Aerobatic Tandem Two-Seater

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Radicaldude1234

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Please explain.
The high rpm of the turbocharger's impeller causes high stresses on the bearings. That and the chance of intermittent oil flow during maneuvers would drastically shorten the life of the supercharger. The same thing could be said of engines, but I'm erring on the side of caution and simplicity by avoiding turbo/supercharging and non-geared engines.


As do I. It has a bug that I discovered and which was verified by SW: an external centerline-symmetrical flow analysis has significant errors along a significant strip of cells near the centerline. Cells away from the centerline are ok. (I've run a 767 half model at work with most excellent results for airflow along a sensor window away from the centerline.)

Even though a full model has more cells, it will solve faster (~2.5X to 4x fewer iterations!!!) than the half model. (Some of my half models had goals that never converged; I had to terminate the runs.)

When I told SW I'd update my seat if they fixed it, they said weren't going to. Notice the caveat in the instructions: don't use symmetry unless you know it won't cause errors.

This error will affect your analysis: you will probably get nose down pitching from your canopy and nose up pitching from the canopy downwash onto the aft fuselage. These numbers need to be correct to get a correct pitching moment. You can SWAG the canopy effect by hand calcing it as a little fat wing. Multhopp's method probably won't catch this.

If you doubt what I'm saying, run a test for yourself: run a hershey bar wing all by itself with centerline symmetry and full span. Notice the flow anomaly near the centerline. Compare the lift, drag, cp, and pitching moments, both to each other and to the hand calc numbers.
I have a 16 core processor with 32GBs of RAM, a couple more iterations doesn't take that much longer :grin:

I did extend the analysis to the whole aircraft though. My problem, however, is that the fuselage drag and overall lift values look correct....but the induced drag on the wing is about 4 times what it should be. I was wondering if you had any remedies? I've tried increasing the number of cells on the mesh for the wing to no avail. I have heard that the drag values on Flow Simulation break down for Reynold's #s larger than 6,000,000, so that might be the case...

An accurate pressure map and forces would assist greatly in structural design, though I could get around it. I was, however, going to depend on CFD for fine tuning control forces.
 

wsimpso1

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One big point.

Your design is really spread out. The back seater is fully aft of the front seater, which has tails. There is excess space between the front seaters feet and the engine. The back seater has big influence on CG, as does the aft baggage. This also adds length to the fuselage immediately, raising overall weight and reducing performance. The fix for all of this aft weight is a bigger tail and a more forward engine than you might otherwise have had, further adding wieght, which further reduces performance.

Fix? A couple things and then a bunch of iterating.

Move the back seater forward, putting the rudder pedals for the back seater next to the thighs of the front seater. That slides the backseater and aft baggage forward by about 30 inches, reducing the aftmost CG by a bunch, reducing the bending moments and weight, and improving the difficulty in making it work and have utility too.

Next, once you have the people closer together and the aft baggage less aft too, now start looking more seriosuly at the range of cg you have and start iterating the locations of the engine forward of the firewall, people and bags aft of the firewall, maybe play with splitting some of the bags forward and having a movable battery, balanced angainst how big the horizontal tail has to be to keep it all controllable and landable over the whole range of weights for fuel, peopel, and bags.

Once the people are closer together and you start looking at CG and tail size needed to flare while keeping enough static margin, you can come up with a shorter canopy with a more streamlined after section and perhaps take out some weight in both the fusealge and tail. You may still need to extend the engine mount a little to make it all work, but that is not the end of the world, it makes for more forward baggage, which lets you reduce your aft baggage, further streamline the bird aft of the wing, annd further reduce the tail area needed...

In all, you can take out weight while improving utility and performance.

Billski
 

Radicaldude1234

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One big point.

Your design is really spread out. The back seater is fully aft of the front seater, which has tails. There is excess space between the front seaters feet and the engine. The back seater has big influence on CG, as does the aft baggage. This also adds length to the fuselage immediately, raising overall weight and reducing performance. The fix for all of this aft weight is a bigger tail and a more forward engine than you might otherwise have had, further adding wieght, which further reduces performance.

Fix? A couple things and then a bunch of iterating.

Move the back seater forward, putting the rudder pedals for the back seater next to the thighs of the front seater. That slides the backseater and aft baggage forward by about 30 inches, reducing the aftmost CG by a bunch, reducing the bending moments and weight, and improving the difficulty in making it work and have utility too.

Next, once you have the people closer together and the aft baggage less aft too, now start looking more seriosuly at the range of cg you have and start iterating the locations of the engine forward of the firewall, people and bags aft of the firewall, maybe play with splitting some of the bags forward and having a movable battery, balanced angainst how big the horizontal tail has to be to keep it all controllable and landable over the whole range of weights for fuel, peopel, and bags.

Once the people are closer together and you start looking at CG and tail size needed to flare while keeping enough static margin, you can come up with a shorter canopy with a more streamlined after section and perhaps take out some weight in both the fusealge and tail. You may still need to extend the engine mount a little to make it all work, but that is not the end of the world, it makes for more forward baggage, which lets you reduce your aft baggage, further streamline the bird aft of the wing, annd further reduce the tail area needed...

In all, you can take out weight while improving utility and performance.

Billski
Thanks for the input.

That was looked at: there actually is no excess space between engine and the front pilot. What I didn't model were all the accessories behind the engine.With those, there is less than 12" between the engine and the firewall. The pilot's feet are 6 inches from the firewall, which is exactly how much space he/she needs for pedal movement.

For the rear pilot, I am adamant about there being two full sets of controls. I've flown in aircraft with the back rudder pedals spread out to the side and we're not doing that. So the placement stays. Adjustable seats are planned and the humans depicted are 6' tall; the seats move up/forward and the pedals back to accommodate shorter pilots.

Based on current component placement, the static margin (which is only a rough estimate of stability) ranges from 14% with only the forward pilot and no baggage, and 9% with rear pilot and 50lb luggage.
 

gtae07

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For the rear pilot, I am adamant about there being two full sets of controls. I've flown in aircraft with the back rudder pedals spread out to the side and we're not doing that. So the placement stays. Adjustable seats are planned and the humans depicted are 6' tall; the seats move up/forward and the pedals back to accommodate shorter pilots.
Having ridden in the back of an RV-8 and -8A, which are set up with the "pedals" (such as they are) on either side of the front-seater, I can understand you. Some people are fine in that position, but both times I was quite anxious to get out after only half an hour. In that seating position you can't much do anything with your legs but push pedals. The lack of freedom of movement was killing me.
 

TFF

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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. .
 

wsimpso1

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Thanks for the input.

That was looked at: there actually is no excess space between engine and the front pilot. What I didn't model were all the accessories behind the engine.With those, there is less than 12" between the engine and the firewall. The pilot's feet are 6 inches from the firewall, which is exactly how much space he/she needs for pedal movement.

For the rear pilot, I am adamant about there being two full sets of controls. I've flown in aircraft with the back rudder pedals spread out to the side and we're not doing that. So the placement stays. Adjustable seats are planned and the humans depicted are 6' tall; the seats move up/forward and the pedals back to accommodate shorter pilots.

Based on current component placement, the static margin (which is only a rough estimate of stability) ranges from 14% with only the forward pilot and no baggage, and 9% with rear pilot and 50lb luggage.
Did not mean to offend, but to inform. Allow me to detail them.

The rudder pedals beside the front seat do not have to be the awful thing that you have experienced, but if you have decided on full space for the back seater, you will have compromises to make surrounding that choice. Having considered tandem, and done the calcs, I recommend you look into several issues carefully:

First is 50 pounds of baggage. I hope you are planning for more than that. Yeah, our EFB make it easier, but that still won't allow two people to fly and camp at fly-in's unless you go really light and small on the tent, the bedding, and range and amount of clothing. Most of us will need substantially more baggage capacity;

Second is pitch stability. If 9% static margin is without considering the prop, you may have some big surprises with regards to stability when the back seat and rear baggage bay are filled. If you included the effect of the prop, kudos to you. Otherwise, look hard at how much static margin is typically used up when a tractor prop is pulling, and then revisit tail area and stability issues;

Third is yaw stability. Ditto from pitch stability, but usually less of an issue;

Fourth and my last (for right now) is ability to flare at forward CG. You really need to look hard at the ability of the tail to lift the nose at stall speed, at forward CG, with flaps down, with flow over the tailplanes reduced by an idled prop, and with ground effect removing almost all downwash from the wing onto the tail. It can cause you to either accept higher landing speed (relative to stall speed), or if you try to pull on the stick to get down near stall speed, it runs out of pitch control and drops the airplane on. If you have done this work, and like the result, great.

If you have fully gone through all of these, and are happy with the airplane, great. You are the only one who has to be happy with your bird. But if any of these items sound new to you, you have more work to do make sure your bird covers your CG range and flight conditions. I still suspect your bird will be a better airplane with better performance in takeoff, climb, cruise, and landing if you find a way to comfortably scoot the back seater and aft baggage forward.

Billski
 

wsimpso1

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The biggest objection to my proposed adjustments seems to be the issue of some airplanes with awful ability for the back seater to stretch legs or otherwise be comfortable. I have had that problem in side-by-side birds too. To me, poor design is the issue, not the gross layout. Never felt cramped in the back of a Super Cub, but I sure have in the RANS S6.

I believe that the answer lies in opening the space for the back seater's feet and making the rudder pedals adjustable. If you have two inches under the floorboards, you can make the rudder pedals able to slide fore and aft. You can also do it by making the front seater's seat run on a somewhat narrow set of tracks and put the pedal adjustment mechanism under the edges of the seat with the pedals extended from the side. The rest of the scheme is to not constrain the back seater's legs and feet. Make that space open like we find in Super Cubs and the like.

Of course why bother if all of the other issues I have brought up are under control with only minor penalties...

Billski
 

Toobuilder

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...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...
Yep, it's going to be a big bird. Just took the tape measure to the fuselage of my L-39 (a diminutive thing, for a jet), and it is over 10 feet between the forward bulkhead and the rear. That's 10+ feet of cockpit floor. That's a true dual cockpit, with essentially identical control stations front and rear, with no overlap.
 

Radicaldude1234

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Been working on the CFD process to better reflect sizing equations. Values are now within 10% of those equations and is good enough to verify calculations.

The preliminary model is not trimmed and run to test numbers at an airspeed of 100knots:

2018_0310_CFDprelim2.jpg

Some observations:

- The CFD values are in line with the equations in Raymer's books.
- SL max speed will be ~185 knots with O-470; ~205 knots with IO-720
- Unexpectedly, the fuselage creates ~275lbf of lift for ~45lbf of drag. Most of this is because of the canopy bulge, so the mounting will have to be reinforced.
 

Radicaldude1234

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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:

2018_0311_TailDragBubble.jpg



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.
 

tspear

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Curious, why only two lifting surfaces?
Why not follow the Piaggio Avanti and have a small wing in front? Make it adjustable just a few degrees it can handle the CG issues caused by having a passenger or flying solo.
When I asked an aeronautical engineer before the answer was, it will look funny :D
If the lift required is small and this is not to be a primary load carrying wing, then it likely has less drag then the larger tail and main wing to handle the large CG envelop.

Tim
 

flyboy2160

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Looks like there's some flow separation ...
Be careful with flow separation predictions using Flowsim with the out of the box settings.

I purchased this program for home use after it gave exceptional correlation at work to wind tunnel tests and to flight tests for separated flow off a bluff body and for boundary layer flow.

After I encountered some discrepancies with separated flow off surfaces at much shallower angles to the flow, I was advised that the program has internal parameters that are set to more accurately predict bluff body separation at the expense of shallow angle separation. I was told the only way to tweak these is by a series of tests comparing CFD to real flow results. I had neither the time nor the money to do that.
 
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Radicaldude1234

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Be careful with flow separation predictions using Flowsim with the out of the box settings.

I purchased this program for home use after it gave exceptional correlation at work to wind tunnel tests and to flight tests for separated flow off a bluff body and for boundary layer flow.

After I encountered some discrepancies with separated flow off surfaces at much shallower angles to the flow, I was advised that the program has internal parameters that are set to more accurately predict bluff body separation at the expense of shallow angle separation. I was told the only way to tweak these is by a series of tests comparing CFD to real flow results. I had neither the time nor the money to do that.
Sounds like I have an additional item to verify during the test flight via a tuff test...

What parameters were you told to tweak?
 

Radicaldude1234

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After staring at Excel spreadsheets, on a day off no less, I decided to make something.

Ended up making 2 models:

First, I 3d printed what I call my "motivational model". As I have a lot of 1:18 warbird models back when they were the hottest thing, that's the scale I went with. Just going to put it where I walk through everyday so remind myself of the project.

IMG_2483.jpg

Next, as my spreadsheets were telling me that everything was fine, thought I'd gain an understanding for what it all meant. So I fired up X-Plane and did a rough approximation of the design:

2018_0613_XPlane6.jpg

2018_0613_XPlane10.jpg

As I'd expected, performance isn't spectacular with the 260hp I'm planning. At low speeds it handles like any other GA airplane. Above 100 knots, however, it comes alive. Control becomes noticeably crisper and the airplane holds onto energy pretty well when maneuvering. It seems to live in the 120-170 knot speed range.

Will test with 300hp (O-520/540/550), 400hp (IO-720), and just for fun 750hp (PT-6)
 

Riggerrob

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My guess is that (H stab root) flow separation is caused by interaction between aft fuselage and vertical fin airflow. Adding the H tail nearby further complicates airflow.
As a I suggested a while ago, try raising the turtle deck. If the turtledeck is high enough to include the H stab roots, then you reduce surfaces to two in that neighbourhood.
 
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