Decalage angle

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poormansairforce

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Read these three posts from the VP turtledeck options thread:
In your stability calculations, have you accounted for the downwash from the canopy impinging on the flat top of the aft fuselage boom? It will give you a pitch up moment you need to accommodate. A 'standard' stability analysis will miss this. Notice how the PIK has a turtledeck that runs all the way back to the tail. One of the things that does is reduce the pitching moment I just described. The wider the cockpit, the worse this pitching moment becomes.
Drag on the fixed landing gear also tends to add a pitch down effect with increasing speed, so I'll be interested to see if that canopy downwash has any noticeable effect in practice.
Yes, it does. I changed my plane from a flat-topped aft boom like the one shown to a traditional turtledeck because of the nose up pitching moment. I didn't see it in several stability hand calcs. I didn't see it until I ran viscous CFD.
This boom pitching moment was greater than the pitching moment from the landing gear drag.
This is what your fighting except inverted.
 

Doggzilla

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He's just saying your landing gear drag is fighting your high center of thrust and it's causing a nose down.

This happens with aircraft like the "Lake LA-200" as well, especially with gear down.
 

Doggzilla

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I wonder if anyone has tried a gull wing to balance the drag. A gull wing projecting above the engine would be very stable and even out the drag significantly.
 

Eugene

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Personally I agree with my russian friends, who believe that engine without cowling producing only 10% of total drag. Maybe because I am russian as well. And I don't agree with my designer and Peter Garrison, that my engine is 50% of total drag.

So, 90% of the drag on Skyboy is hanging below engine and producing pitching moment. Only one thing that counter balancing this moment is undersized horizontal tail with Vh=0.34. This tail to generate enough force needs to have aggressive AOA = 6-8° in level flight. This tail have drag that is position below engine as well and pithing moment going up again.

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This large moment is additional load to the wing and I see 4° AOA in level flight to the bottom of the wing. On NACA 4412 airfoil this angle corresponding to 6° AOA measuring to the chord line. So, wing is big drag generator in level flight with 6° AOA and this drag is below engine as well!!!

All of this making this airplane in to perfect flying drag bucket with glide ratio 7:1.

So, good thing, is that on this airplane every time drag is reduced = pithing moment reduced = Tail AOA can be reduced = wing will get in to smaller AOA with less drag again.

So, All I need to do is simply reduce drag! That is all!!! How hard can that be???.....................................................

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poormansairforce

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So, good thing, is that on this airplane every time drag is reduced = pithing moment reduced = Tail AOA can be reduced = wing will get in to smaller AOA with less drag again.
+1
Build yourself a manometer and tape the tube to the center of bottom of the fuse with the end just beyond the apex of the upward curve and see how much the pressure drop is at that location. Try it at several locations.
 

BJC

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+1
Build yourself a manometer and tape the tube to the center of bottom of the fuse with the end just beyond the apex of the upward curve and see how much the pressure drop is at that location. Try it at several locations.
No, it just measuring the pressure drop at that location. But don't point it into the airflow.
pmaf:

Pressure drop between which two points. Assuming that you mean static pressure?


BJC
 

poormansairforce

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pmaf:

Pressure drop between which two points. Assuming that you mean static pressure?


BJC
Ideally he would have a flush port perpendicular to the fuse surface at each location but drilling holes in the fuse would be tough to advocate. I'm trying to determine how much adverse pressure drop there is acting down and backward.

Notice that the area affected is in front of the CG and the reflex area that would create lift is behind the CG. It all leads to a significant nose down torque being applied to the airframe. And the drag acts well under the CG.

A couple of tests on the flat area aft of those locations would be good also but should be of the flush port variety. Which is probably tough to do.
 

Eugene

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Yes, in my world of forced air heating and cooling, when we need to measure static pressure, port needs to be perpendicular to the airflow. I will try to come up with something. I know now what we are looking for. It would be interesting to see pressure drop in different location of the airplane. Thank you !!!
 

Doggzilla

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You are correct.

In fact, reducing drag on the cowling will actually make the pitch trim very slightly worse, as the drag is partially cancelling out the thrust that is causing the pitching.

If I was going to reduce drag, I would start at the landing gear. It sticks out the furthest from the centerline, so it has the strongest moment arm. The drag on the gear produces more than twice the moment arm of the fuselage.

It's just like weight and balance calculations, but turned vertical. The wheels clearly have the worst moment arm for their drag to cause a pitch problem.

A "thin leg" Cessna 172 type landing gear would greatly reduce the pitch from drag.
 

Eugene

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If I was going to reduce drag, I would start at the landing gear. It sticks out the furthest from the centerline, so it has the strongest moment arm. The drag on the gear produces more than twice the moment arm of the fuselage.
Yes, some guys , I talked to believe that landing gear positioned at pretty big distance from engine = producing very large moment

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AdvenJack

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Now I've read through the entire thread from post # 1 starting four days ago.
I am curious Eugene, how much shape changing to the various parts of this
plane are you willing to carry out? It seems from your ongoing efforts, that you
would be willing to turn your airplane into one that no longer even resembles,
the plane that the factory originally sold. Your motivation to achieve your goals
is very, very well established!
 

Eugene

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Now I've read through the entire thread from post # 1 starting four days ago.
I am curious Eugene, how much shape changing to the various parts of this
plane are you willing to carry out? It seems from your ongoing efforts, that you
would be willing to turn your airplane into one that no longer even resembles,
the plane that the factory originally sold. Your motivation to achieve your goals
is very, very well established!

Improving flying characteristics, this is what I am after. Sure, airplane Will possibly look somewhat different. That doesn't make any difference to me. I have seen many guys at Oshkosh every year doing bigger and better things in aviation by spending a lot more time and money. So, I'm just a beginner! And this is how I see experimental aviation. To my surprise most people at EAA comunity see it differently. I don't see experimental part in how they fly. But in return they call me crazy Russian for changing wing position by 2°. And I am wasting my time, and they having a good time and so on ....................
 

poormansairforce

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Yes, in my world of forced air heating and cooling, when we need to measure static pressure, port needs to be perpendicular to the airflow. I will try to come up with something. I know now what we are looking for. It would be interesting to see pressure drop in different location of the airplane. Thank you !!!
I totally forgot you were in HVAC so you get what I'm going after! Flush would be better but I wasn't going to ask you to drill a hole in your fuse! I'm a GC so had many conversations about fans, air ducts, etc;)

Having an inverted wing on your plane is just wrong so it may take some fairing to get it right. I like the cockpit and have looked at some of the Skyboys so I'm hoping you get this figured out so I can buy one cheap before everyone else finds out;)
 
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