|Jul 21, 2009, 08:28 AM|
Joined Mar 2008
Wing area and elevators on swing-wing jets
I'm trying to decide on a pusher prop jet to design and build (balsa). I'd like to pick something with a decent wing area to help make it a good park flyer with around 10-12 oz wing loading.
It seems that some of the delta wing types like Mirages, Draken etc have very large wing areas and many of the swing-wing types like F-111, Tornado have relatively small wing areas.
However when the wings on these planes are fully swept (aft) you could sort-of consider the wings and elevators to be one large delta design then suddenly you have plenty of wing area.
Is it incorrect to count the elevator on a swing-wing into the total area?
|Jul 22, 2009, 11:37 PM|
You Know, that's a good question !! I've wondered that for years myself !
How about it Steve or Benie ? Any thoughts and what about C.G.?
Blue Skies................R Dub
|Jul 23, 2009, 07:59 AM|
Joined Mar 2008
And maybe I can extend my question a little further too... I found a very interesting link:
which talks about angle of attack and lift etc which makes a lot of sense to me. Based on this I reckon that elevator area could be considered part of the "wing area" as the surface is surely adding to lift when at a positive angle of attack.
Then I guess things like strakes (LEXs) and very flat fuselages should also add lift? So a plane like F-111 with big strakes and a nice wide flat fuse probably ends up having a really big "lift" area.
|Jul 25, 2009, 09:39 AM|
lift force and CG position
this is more academic discussion. I feel we're not scientists, but more engineers. To undrestand all this things we should go back to university for 3 years.
We have big advantage the real planes are flying, so we just copy them, although they are designed for totally different speeds.
The principle of the swing wing on the real plane is that for lower speeds it simulates "sailplane" (big wing span, low profile depth, good manoeuvrability) and for higher and supersonic speeds it simulates "delta wing" (high profile depth, lower wing span).
So You're right suppose it'll creates a delta, but the main reason is the lower aerodynamic resistance. But the delta on plane without elevator is causing problems with manoeuvrability at the lower speeds.
The wing swing planes don't need so big wing area, cause when wing is fully front, the wing has big relative thickness (ratio between the thickness of the wing and the wing profile depth) and produces sufficient lift force with smaller wing. Sailplanes has relative small wing area, but big wing span - high thickness and they can hold bigger wing loadings.
CG position on the real plane is always in front of the total lift force position for stability reasons.
Generally works following:
Lift force position for wing only is approximately in 1/4 of wing chord (depth of profile) for our speeds.
Of course the elevator - depends on actual position and the fuselage - depends on shape, produces also positive or negative lift force.
Let's suppose the straight flight only for design and forget the fuselage.
The "wing area" is whole area of wing including the part "hidden" in the fuselage (compensation for fuselage - also produces lift).
Wing loading is a ratio between the weight and the "wing area".
You should not count the elevator, cause elevator itself produces positive or negative lift force depends if You go up or down.
On the wing swing model you need to compensate the changing lift force position using elevator UP when wing going aft. So totally You produce a negative force forcing tail down - front up and all model weight is hold only by the main wing-fuselage lift force. This is experience I did with F-14. When I change wing position fully aft, the model speed incerases but it's not so stable and very small deflections causing big moves.
If You want to find CG of the park jet, start to put it 1/2 inch in front of the theoretical wing lift force position (1/4 of the profile depth from the leading edge).
With my L-159 I started exactly at the wing lift force position and I had to go closer to the front.
I think it's not only about wing loading, but also depends on the wing profile thickness, depth and wing span.
Thicker and small depth profiles can have higher wing loading. Thin and deep profiles (like delta) needs lower wing loading. So if You want, You can count the elevator, but it will not help.
Theese were my experiences and may be something is not fully correct according to the "aviation terms", but may be this explanation of the mechanical engineer is more simple to understand for those are not experts.
I kindly ask experts to be forgiving.
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