Problems in designing a small aircraft.

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Sraight'nlevel

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I wonder if others have come up with same the problems.


1. To get the spar carry through the fuselage will severely seem to cause plane to be bigger than intended.

2. There seems to be no way to get into the cockpit if everything else seems to be nice and tight.

3. Without pilot inside the plane seems to drop to its tail albeit being a nose wheel design.

---------------------------------

Many have bee able to by pass these problems in various ways.

Quickie is a good example...three wing design could be another.

BD-5 is a low wing and gets the wing into right place although the CG is a bit critical.

---------------------------------

Any ideas how to solve these "problems" ?
 

TFF

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You study everyone’s airplane. You catalog everything that they made work. Unless you are a Burt Rutan, you are going to have to pick a way it’s been done before. Even Burt’s didn’t invent foam and fiberglass together. Designs get revised. There is perfect, which is not obtained, the final product, and how many iterations it took to get there.
 

Sraight'nlevel

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Yes thank you for the comments !

I figure...in order to create something special for the electric flight...a new kinda aircraft has to be developed, but since aeroplanes have been around nearly 120 years by now...anything breathtakingly new is very very difficult to develope.

The new has to be in the numbers.
 

Riggerrob

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Yes thank you for the comments !

I figure...in order to create something special for the electric flight...a new kinda aircraft has to be developed, but since aeroplanes have been around nearly 120 years by now...anything breathtakingly new is very very difficult to develop.

The new has to be in the numbers.
Electric airplanes offer the advantage of lighter powerplants allowing for a greater variety of locations that still balance. For example. To build a light single-engine like a Stemme or Bolkow Junior, you can mount the prop and entire motor in the nose and still mount both pilots forward of the main wing spar. The advantage is no vibrating drive shaft like the Stemme, nor a forward swept wing spar like the Bolkow.
Light-weight electric motors might finally make the "prop on tp of fin" configuration (e.g. Sea Wind flying boat) practical.

Your final weight bill for propulsion will be about the same as a gasoline-powered airplane, but you can mount motors in a wider variety of corners of the airframe.
 

Sraight'nlevel

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Messages
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Electric airplanes offer the advantage of lighter powerplants allowing for a greater variety of locations that still balance. For example. To build a light single-engine like a Stemme or Bolkow Junior, you can mount the prop and entire motor in the nose and still mount both pilots forward of the main wing spar. The advantage is no vibrating drive shaft like the Stemme, nor a forward swept wing spar like the Bolkow.
Light-weight electric motors might finally make the "prop on tp of fin" configuration (e.g. Sea Wind flying boat) practical.

Your final weight bill for propulsion will be about the same as a gasoline-powered airplane, but you can mount motors in a wider variety of corners of the airframe.
That is a very important aspect. Electric motors are very light, simple and small.
 

Riggerrob

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" ... 1. To get the spar carry through the fuselage will severely seem to cause plane to be bigger than intended. ... "
Yes, when you need to locate both the pilot and main wing spar carry through near the center-of-gravity, it can become a packaging hassle that can create fuselages that are too deep. Fortunately, several designers have already invented solutions. some squeeze from the top, while others squeeze from the bottom while others juggle components forward or aft for balance.

See and avoid is a huge safety issue at the uncontrolled airports favored by small airplane pilots. so being able to see other airplanes is the best way to avoid collisions. IOW Big windows are a cheap form of insurance.

Many high wing monoplanes need to shift the spar considerably above the pilots' eye line to improve visibility (especially sideways into the turn while turning onto final approach, creating cockpits that are too deep for streamlining. The challenge is to keep boundary layer airflow attached as the fuselage decreases in cross-section aft of the wing.
A few Cessnas (e.g. military O-1 Bird Dog) added cabin roof windows, while Zenith gets around this by making the entire cabin roof out of Plexiglass. Zenith also improve slant visibility (into the turn) by tapering wing roots. Chris Heintz did not invent this configuration, he just adapted a 1930s German design (Fieseler Storch). This "cheat" works best with external strut bracing that allows for thin spar carry through tubes (usually small diameter steel tubing).
Cessna tried to "cheat" this way on their cantilever, high-wing 177 Cardinal and 210 Centurion, but by the time they got the pilots' eyes far enough forward they developed balance problems. Cardinal needed two iterations of horizontal stabilator to cure pitch problems. 210 Centurion got a straight leading edge and forward swept trailing edge (which equals a minor forward sweep in the main wing spar). This caused a fierce debate between 210 engineers and salesmen. Engineers wanted to install even more dihedral - to improve roll stability - but salesmen out-voted them to limit dihedral.

Mid-wings are the most difficult to balance. ergo there are few small mid-wings outside of hard-core aerobatic competitors or Formula One racing circles. Aerobats prefer mid-wings for equal stability when upright or inverted. Racers prefer mid-wings for their minimal trim drag changes at different throttle settings. Most Formula pilots just accept cramped cockpits and awkward entry as part of the price of flying fast, so they sit between the front and rear spars.
If you want a mid-wing or shoulder wing, you can copy the Sisler Cygnet or Bolkow Junior's wing which is slightly swept forward like many two-seater sailplanes. Bolkow passes the spar carry-through just behind the pilots' shoulders. A mildly forward-swept wing (say 10 degrees) makes little aerodynamic difference but can be annoying to build.

Most low-wing monoplanes just sit the pilots' buttocks on top of the main wing spar, but this can force the pilots' heads so high that huge bulged canopies (aka. cockpit ceilings) look silly. This styling challenge is most noticeable in light, single-seaters where the height of the pilot's spine does not change as the fuselage gets shorter. Rockwell 112 found an elegant solution in a laminar airfoil section and forward swept wing (straight leading edge matched by a forward swept trailing edge) that routed the main spar below the rear seats.
RV adapted a variation on this theme to reduce cabin depth on their RV-12. The RV-12 structural configuration is similar to the Bolkow Junior - except that the spar passes behind pilots' buttocks. RV also "cheated" balance by using the lightest engine available (Rotax 900 series) along with moving the fuel tank and battery aft of the wing spar. "Cheating" on balancing engine weight allows RV-12 to eliminate the forward-swept wing.

Separating the lift surface (aka. main wing) into two components (ala. tri-surface Piaggio Avanti) can allow you to route spar carry-throughs forward and aft of the cabin pressure vessel, but complicates aerodynamics. Since Avanti's main wing is straight and aft of the cabin, they needed to add more lifting surface (canard) forward of the cabin to balance. Avanti's canard only provides lift since it is fixed. Avanti uses a conventional horizontal tail for pitch stability and control.
Back during the 1960s Hansa suffered a similar balance problem when designing their biz jet, but solve the balance problem by bending considerable forward sweep into their main wing.
 

Riggerrob

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" ... 1. To get the spar carry through the fuselage will severely seem to cause plane to be bigger than intended.

2. There seems to be no way to get into the cockpit if everything else seems to be nice and tight. ... ?
Ease of entry is a balancing act between ease and structural weight.
Any door/hatch/canopy creates a hole in the fuselage structure that has to be reinforced.

Reinforcement is easy if the cockpit structure is already made of welded steel tubing (PIper Cub or Galstar STOL). You just shift a few tubes to create a human-sized hole. You might need to add a couple of extra pieces of steel tubing to maintain triangulation (did I just invent a new word?). In this case, steel tubes just prop the various components (engine, landing gear, wings, tail, etc. in the correct formation. Steel tubing also makes for great roll cages that protect occupants during "less than perfect landings."

OTOH Structural design becomes far more complicated with monocoque stressed skin fuselages. Since the ideal monocoque is a thin-shelled egg shape, any hole is going to weaken it. You need to add reinforcements around every door, window, access pane, etc. All those reinforcements add weight. The most popular method is simply to build the structure as a canoe shape with stiffened gunnels. This configuration is most popular with the Walter Mitty types who really want to be fighter jocks. Since the cockpit canopy is already a thin, optically pure Plexiglass dome, it does not add significant weight. A disadvantage is all the extra external steps, handles, non-skid and special boarding procedures.

However, the average passenger has grown accustomed by a life or riding in cars, buses, rail cars, airliners, ferry boats, etc. so will balk at any fancy boarding procedures. In the worst case scenario, a clumsy passenger ignores special boarding procedures and steps through the wing. Grrrrrr! (Sound of mechanics' opinion of the passenger).

The vast majority of passengers will only be comfortable with side doors that can be entered in a single step from standing on asphalt. Side doors are well understood - structurally - but still heavier.

Better suited to the boating crowd are the hinged bow windows seen on small flying boats (Lake Amphibian, Republic Sea Bee, etc.) because these allow experienced passengers to help with tossing ropes to docks, etc.

Finally, we get to the novel belly hatch installed on Barnaby Wainfain's odd-looking Facetmobile. Since his structure is mostly aluminum tubes, he could mount a hatch or door anywhere, but decided to simplify boarding by just crouching underneath and standing up into the cockpit. with controls mounted to side walls or the instrument panel, this leaves the floor clear for a large hatch.

Now for my challenge: I want to design a shoulder-winged light plane with two seats (Bolkow Junior or Sisler Cygnet) and side doors because as my knees get older, I get less enthusiastic about climbing up the side of an airplane. I want to be bale to walk up to the airplane, sit my by butt on the seat and swing my legs onboard with a minimum of grunting and straining. Breguet did build a few airplanes like this, but their doors were tiny and better suited to small soldiers. The challenge is "bridging" structural loads from the main wing spar to the firewall ???????? Its possible with a multitude of steel tubes, but I was thinking more in terms of large diameter aluminum tubing or wide cross-section composite tubes and a thick floor. I wonder how much of the load can be routed through the canopy frame???????
 

rotax618

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The two constraints, butt level seat and shoulder wing, will require some very clever ideas. Once your butt is on the seat your hip joint should be level with the CG ( for a properly balanced very light airplane). A solution would be forward sweep which introduces aerodynamic and structural problems.
I am also a very inflexible geriatric, I can put my butt on the seat of my Savannah, I wish the doors were wider, I still have some flexibility issues bending my knees.
The Savannah has a clear roof and it is not difficult to lean forward to see forward of the leading edge.
Make sure that the specifications doesn’t make your project either unbuildable or so compromised that it becomes overweight or unsafe.
 

llemon

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Apr 29, 2021
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122
Take a look at the mini-bulle;

Another thing to consider is that in some ways an electric is like a 2-stroke in terms of engine weight. So some of the configs that make sense in 2-strokes probably make sense in electric as well.

Also I'd strongly suggest doing some preliminary CG calculations. I did similar calculations recently for a sub -660lb and was surprised that I could get a bit better travel than what I though. Pilot weight is probably better thought of as 0-125 instead of something like 125-250.
 

Riggerrob

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" ... 3. Without pilot inside the plane seems to drop to its tail albeit being a nose wheel design. -- Any ideas how to solve these "problems" ?"
Many cargo planes install a temporary pogo-stick under the aft fuselage during loading operations. It prevents the plane from tipping back onto its small and delicate tail bumper.
Pogo sticks are usually hinged to allow them to simply drag behind the airplane if you forget to remove it before flight. Attachment fittings are also flimsy enough to break before the pogo stick interferes with flight.
If you want to get fancy, you could install a manually retractable pogo stick. Make the pogo stick operable from inside the cockpit. Ideally, the pogo stick lever would be beside the parking brake handle to remind you to raise the pogo stick before releasing the parking brake.
 

Dan Thomas

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Light-weight electric motors might finally make the "prop on tp of fin" configuration (e.g. Sea Wind flying boat) practical.
I've never been able to figure out what benefit that arrangement might have. The fin has to be stronger, meaning heavier, to take the thrust and gyroscopic loads, neither of which are insignificant, and added weight for no real gain is never wise.
 

Niels

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Messages
242
For a single seat:
A 8kg 30kW electrig motor up front
and two 15kw/10 kg range extenders from battery cars,one on each wing.
Pilot somewhere in between.
People at TU Graz in Austria are seaching for best range extener configuration.
Results to be shown on a two stroke conference in september on a conference in Sevilia,Spain.
My sidevalve two stroke was favourite in 2020 so I am biased.
 

John.Roo

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Location
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Hello!
Pls. could be possible to get more info about:
"15kw/10 kg range extender from battery cars"?

Thanks for info about TU Graz range extender development 👍
Range extender with reasonable weight and output power 10-15 kW would be really great ;)

Best regards!
Martin
 

Sraight'nlevel

Well-Known Member
Joined
Nov 4, 2021
Messages
388
Yes, when you need to locate both the pilot and main wing spar carry through near the center-of-gravity, it can become a packaging hassle that can create fuselages that are too deep. Fortunately, several designers have already invented solutions. some squeeze from the top, while others squeeze from the bottom while others juggle components forward or aft for balance.

See and avoid is a huge safety issue at the uncontrolled airports favored by small airplane pilots. so being able to see other airplanes is the best way to avoid collisions. IOW Big windows are a cheap form of insurance.

Many high wing monoplanes need to shift the spar considerably above the pilots' eye line to improve visibility (especially sideways into the turn while turning onto final approach, creating cockpits that are too deep for streamlining. The challenge is to keep boundary layer airflow attached as the fuselage decreases in cross-section aft of the wing.
A few Cessnas (e.g. military O-1 Bird Dog) added cabin roof windows, while Zenith gets around this by making the entire cabin roof out of Plexiglass. Zenith also improve slant visibility (into the turn) by tapering wing roots. Chris Heintz did not invent this configuration, he just adapted a 1930s German design (Fieseler Storch). This "cheat" works best with external strut bracing that allows for thin spar carry through tubes (usually small diameter steel tubing).
Cessna tried to "cheat" this way on their cantilever, high-wing 177 Cardinal and 210 Centurion, but by the time they got the pilots' eyes far enough forward they developed balance problems. Cardinal needed two iterations of horizontal stabilator to cure pitch problems. 210 Centurion got a straight leading edge and forward swept trailing edge (which equals a minor forward sweep in the main wing spar). This caused a fierce debate between 210 engineers and salesmen. Engineers wanted to install even more dihedral - to improve roll stability - but salesmen out-voted them to limit dihedral.

Mid-wings are the most difficult to balance. ergo there are few small mid-wings outside of hard-core aerobatic competitors or Formula One racing circles. Aerobats prefer mid-wings for equal stability when upright or inverted. Racers prefer mid-wings for their minimal trim drag changes at different throttle settings. Most Formula pilots just accept cramped cockpits and awkward entry as part of the price of flying fast, so they sit between the front and rear spars.
If you want a mid-wing or shoulder wing, you can copy the Sisler Cygnet or Bolkow Junior's wing which is slightly swept forward like many two-seater sailplanes. Bolkow passes the spar carry-through just behind the pilots' shoulders. A mildly forward-swept wing (say 10 degrees) makes little aerodynamic difference but can be annoying to build.

Most low-wing monoplanes just sit the pilots' buttocks on top of the main wing spar, but this can force the pilots' heads so high that huge bulged canopies (aka. cockpit ceilings) look silly. This styling challenge is most noticeable in light, single-seaters where the height of the pilot's spine does not change as the fuselage gets shorter. Rockwell 112 found an elegant solution in a laminar airfoil section and forward swept wing (straight leading edge matched by a forward swept trailing edge) that routed the main spar below the rear seats.
RV adapted a variation on this theme to reduce cabin depth on their RV-12. The RV-12 structural configuration is similar to the Bolkow Junior - except that the spar passes behind pilots' buttocks. RV also "cheated" balance by using the lightest engine available (Rotax 900 series) along with moving the fuel tank and battery aft of the wing spar. "Cheating" on balancing engine weight allows RV-12 to eliminate the forward-swept wing.

Separating the lift surface (aka. main wing) into two components (ala. tri-surface Piaggio Avanti) can allow you to route spar carry-throughs forward and aft of the cabin pressure vessel, but complicates aerodynamics. Since Avanti's main wing is straight and aft of the cabin, they needed to add more lifting surface (canard) forward of the cabin to balance. Avanti's canard only provides lift since it is fixed. Avanti uses a conventional horizontal tail for pitch stability and control.
Back during the 1960s Hansa suffered a similar balance problem when designing their biz jet, but solve the balance problem by bending considerable forward sweep into their main wing.
Would it be possible not to carry the spar through the fuselage at all ?
 

Sraight'nlevel

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Joined
Nov 4, 2021
Messages
388
Many cargo planes install a temporary pogo-stick under the aft fuselage during loading operations. It prevents the plane from tipping back onto its small and delicate tail bumper.
Pogo sticks are usually hinged to allow them to simply drag behind the airplane if you forget to remove it before flight. Attachment fittings are also flimsy enough to break before the pogo stick interferes with flight.
If you want to get fancy, you could install a manually retractable pogo stick. Make the pogo stick operable from inside the cockpit. Ideally, the pogo stick lever would be beside the parking brake handle to remind you to raise the pogo stick before releasing the parking brake.
Isn't it why many are tail draggers ?
 

Niels

Well-Known Member
Joined
Aug 15, 2019
Messages
242
Hello!
Pls. could be possible to get more info about:
"15kw/10 kg range extender from battery cars"?

Thanks for info about TU Graz range extender development 👍
Range extender with reasonable weight and output power 10-15 kW would be really great ;)

Best regards!
Martin
You are closer to Graz than me and Czech people have some very smart engine designer/manufacturers and very good electric people also.

My information comes from papers for session 5 from 2020.

smart two strokes
 

Dana

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Joined
Apr 3, 2007
Messages
10,960
Location
CT, USA
I've never been able to figure out what benefit that arrangement might have. The fin has to be stronger, meaning heavier, to take the thrust and gyroscopic loads, neither of which are insignificant, and added weight for no real gain is never wise.
If it's an amphibian, it means you don't need an additional pylon to carry the engine. It also puts the prop noise back behind the cockpit.
Would it be possible not to carry the spar through the fuselage at all ?
Sure, but you then need external wing bracing or very strong fuselage sides.
 
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