Quantcast

How to do what I want without using a 3LS design ?

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

rbarnes

Well-Known Member
Joined
Aug 28, 2015
Messages
312
Location
Texas
In the initial design conception of a homebuilt project I want to start.

Concept: 2 person twin engine side by side with easy egress, e.g. Velocity / Cessna single, and a hard core approach to flight safety and operations.
We can argue all day about 1 engine vs 2 engine. I want 2 engines when flying at night. Period. I hate flying at night in a single engine airplane.

That locks in 2 engines into the design. Now we have the danger of VMC in a twin and the whole reason for the old saying "the 2nd engine is out there to fly you to the scene of the accident"

A twin engine canard can be designed easily to pitch buck before VMC is reached, e.g. Twin Velocity - but I dont like the safety you give up with the flat relatively high speed approaches of a canard.

So we end up with a 2 engine pusher 3LS design to get the no VMC safety of a canard and the low speed efficiency of being able to put flaps on the canard and main wing and a traditional elevator out back.
Think twin engine Eagle 150 that isn't fugly

Having come up with that brilliant idea I've been researching the web and this site and reading everything I could on the subject and it seems like I have the right idea, but implementing it will be hella hard to near impossible for someone of my non-existent engineering experience.

And finally to the point .... how else could you make a twin with no VMC in a conventional design with cabin accessibility of Cessna 177RG cabin ?
I know the Boomerang, but something like that looks even hard to design, fabricate, and make work than a 3LS...
 

Mad MAC

Well-Known Member
Joined
Dec 9, 2004
Messages
685
Location
Hamilton New Zealand
Any idea on what Tecnams P2006 Vmc is? configuration wise its what you want.

Canards maybe worst than normal tractor engines due to the reduce lever arm of the rudders.

Noting that all aircraft have Vmc, its normally only twins that it kills people (for those of us that don't fly warbirds).

Other options for reducing Vmc issues:
  • Add a 3rd engine
  • A really big fin / rudder with a LEX (undr fuse strake too maybe), you can have any Vmc you like on a twin its just how bigger fin / rudder
  • Separate high lift and drag devices (improves ability to stay above Vmc)
  • Beech use a vacuum bais system to produce rudder inputs, in th event of an engine failure.
  • push me pull me configuration.
 

Victor Bravo

Well-Known Member
HBA Supporter
Joined
Jul 30, 2014
Messages
8,025
Location
KWHP, Los Angeles CA, USA
I was going to say that the Partenavia P-68 Victor series is probably a good starting point for a configuration.

Depending on whether you want more XC speed or more traveling in luxury, the Short Skyvan congiguration with a rear ramp entry/egress can be turned into a little flying Winnebago or camper.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,217
Location
Saline Michigan
Boomerang style twin is the best twin configuration we know of. Its total lack of symmetry bothers some folks. Symmetry is over rated - no human has symmetry.

Hard to design? Harder than a 3 LS? I seriously doubt it. Three surface ships have stability and control to manage over the entire speed range, CG range, and range of power settings. It always has to stall nice, so flaps will all have to be interconnected and stone reliable, then be able to adjust the ratios on them. Then they may have to be "fixed" so that stability is satisfied while ensuring canard stalls first. At no point will a 3LS main wing be taken to stall, so the stall speed will be higher for any whole airplane wing loading than with a conventional bird.

You will need to set everything up in Excel so you can do a bunch of iterating with any design. The usual design of a engine-in-front, tail-in-back airplane requires CG estimation, neutral point estimation, tail sizing, foil placements, landing gear placement, etc. Criteria are sum of vertical forces = zero, sum of moments in pitch axis = zero, sum of moment first derivatives < 0 at aftmost CG, airplane can reach first stall with downwash nulled out. Usually these are all in the FS-WL plane and a bunch of estimation. A Boomerang config will require also running rolling moment = zero in the WL-BL plane. Remember to include rolling moments from engine torque over the speed range. Yeah, roll trim may have to be powerful, but all twins have that need.

Set up your calculation of wing lift based upon elliptical lift distribution - this means the wing ends up based upon where the wing tips are, and centerline of the wing must be close to CG. So you set up your CG estimation using positions of stuff in the all three axes instead of just two axes. Then you can iterate your design, update all the weights, centerline of lift, and fine adjust just where the wing tips are to center lift on the lateral CG as well. Tail centerline is easy, take a page from Burt and be prepared to extend the tip on one side or the other.

Controls are not as fussy as you might imagine. Ailerons and flaps are conventional. Elevator and rudder control runs only in the main fuselage - you usually do not need a rudder in the second fuselage, but if you think you do, run a linkage across the horizontal stabilizer.

The only fussy part I see is that Burt went with some negative sweep to put the spar through the main fuselage in a convenient place. You may need to fuss with that too, or maybe you won't. But if you set up everything in spreadsheets, as you refine details of weights and positions, you can also iterate wing geometry using positions of MAC and sweep about that to get the spar in a convenient spot in the main fuselage.

One thing to note about 3LS ships. They are always "fixed" to get stall behaviour right over the range of W&B, flaps settings, etc. Aero cures exist around them and they also usually require adjustments to canard spans, trim systems, etc during early flights. Catbird was fussed over repeatedly... First time I saw it at OSH was 1992, and the right side of the canard was sawn off and sanded smooth, then epoxied over. Since then the massive core canard has been removed and the somewhat heavier molded canard has been installed and trimmed with CG limits adjusted. Aerodynamically, the Boomerang has none of that visible on the outside. And the people who fly the Boomerang say it is just an easy to fly airplane that happens to have two throttles and a gear switch.

Sounds doable to me,

Billski
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,318
Location
US
And finally to the point .... how else could you make a twin with no VMC in a conventional design with cabin accessibility of Cessna 177RG cabin ?
Do it just like Cessna did with the 337 (Skymaster)? If the strut hampers accessibility, just use a cantilever wing.
- Conventional design? Check
- No VMC? Check.
 

rbarnes

Well-Known Member
Joined
Aug 28, 2015
Messages
312
Location
Texas
Thanks guys. Let me play with my imagination and asymmetrical engine layouts. I had my heart set on a 3LS, because they look cool and sexy, but the more I read stuff like the "7 sins of aircraft design" I realized I was making one of those mistakes and shoe horning myself into a corner instead of focusing on the main goal of an idiot proof twin engine cross country cruiser.

I was thinking about a front/back engine layout circa 337 and Adams 500, but the couple of times I flew in a 337 I didn't like it. Two engines mounted on fuselage was really loud and I've heard of the horror stories of shoving all those system for two engines in the fuselage. Then there are the structural consideration of booms off the wings for the tail. Push-pull twin is really not appealing, but I'm still toying with that as well.

I had never seen that Britten-Norman tri-engine plane before.... hmmm, these are the kind of ideas I need.

Keep the ideas coming. I realize this is going to be a 15 year endeavor but I'm looking for a life challenge for my mind. Life has gotten into a bit of a pay check to pay check rut.
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
5,318
Location
US
I was thinking about a front/back engine layout circa 337 and Adams 500, but the couple of times I flew in a 337 I didn't like it. Two engines mounted on fuselage was really loud and I've heard of the horror stories of shoving all those system for two engines in the fuselage. Then there are the structural consideration of booms off the wings for the tail. Push-pull twin is really not appealing, but I'm still toying with that as well.
For consideration as you think about options:
- Depending on the propeller length (which, in turn, may depend on the HP of the engines you install and the relative importance of short takeoff/high climb rate vs cruise speed), you may find that a single low boom or booms at the fuselage lower corners can be used. This simplifies control runs.
- Fixed pitch props? They reduce costs lot. OTOH, controllable pitch props give you the best climb and cruise speeds and being able to feather the prop on a dead engine will be increasingly important if your engines aren't on the centerline. Rutan's Defiant achieved 200 mph cruise as well as good single engine climb using fixed pitch props, it can be done. Eliminating the prop pitch control does simplify emergency actions required, and can be seen as a safety advantage IF safe single engine climb can still be accomplished with fixed pitch props.
 

rbarnes

Well-Known Member
Joined
Aug 28, 2015
Messages
312
Location
Texas
For consideration as you think about options:
- Depending on the propeller length (which, in turn, may depend on the HP of the engines you install and the relative importance of short takeoff/high climb rate vs cruise speed), you may find that a single low boom or booms at the fuselage lower corners can be used. This simplifies control runs.
- Fixed pitch props? They reduce costs lot. OTOH, controllable pitch props give you the best climb and cruise speeds and being able to feather the prop on a dead engine will be increasingly important if your engines aren't on the centerline. Rutan's Defiant achieved 200 mph cruise as well as good single engine climb using fixed pitch props, it can be done. Eliminating the prop pitch control does simplify emergency actions required, and can be seen as a safety advantage IF safe single engine climb can still be accomplished with fixed pitch props.
This is one area that technology has finally caught up with us dreamers. The rotax 915 offers a true single lever FADEC automatic constant speed prop system from a certified engine now. Jet engine like control simplicity. It's also a big part of the dream. Lose an engine and you pull one lever back. Everything wired into a Garmin or Dynon that will flash warnings at you as it monitors engine stats. Pair of turbo 140hp engines on a 2 seat twin is plenty of power for excellent performance on two or one engine.

Definitely going to take a rethink at the push/pull set-up. How hard is it do a cantilever wing with booms ? Is designing a structure that can pass through all the different loads the wing spar now has to handle hard to deal with ?
 

narfi

Well-Known Member
Log Member
Joined
Feb 23, 2016
Messages
942
Location
Alaska
Two engines mounted on fuselage was really loud and I've heard of the horror stories of shoving all those system for two engines in the fuselage. Then there are the structural consideration of booms off the wings for the tail. Push-pull twin is really not appealing, but I'm still toying with that as well.
Is the traditional Dornier layout too draggy for you?

Their new Seastar is a fairly unique and sexy look for today.
 
Last edited:

rbarnes

Well-Known Member
Joined
Aug 28, 2015
Messages
312
Location
Texas
Not necessarily, and the DoubleEnder STOL plane had me doodling some ideas as well. I wonder how bad the pitch changes are with power inputs though with the engines up that high ?

1806_4.jpg
 

poormansairforce

Well-Known Member
Joined
Mar 28, 2017
Messages
1,068
Location
Just an Ohioan
Note that the Trislander has some pretty serious down thrust built into the rear engine. That works because it's behind the center of gravity. Why not a twin pusher Velocityish design with the engines canted inward?
private-jet-193.jpg
 

Brünner

Active Member
Joined
Apr 12, 2020
Messages
30
Location
Beer country
I honestly fail to see the advantage of placing the engine up there when you could install it on the nose. Shouldn't the weight distribution be easier, better as well?
 
Top