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Hopefully my comments are not way OT. I was reading through this thread and noticed a few comments of wood not being cost effective anymore versus Al tube. While the price of (all grades) of wood have gone up significantly recently, I was surprised to find aircraft spruce still appears to be very cost competitive and in many cases even cheaper than Al tube.
From Aircraft Spruce
For example typical longerons:
A/C spruce 0.75" square are about $2 per ft.
6061 1" dia and 0.058" wall is $3.85 per ft
4130 steel 0.75" did 0.035" wall is $5.45 per ft
Spars:
6061 2.5" dia 0.065 is about $15.50 per ft
A spruce spar that carries about the same bending load (3.75" x 0.75") is $7.50 per ft
Sheet
Plywood AC Birch (0.1") 8' x 4': $111
6061 T6 8 'x 4' : $113
2024 T3 8' x 4' : $131
Sure, Al tubes can be found more readily (locally) and maybe found cheaper online but so can a/c quality wood if one is patient enough to search. Also shipping adds significantly cost but that is really an issue for long pieces (over 8') and applies to all materials. Wood is a lot of fun to build with and still is very much an option for the plans built and one of design airplanes.
From Aircraft Spruce
For example typical longerons:
A/C spruce 0.75" square are about $2 per ft.
6061 1" dia and 0.058" wall is $3.85 per ft
4130 steel 0.75" did 0.035" wall is $5.45 per ft
Spars:
6061 2.5" dia 0.065 is about $15.50 per ft
A spruce spar that carries about the same bending load (3.75" x 0.75") is $7.50 per ft
Sheet
Plywood AC Birch (0.1") 8' x 4': $111
6061 T6 8 'x 4' : $113
2024 T3 8' x 4' : $131
Sure, Al tubes can be found more readily (locally) and maybe found cheaper online but so can a/c quality wood if one is patient enough to search. Also shipping adds significantly cost but that is really an issue for long pieces (over 8') and applies to all materials. Wood is a lot of fun to build with and still is very much an option for the plans built and one of design airplanes.
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Did Bates provide you with a digital version of his 1/4 scale P-39 plans? My 70% P-39 mock-up was built from enlarged Pepino plans and I have posted a photo of a rear engined, 1/4 scale driveline model built from Bates plans. What scale is the airframe in the photo? How did you plan on dealing with the scale door size being too small for anyone over 5' 2" to get into? Were you planning on a mid or front engine location?
Vince Homer
freerangequark
Well-Known Member
As one of the designers of the Timber Tiger Aircraft Ryan ST-L replica, 95% scale, I'm going to chime in here... since the prototype is in US Army PT-16 markings, I think it fits right in here in the forum category. 
We are commonly asked, "Why 95%?"
Short answer... cost, simplicity, and performance...it's simple as that.
Long answer:
1. Inverse cube law as it applies to area versus volume
According to the inverse cube law, a 5% size reduction = 14.37% volume decrease. Equate this to mass. In the case of the Timber Tiger Ryan ST-L, the 5% size decrease amounted to a 20%, or 200lbs weight savings.
This is what allowed us to use a 100hp engine. (BTW, the resonator on the aircraft cuts off a lot of high frequency noise so the airplane doesn't sound like a sewing machine.
Here is a sample of how it sounds.
2. The big challenge of scaling down an airplane is that pilots don't shrink, in fact, they get larger, especially since the original was first built in 1934 when people were generally of a smaller stature. We addressed this a few ways:
a. The fuselage width is actually 100% scale. Its difficult to tell though.
b. The outside of the plane is scale, but the seating pedal placement has been changed to make the plane more ergonomically friendly than the original. This has been confirmed through comparison to original Ryan ST-A's and STM's by pilots who have sat in both aircraft.
c. Since evaluating the prototype, further refinements to the design have been made including more legroom in the rear cockpit and significantly more legroom in the front cockpit allowing us to add brakes to the front rudder pedals as well.
3. The plane actually features more shoulder room than the original as well.
4. Lower cost
5. Lower wing-loading
6. Higher performance via higher power to weight ratio
7. Allows use of modern engines and maintain the scale look.
Here is a pic of me taxing the plane at AirVenture last year. (BTW, I'm just shy of 6'0").
Here is me standing next to the plane at Poplar Grove
Lastly, here is the Timber Tiger Ryan ST-L nose to nose with a PT-22. (The PT-22 is an entirely different airplane than the Ryan ST series and is often confused as being similar)... anyways it was photo opportunity I had at Broadhead, WI last year that I didn't want to pass up.
Length:
Original Ryan ST-A = 21'-6"
Timber Tiger ST-L (95%) = 20'-5"
PT-22 = 22'-5"
Also, here is some more about the airplane from EAA. This came out along with the article in the March 2022 edition of Sport Aviation Magazine.
Blue skies!!
Glenn
We are commonly asked, "Why 95%?"
Short answer... cost, simplicity, and performance...it's simple as that.
Long answer:
1. Inverse cube law as it applies to area versus volume
According to the inverse cube law, a 5% size reduction = 14.37% volume decrease. Equate this to mass. In the case of the Timber Tiger Ryan ST-L, the 5% size decrease amounted to a 20%, or 200lbs weight savings.
This is what allowed us to use a 100hp engine. (BTW, the resonator on the aircraft cuts off a lot of high frequency noise so the airplane doesn't sound like a sewing machine.
Here is a sample of how it sounds.
2. The big challenge of scaling down an airplane is that pilots don't shrink, in fact, they get larger, especially since the original was first built in 1934 when people were generally of a smaller stature. We addressed this a few ways:
a. The fuselage width is actually 100% scale. Its difficult to tell though.
b. The outside of the plane is scale, but the seating pedal placement has been changed to make the plane more ergonomically friendly than the original. This has been confirmed through comparison to original Ryan ST-A's and STM's by pilots who have sat in both aircraft.
c. Since evaluating the prototype, further refinements to the design have been made including more legroom in the rear cockpit and significantly more legroom in the front cockpit allowing us to add brakes to the front rudder pedals as well.
3. The plane actually features more shoulder room than the original as well.
4. Lower cost
5. Lower wing-loading
6. Higher performance via higher power to weight ratio
7. Allows use of modern engines and maintain the scale look.
Here is a pic of me taxing the plane at AirVenture last year. (BTW, I'm just shy of 6'0").
Here is me standing next to the plane at Poplar Grove
Lastly, here is the Timber Tiger Ryan ST-L nose to nose with a PT-22. (The PT-22 is an entirely different airplane than the Ryan ST series and is often confused as being similar)... anyways it was photo opportunity I had at Broadhead, WI last year that I didn't want to pass up.
Length:
Original Ryan ST-A = 21'-6"
Timber Tiger ST-L (95%) = 20'-5"
PT-22 = 22'-5"
Also, here is some more about the airplane from EAA. This came out along with the article in the March 2022 edition of Sport Aviation Magazine.
Blue skies!!
Glenn
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Thank you for this, Glenn! I've found that most people who criticize scale repros of early aircraft have little to no flying experience, nor knowledge in aircraft design. They built styrene models from boxes and think that made them experts, but have never sat in a cockpit themselves. Alan James built an Issaacs Spitfire and someone posted a video on it on Youtube and the nitwits came out of the woodwork to nitpick it not looking exactly like a styrene dust collector. We build these to fly in because it's supposed to be fun, not to impress tiny minds. I've flown in enough cramped cockpits to wonder when the fun is supposed to start. My '41 Porterfield Collegiate is as cramped as I care to be. BTW, the Singer 212 I use to sew hotair balloon envelopes still sounds more like a Menasco than your resonated Rotax, but good attempt!
freerangequark
Well-Known Member
One more sound byte...
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Have you made any progress on the final design? What scale are you planning to use? Is it going to be a genset/batteries/motor arrangement?
Vince Homer
EpoxyCowboy
Member
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Hey Vince!
I saw your article in the Replica Fighters Association Magazine. Nice! I would love to see more on your B-25 to compare notes. I started in this hobby with my own B-25 design, but soon realized, that I was a bit over my head as a rookie back in 1990-something.
Sadly, no, not much progress. My grandfather was preparing to sell me a property back in Michigan, so I moved my shop & aircraft up there. I wont go into details, but my 100 year old grandfather passed away before he could sign my contract, and my Aunt put an end to the purchase. Greed and Evilness... So all the time and money I put into the new shop was waisted and I have been moving back to Ohio since.
I should get back to building this month.
Next step is to:
- Redesign the tail surfaces.
- determine the Final Scale
- Reshape the fuse based on FS
- Start the FS Build. (I will post here)
As for Battery, I'm not sure until I decide on the motor. Right now I would prefer the new H3X. They are promising a lot now and in the future. But will they deliver or go the way of DGEN? I have not been in a hurry because I am still working on structure, but once I start releasing photos of the FS build, I'm going to get asked a lot about the power plant and I would prefer to have that figure out before then. So, if H3X wants to be my solution, they need to deliver something to the market soon.
Thanks Vince!
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75% would make for a large aircraft, about 47 ft. wing span and a wing area of 343 ft2. It would allow for side-by-side seating and a large enough area behind the pilot/copilot seating for a couple passengers. With a reasonable wing loading around 10 lb/ft2 you get a 3400 lb aircraft. This would be a very big project. Since it is a twin and sufficient engine hp for single engine operation must be provided you end up with 175+ hp engines.
At 50% a tandem cockpit works well. The passenger's back is up against the main spar makingthe passenger/no passenger weight and balance a non issue. Somewhere around 55% scale even the nacelles are right for Verner or Rotec radials. A 75% scale version might use Continental 225 radials, although they are a bit heavy and quite dated. Unfortunately, there's not any new radials available in the 200 hp range.
I am currently involved with a project using electric and applying this experience to something like an all electric B-25 tells me it would be a very big development effort. The B-25 airframe isn't optimized for electric power, something alot more aerodynamic and single engined would seem more appropriate. To me the appeal of a B-25 was the possibility of two Verner or Rotec radials immatating a real B-25.
I had a look at the H3X website and their motors would be impressive. So would the batteries to power them at 200 kw each. At a very optiistic 300 wh/kg a 1 hour battery would weigh in the neighborhood of 1000 kg for a take-off and 3/4 throttle for an hour flight with no reserve. I'm not saying it couldn't be done, it would change the mission, no long cross country flights.
No matter how you proceed, conventional or electric, 50% or 75% this is a large project and I encourage you to get some additional hands working on it so you can have a shot at completion. The Bally B-17 took many years to complete and he had help.
Vince Homer
At 50% a tandem cockpit works well. The passenger's back is up against the main spar makingthe passenger/no passenger weight and balance a non issue. Somewhere around 55% scale even the nacelles are right for Verner or Rotec radials. A 75% scale version might use Continental 225 radials, although they are a bit heavy and quite dated. Unfortunately, there's not any new radials available in the 200 hp range.
I am currently involved with a project using electric and applying this experience to something like an all electric B-25 tells me it would be a very big development effort. The B-25 airframe isn't optimized for electric power, something alot more aerodynamic and single engined would seem more appropriate. To me the appeal of a B-25 was the possibility of two Verner or Rotec radials immatating a real B-25.
I had a look at the H3X website and their motors would be impressive. So would the batteries to power them at 200 kw each. At a very optiistic 300 wh/kg a 1 hour battery would weigh in the neighborhood of 1000 kg for a take-off and 3/4 throttle for an hour flight with no reserve. I'm not saying it couldn't be done, it would change the mission, no long cross country flights.
No matter how you proceed, conventional or electric, 50% or 75% this is a large project and I encourage you to get some additional hands working on it so you can have a shot at completion. The Bally B-17 took many years to complete and he had help.
Vince Homer
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I forgot to add this background material:
EpoxyCowboy
Member
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Actually the 75% with H3X motors was for the P-39. My bad for not being specific. My B25 was planned for 50% with Rotecs as Verner was unknown to me back then. If I ever restart the B25 project I would only change the engines for Verners… maybe and scale to 55%.
So what scale would you suggest for the 39?
So what scale would you suggest for the 39?
The P39 was such a small fighter, I don't see why you couldn't stay very close to full size and still have manageable dimensions, much like the Timber Tiger Ryan. Personally, I plan to draw it up at no less than 85%.
EpoxyCowboy
Member
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Ok then! If I go to 82%, it will be a good match for Corsair82. It would be nice to see them side-by-side. )))
Joseph Labert
Joseph Labert
Except the Airacobra starts out 17% smaller and 48% lighter than the Corsair so the two aren't really starting from the same place.
Yeah, but I don't even look at the weights. By the time you subtract the weapons load, the armor, the giant engine, the long rang internal fuel, these aircraft's weights are anybody's guess. I suppose you could look at what they weighed as Reno Racers, but I don't know how common that knowledge is.
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The P-39 mock-up I built was 70% with 4 inches added to the cockpit width and 2 inches to the height. I doubt anyone would notice the "Fattening" of the cockpit area. Once you got into the thing it was very roomy and the visibility was good. I also made the doors a little larger than scale, but they were still too small for easy entry and safe egress. If I were to do it again i would make the whole aircraft 75% and hinge the top of the canopy. Photo of the right door below.
The genset/battery with a nose motor approach would solve a lot of mechanical problems. Based on my recent experience with electric airplanes it would be very noce to find a company that could supply a turn-key system. I have made a couple P-39 systems, one in 1/4 scale, with the midship engine, drive shaft and nose mounted reduction unit. It turns out not to be an easy task. Things tend to want to shake apart, especially with a single cylinder, 2 stroke engine.
The photos below: The engine has a water cooled head with water pump and radiator from a computer CPU cooler. The exhaust is piped to dual six port headers, The dirve shaft is carbon fiber with RC truck universals on each ent. There's a cog belt reduction unit in the nose and provisions for an electric starter. The prop hub is a copy of a ful scale commercial one that i have and the blades were harvested from two blade war bird props. The pitch is ground adjustable.
Safe emergency egress is always an issue. The 50% B-25 with tandem seating worked well except you had to get into the thing from the top making inflight egress very tricky. The only solutioon I could see was to make the passenger seat fold up and put a hatch under it so you could get out through the belly in flight. This all hinges on wearing a slim backpack parachute, something I never do in my Sonex or Onex.
Vince Homer
The genset/battery with a nose motor approach would solve a lot of mechanical problems. Based on my recent experience with electric airplanes it would be very noce to find a company that could supply a turn-key system. I have made a couple P-39 systems, one in 1/4 scale, with the midship engine, drive shaft and nose mounted reduction unit. It turns out not to be an easy task. Things tend to want to shake apart, especially with a single cylinder, 2 stroke engine.
The photos below: The engine has a water cooled head with water pump and radiator from a computer CPU cooler. The exhaust is piped to dual six port headers, The dirve shaft is carbon fiber with RC truck universals on each ent. There's a cog belt reduction unit in the nose and provisions for an electric starter. The prop hub is a copy of a ful scale commercial one that i have and the blades were harvested from two blade war bird props. The pitch is ground adjustable.
Safe emergency egress is always an issue. The 50% B-25 with tandem seating worked well except you had to get into the thing from the top making inflight egress very tricky. The only solutioon I could see was to make the passenger seat fold up and put a hatch under it so you could get out through the belly in flight. This all hinges on wearing a slim backpack parachute, something I never do in my Sonex or Onex.
Vince Homer
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One way to look at aircraft weight is to assume a wing loading and work backwards from there. If you look back a few responses to my one staing the 75% B-25 would have a scale wing area of 343 ft2 and you assume a general aviation range for wing loading os 10 lbs/fts you get a 3400 lb airplane. A 75% P-39 would have a 120 ft2 wing and a gross weight of 1440 lbs at 12 lb/ft2. If you're willing to accept a higher stall/landing speed it could be considerably heavier.
Vince Homer
Yeah, that approach makes sense, and so, the actual weights of the original have little bearing on the replicas. I don't know that there's much to gain by greater weight. If MOSAIC changes the way the limits for LSA are figured, I think it opens the door for a lot of very capable designs approaching 1500lbs. There's always VGs to help mitigate increasing stall speed. My thoughts have always been to keep the aircraft comfortably within the skills of sport pilots. I don't want a large pucker factor for any replica I'd look at owning.
I really admire your work, you're an excellent experimenter! Do you ever wear a chute in a sport plane?
