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Aerobatic Tandem Two-Seater

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Bigshu

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Jun 7, 2020
Messages
140
No, I can set you up. Not the same as designing your own plane, so not expecting you to bite or anyone else, really. I do have a friend who's time has come flying and he is getting rid of it.
What's he asking for the RV4?
 

Yellowhammer

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Joined
Feb 21, 2020
Messages
163
So personally, I think the CAD exercise at my other thread CAD Exercise: Mustang Outer Mold Line has run its course. I can finish the model, but as I have no desire to build a P-51 replica given there is no viable engine and there are a bazillion of them around anyways, I've decided to go back to a design that I've been working on awhile back.

This is my second draft of the design using the techniques I've learned in the aforementioned thread.

Intended Equipment:
-Engine: (I)O-470 in prototype. IO-520/IO-540/IO-550/IO-720 in later versions. Maybe O-360 in fixed gear version
-Avionics: IFR Capable
-Other: Retractable Landing Gear; electric or electrically powered hydraulics

Goals:
-Performance: 7.5G Load limit; However fast a 230-300hp engine will take it; <55kt stall
-Crew Accommodations: "Functionally Comfortable" cockpit (28" wide at shoulders); Harmonized control system; Sliding Canopy
-Ease of Construction: ~1000hr build time; Composite cowl; Sheet metal construction for majority of airframe

First Pass Design:

View attachment 67429

View attachment 67430

Personal Critiques:

-Too blocky
-Canopy could have caused aerodynamic issues
-Slab sided fuselage looks awkward

Second Pass (Current) Design:
View attachment 67427

View attachment 67426

View attachment 67428

More to come....

Ooooh! I like the hell out of this! Good luck and please keep us well posted!
 

Yellowhammer

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Joined
Feb 21, 2020
Messages
163
Looks like there's some flow separation with the intersection between the horizontal and vertical tails that appeared after I gave the H-tail the correct incidence:

View attachment 70260



The horizontal stabilizer is where it is for spin recovery; with the current configuration, a significant part of the vertical tail (especially the rudder) is not blanketed by the H-tail.

My working hypothesis is that the airflow is sufficiently de-energized from flowing over that highly bulged canopy that the flow separates when it hits that weird intersection. As the H-tail is producing lift downwards, the air below is further de-energized as it is accelerated, causing separation.

I've tried moving the horizontal tail slightly up and down to no avail. Possible solutions that I'm looking at are a tail fillet or vortex generators. Both encourage the flow to stay attached.

Lastly, this is exactly what wanted CFD to predict as otherwise stuff like this would have only presented itself during a wind tunnel test (the guys I know working there at the college graduated) or during an actual test flight.
I would much rather see the Horizontal Stabilizer further back. I think it would make production easier and still provide the spin recover you seek or am I wrong?

Thanks,
Yellowhammer
 

Yellowhammer

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Joined
Feb 21, 2020
Messages
163
being a passenger on long flights in a RV7 I get the same thing; no place to stretch. Its seat positions are made to be flown or non flying be short; Im 5-10 or so. Pretty much most planes without a sliding seat are this way. If you do slide a tandem rear seat all the way clear of the front seat, you will have a very long fuselage. Lots of extra to carry around. If you and your significant other were going to takeoff for a year it would be great to have room and the stability of a long fuselage. If rear is rarely flown from, put the baggage between the positions. Its not going to be a nimble airplane though in aerobatics, but would probably be graceful. .

Couldn't he raise the rear area like on the tucano ?
 

Radicaldude1234

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Joined
May 30, 2009
Messages
390
Location
Front Range, Colorado
Apologies guys, I let these replies slip by me as I'm not on here too much.

I have tried around in Solidworks FlowSim and the flow trajectories beautifully show how the air behaves, but I neither managed to get meaningful lift/drag values (2800N drag and 1300N lift seems strange for cruise condition), nor could I get this separation bubbles.

What did you use for the visualization to show this separation? Can you elaborate on that RadicalDude?
I would also be glad if you said how you set up the mesh parameters.

BR, Andreas
It's been awhile since I've toyed around with FlowSim, but I think I was monkeying around with the surface roughness numbers to get it close to "realistic" numbers. I also got the lift/drag numbers by setting Goals on select surfaces rather than the global values.

With that being said, I pretty much haven't use CFD analysis results in the design as of yet. All of my analysis was done with good ol' fashion equations and XFLR5 if I need to "cheat" a bit.

I would much rather see the Horizontal Stabilizer further back. I think it would make production easier and still provide the spin recover you seek or am I wrong?
Moving the H-Stab back would require angled notches on the elevator to enable the rudder pivot, which would require the H-Stab be enlarged and negate any advantage of increasing the elevator moment. Furthermore, and this might be a typo on my part, spin recovery ability is based on the Vertical stabilizer area, specifically the area underneath the Horizontal stabilizer and is therefore not blanketed at high AoA. Of course, a H-Stab set too high, such as in a T-Tail, can also be blanketed by the wing...
 

Lendo

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Joined
Feb 6, 2013
Messages
514
Location
Brisbane
RAD 123, Moving a Horizontal Stabilizer back, involves less surface area than making the HT Bigger, this also affects the Vertical Stabilizer similarly, however this depends upon your design choices/ aesthetics etc. on which way you want to go, personally I choose less area and less drag.
 

Lendo

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Joined
Feb 6, 2013
Messages
514
Location
Brisbane
RAD 123, My suggestion was to extend the Lever Arm, by way of extending the Rear Fuselage Length, not just the HT. Everything can still be in the same position to each other, just further rearward - sorry for any confusion.
George
 
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