already addition of t-Tail and reflexor arrangements.

So far most of the comments have been centred around the Q2 style is designed to go fast. Granted that economic speed may have been the forefront in the minds eye of the designer I think we can also say the same of any fast plane and the faster they go the more difficulty they have slowing down to land.

The driving force for those who would like these planes to fly slower surrounds the nature of the landing strips available. Funnily enough this plane is still at West Melton near Christchurch which I hope to take delivery of the coming weeks and bring it up to Auckland. The previous owners seem to have had no difficulty flying in and out of West Melton and it does seem that the same is true of the other Q2s around that area. In fact most airstrips would be comparatively short by your standards and certainly less than 1800 feet in all but exceptional airstrips which even then would be much less than 3000 feet.

The problem seems to lie in increasing wing pressure by increasing either wingspan or the cord or both. Going back to the high-speed planes and the original spirit of these twin wings which is to fly fast while not forgetting the necessity to land we would not really want to change too much to the wings themselves as that will alter the high-speed characteristics which means we should do what high-speed planes do and that is design and built wings that change the shape. Another member suggested Fowler flaps.

Fowler flaps, which is a doable solution, seems to be the way to go.

I doubt very much of anyone has incorporated fowler flaps into a Q2 but of course it has the added complexity of maintaining their control service function particularly in the main wing. As a mechanical design engineer I've already thought about this and I don't think it is a very difficult problem.

The outstanding question of course is back to the actual calculations of wing area, angle of the flaps and suchlike in order to make such a project worthwhile.

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