Twin-VW engine Push-Pull design idea (The "Beetlemaster")

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,021
Location
USA.
Start to sound doable, one thing! important to have an airfoil with low drag at CL 1.0 for better SE climb.

Did a check with 2x D-motor 88 HP FP prop, some 60 lb lighter. so 1600 as I was
SE climb @ 80 MPH 480 fpm (710 with CS prop non fethered)
300 fpm @ 3400´
100 fpm @ 9400´

The climb propeller we pick for 2 Engine drive, will become a bit more then a speed propeller with one Engine, the software calculate a climb, std, Cruise and a speed pitch prop, it is about 2" increase in pitch between each.

The real installed Power with the Revmaster might be higher then we calculated with here, but better be safe now then sorrovw later.
But for sure we give up a lot of performance in both ends of the speed spectra with FP props, in this case.
The Cessna 337 had a slightly better single engine climb rate on the rear engine if I remember correctly.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,021
Location
USA.
Wing platform for my Beetlemaster . Span-- 34 ft
Cord at fuselage to boom --- 54"
Wing tip cord---- 42"
Total wing area ----- 140.25 sq ft
Wing taper at TE of wing from boom to tip.
By Vigilant1's fuselage dimensions that I am using, it puts the pilots eyes at the leading edge of the wing, not 6" forward as I was thinking before. This is with the TE of wing at the rear firewall.
 
Last edited:

autoreply

Well-Known Member
Joined
Jul 7, 2009
Messages
10,763
Location
Rotterdam, Netherlands
Okay, I understand now. Do you have any preferences on the length of the tail arm (acft CG to ruddervator center--now I've got it at 10 feet). Also, should I keep the total tail surface size the same as if it were a conventional tail, or do you believe the inverted V allows reduced size/wetted area? Actually, given the vertical stab "plates" at the end of the Cessna 337-style tail, Pops and I could probably reduce the size of the H-stab a little.
Just keep it the same (for both). V-stab will fly the same as a conventional tail if area is the same.

Note that for high spans the V-angle is going to be very small because you only need a small H-stab. Doesn't work very well, so just go for 90 degrees and accept that your horizontal projected tail is too big. That gives you plenty of CG range and makes flaps a no-brainer.
Agreed flaps are a must. I'm going to keep investigating possible use of split flaps. I know they aren't very popular on new designs, but they might be good here. They leave the top of the wing totally clean, there's no concern about air spillage through the gap, they generate lift/drag about like plain flaps at small deflections, and give more drag than plain flaps at high deflections which could be just what we need here. Bonus: easy to build (an inset flat surface) and the NACA graphs often show applicable data for one on the regular L/D depiction (I guess because it was easy to slap a wood plank at 60 degree deflection on the airfoil model that was already in the wind tunnel).
I'd go for plain flaps. At 90 degrees they're also pretty draggy. Gap sealing on a composite wing is a non-issue and I think building a low-chord flap (15-20%) is easier then split flaps.
What are you planning to do in there? Badminton? Okay. But next thing I'll hear is that you are certain you'll get laminar flow behind that propeller before it flows around that bus-like cabin.:)
Absolutely. But only outside of the BL on the fuselage.
Edited to add: Hmm--with the desire for the high inverted V tail and the wide cabin you could go with low booms from the bottom/near the bottom of your cabin. They'd clear the rear prop disk at that 50" width if they are down there.
Yes. That's the advantage of a pusher boomed tailed by the way. The spar is far enough aft that you can use a mid-wing. Great to have a horizontal mold split and mold the stub wings and tail in the fuselage mold.
You'll need hardpoints at the bottom of the pod edges anyway for the landing gear mains, they could be built into the booms. I know we said "no retractable gear" but it would be very tempting to build some retracts into those boom tubes as part of the cabin. Maybe semi-submerge the tires (in case of failure/forgetting to put the gear down the covers get scrubbed off but you still roll on the tires), and just have relatively short gear legs. Also, the booms/tubes provide additional crashworthiness/protection from intrusion of the cabin from the sides could even be extended firewall-to-firewall to stiffen things up. Probably metal thinking, but it's the only thinking I have . . .

Back to our regular programming.
I've run the trade-off for my own design. Retractable gear in the booms was the draggiest option. Retracting it under the cowling (C337) the next draggiest. Fixed, Wittmann-style gear with folding gear doors (Bede Speedpants like) was the least draggy option. Admittedly, that's a taildragger, nose-gears are draggy hogs.

Retracting the gears in the booms is also pretty complex structurally, whether it's composite or metal.

Why the Wittmann gear? Much simpler to build and probably lighter than your typical telescoping vertical gear legs.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
Correction-- 9.55 just didn't seem right so checked my math, Wrong. Vigilant1 was correct. Real answer is 8.5 ft or 102".
So reducing it down the 6" for road width does make sense. Think I can live with the difference in structure without to much change.
I thought maybe the 9.55 was to fit through your 10' wide shop door. :). With 8' wide booms and a 30" wide x 43" tall cabin, the angle of the lift strut with the side of the fuselage will be about 38 degrees (i.e. distance of 33" from cabin to top of strut on the wing, and a distance of 43" down to the bottom of the cabin). But if you are going to round the bottom edge of the cabin, the lower strut attach point would probably be higher up and so the strut would be closer to 40 or 45 degrees. Regardless, it would look fine.

Wing platform for my Beetlemaster . Span-- 34 ft
Cord at fuselage to boom --- 54"
Wing tip cord---- 42"
Total wing area ----- 140.25 sq ft
So, you'll have a straight wing for the small section inboard of the booms (where the fuel goes) and then taper outboard of that (like the C-337 does). That will look good. I'm thinking (tentatively) of avoiding that bend in the TE. For my spreadsheet, I guessed that your wing would be a 60" root and a 39" tip for the same 34' wingspan and a virtually identical 140.25 sq' area. I think it will be the same for our performance calcs, if you don't mind I'll keep the sheet as it is (the auto-math will get confused in figuring the aspect ratio if I put in your true root and tip chord due to the bend. I'm too simple and lazy to figure out how to put in the right formula). I haven't yet crunched the numbers on the wing volume between the cabin and the booms: I'd like to keep the fuel aft of the spar if possible. If necessary, maybe an aux tank forward of the spar that can be pumped over to the main tank after a few hours, carry gas in the forward tank when necessary. Details . . .
By Vigilant1's fuselage dimensions that I am using, it puts the pilots eyes at the leading edge of the wing, not 6" forward as I was thinking before. This is with the TE of wing at the rear firewall.
That's seems great. The rough weight and balance looked okay like that? Maybe my guess for the boom and tail weights were different (i.e. wrong).
 
Last edited:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,021
Location
USA.
I thought maybe the 9.55 was to fit through your 10' wide shop door. :). With 8' wide booms and a 30" wide x 43" tall cabin, the angle of the lift strut with the side of the fuselage will be about 38 degrees (i.e. distance of 33" from cabin to top of strut on the wing, and a distance of 43" down to the bottom of the cabin). But if you are going to round the bottom edge of the cabin, the lower strut attach point would probably be higher up and so the strut would be closer to 40 or 45 degrees. Regardless, it would look fine.


So, you'll have a straight wing for the small section inboard of the booms (where the fuel goes) and then taper outboard of that (like the C-337 does). That will look good. I'm thinking (tentatively) of avoiding that bend in the TE. For my spreadsheet, I guessed that your wing would be a 60" root and a 39" tip for the same 34' wingspan and a virtually identical 140.25 sq' area. I think it will be the same for our performance calcs, if you don't mind I'll keep the sheet as it is (the auto-math will get confused in figuring the aspect ratio if I put in your true root and cord due to the bend. I'm too simple and lazy to figure out how to put in the right formula). I haven't yet crunched the numbers on the wing volume between the cabin and the booms: I'd like to keep the fuel aft of the spar if possible. If necessary, maybe an aux tank forward of the spar that can be pumped over to the main tank after a few hours, carry gas in the forward tank when necessary. Details . . .

That's seems great. The rough weight and balance looked okay like that? Maybe my guess for the boom and tail weights were different (i.e. wrong).
I was thinking of the wing platform that you stated, 60" cord and 39" tip cord for about the same area but that would put the leading edge of the wing forward and the pilots head behind the LE of the wing. I want to keep the pilots eyes forward of the LE if I can. So I reduced the root cord to the 54".

For the final W&B , I would use the 31" rear engine dimension, but do a W&B to work out the length of the front engine mount to put the CG where I want. I have done this in the past with good success.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,021
Location
USA.
I was thinking of the wing platform that you stated, 60" cord and 39" tip cord for about the same area but that would put the leading edge of the wing forward and the pilots head behind the LE of the wing. I want to keep the pilots eyes forward of the LE if I can. So I reduced the root cord to the 54".

For the final W&B , I would use the 31" rear engine dimension, but do a W&B to work out the length of the front engine mount to put the CG where I want. I have done this in the past with good success.
You will be very close on the angle of the strut. I'll be putting the fuselage attach point as low as possible from attach fitting welded to the lower fuselage longerons along with the heavier cross tube sized for the load.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
Start to sound doable, one thing! important to have an airfoil with low drag at CL 1.0 for better SE climb.
Yes, like the prop tradeoff/compromise, we need a wing airfoil that gives us safe performance when single-engine, but with as much performance as we can get consistent with that for the 99+% of the time we'll be flying with 2 good engines. We did touch on the airfoil selection issue before (Here, among other places). Based on successful use on other planes with similar profiles, I mentioned the NACA 24XX, the NACA 63A4xx, and the NACA 230XX. IIRC, Autoreply was in favor of waiting to work on the airfoil choice (until more was known about the reqmt). Pops likes the 24XX airfoil: As far as I can tell its biggest down-side is that it's not a modern airfoil highly optimized for low drag at targeted CLs. And that's also (IMO) its biggest advantage: no harsh stall, no drag bucket that can give surprising, non-linear performance changes if AoA or airspeed is changed a little bit. The surface doesn't have to be perfect to generate the expected results. I think dull is okay in this area, but I'm sure others can make a very good case for something else.

If our target single-engine climb airspeed is 70 Kts, the required Cls look something like this:
Pops: 1500 lbs: .66 1700 lbs: .74
Autoreply: 1500 lbs: .73 1900 lbs: .92
Vigilant1: 1500 lbs: .73 1700 lbs: .82

At 130 kts (cruise speed? I'm not sure, depends on what thrust the 48" x 57" prop gives us at higher speeds), the required Cl would be
from .22 (Pops at 1500 lbs) to .32 (Autoreply at 1800 lbs).
Obviously, if we let the plane get slow the required Cl for level flight could go all the way up to the Clmax of the airfoil
For thought: Here's what Theory of Wing Sections says about the Cd of a few airfoils at .7 and .9 Cl (SE climb) and CL .25 (approx cruise):
(Approx Re 3 million)
........................Cl .7........Cl .9.......Cl .25 Notes
NACA 2412:....... .007 ... .008 ..... .006 2D Clmax = 1.6 (at 16 deg AoA). Smooth Cl/Cd curve. Cessna piston GA, Buttercup, many others.
NACA 2415:..... .007 ... .008 ..... .006 2D Clmax = 1.4 (at 14 deg AoA). Smooth Cl/Cd curve.
NACA 23015:.... .0085.. .010....... .007 2D ClMax = 1.5 (at 15 deg AoA). Van's, Aerocommander Shrike

Did a check with 2x D-motor 88 HP FP prop, some 60 lb lighter. so 1600 as I was
SE climb @ 80 MPH 480 fpm (710 with CS prop non fethered)
300 fpm @ 3400´
100 fpm @ 9400´
D-Motors would be interesting, but what would you call the plane then:)? I think we could have adjustable pitch props for our 80 HP Revmaster for the price of a D-motor--with no prop. I like the idea behind them for our application (flathead, side valve, etc), I wish I knew more

The real installed Power with the Revmaster might be higher then we calculated with here, but better be safe now then sorrovw later.
Yes, it's best not to count on every HP in the brochure. Better a nice surprise than an unhappy one.

But for sure we give up a lot of performance in both ends of the speed spectra with FP props, in this case.
As a prop guru/perfectionist, it has got to bother you to see all the performance we're missing due to these compromises. But there's this: If a person starts with Revmasters and inexpensive FP props, and if our estimates are close, they'll have a good, fun, safe airplane. And, when they can/want to spend more, or other options become available, they can swap out the props. Their engine investment is still good.

About flight at cruise: if we are running at high RPM but low load/manifold pressure because of the low pitch needed to get good SE thrust at 70 knots, then the engines will be happy. Sure, they are loafing, we are underpropped and they could be making more power. But they are sipping fuel, the heads are cool, the loads on the bearings are low, etc. The plane might need cowl flaps that can be closed to keep the engine temps comfortably warm at cruise.

Yes and no, this is with the added drag from stopped propeller, others it will do some 135 mph.
interesting, in the prop soft, I have laminar and turb airfoils, (equations that change drag with CL) with turb the total Fo goes from 3.2 to 3.6, max theoretic speed goes from 175 to 170 mph
Does the software tell you the expected thrust from the 70 HP engine with a 57" x 48.52443" prop at those higher speeds? 170/175 MPH would be very nice with fixed pitch props that also give acceptable single-engine performance. Roughly speaking, it looks like these designs would require about 150-200lbs of thrust (75-100 lbs per engine) to cruise at 130Kts at 6K' MSL.
 
Last edited:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,021
Location
USA.
"Pops likes the 24XX airfoil: As far as I can tell its biggest down-side is that it's not a modern airfoil highly optimized for one thing. And that's also (IMO) its biggest advantage: no harsh stall, no drag bucket that can give surprising, non-linear performance changes if AoA or airspeed is changed a little bit. The surface doesn't have to be perfect to generate the expected results. I think dull is okay in this area, but I'm sure others can make a very good case for something else".

I spent a year trying to decide on the airfoil for the JMR. I kept coming back to the 24XX . It does not excel in any direction but does everything very well. Bob Barrows kept trying to talk me into a Riblett for the JMR but I stayed with the 2414. The LSA Bearhawk does very, very well with the Riblett airfoil. Bob also uses the Riblett on the Patrol and went to the Riblett on the new 4 seat Bearhawk "B" model, and I think he gained about 5 mph in cruise over the old airfoil, don't know about any other changes.
 

Jan Carlsson

Well-Known Member
Joined
Jan 11, 2009
Messages
1,860
Location
Sweden
Good Morning, you have been busy when I was napping.

First criteria for the airfoil is how is the wing built? will the Surface be good enough for a laminar work?

thin metal or fabric, I see no Point in having a laminar airfoil. with thick metal or Wood, and Composite yes.

I was into the riblett for other Project, looking into the GA-30-412 Riblets "4" is here the design CL rather then the camber, this one is 3.3% at 45% c
it is Close to N 3412 but with less moment, others they are very Close. so riblett wins, those i picked just to compeere. what camber is need is still to determine.
the C-master have 2412 and 2409 with 2deg washout.

the 24xx will be Close to the 34xx and have less moment then the GA riblett -412 I guess a mix of 24xx and 34xx will have the same moment as the riblett.

a check with performance number with the moment load at the tail and wing, with induced drag on both wing and tail, in will help pick a airfoil.

The max speed with FP props is regulated by the pitch and rpm, it will not go much faster then the pitch*rpm even with light load. there is some more pitch Before zero lift line, say there is 6 degree to zero lift line, maybe (i have not checked) we can come to half way there on light load with 2 Engines. so 3 deg more, that is 7" on a 57" prop.
so 48.5 + 7 = 55.5"

max continuos rpm beeing 3050 so 55,5/12x3050/88=160 mph =139 kts
48,5 will be some 140 mph = 121 kts

If we can do 170 mph with a turb airfoil and 150 HP, we will need 83,3% 125 hp at 160 mph. (3050/3350= 91%) so throttle back.
at 140 mph 55,85% = 84 HP
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
Good Morning, you have been busy when I was napping.
Busy, sure. But as my wife would tell you, that's not the same as "productive."

The max speed with FP props is regulated by the pitch and rpm, it will not go much faster then the pitch*rpm even with light load. there is some more pitch Before zero lift line, say there is 6 degree to zero lift line, maybe (i have not checked) we can come to half way there on light load with 2 Engines. so 3 deg more, that is 7" on a 57" prop.
so 48.5 + 7 = 55.5"

max continuos rpm beeing 3050 so 55,5/12x3050/88=160 mph =139 kts
48,5 will be some 140 mph = 121 kts
One of the good things the Revmaster (and other VW derivatives) get from their automotive ancestors (in addition to low price) is a short stroke. Even the R2300 has a stroke of just 84mm, which results in piston speeds 15% less than an O-200 or O-235 at the same RPM. Revmaster gives their "cruise RPM" as "3000 plus or minus 200," and sets a redline at 4200 RPM. So, running at 3200 RPM (remember the engine is lightly loaded approx 40-60 HP each) shouldn't present a problem for the engine. For the prop, the helical tip speeds for a 57" prop at 3200 RPM would be:
130 Kts (150 MPH): Helical tip speed= 865 FPS______Mach .77 at SL_____Mach .79 at 6K' MSL (std temps)
150 Kts (173 MPH): Helical tip speed= 874 FPS _____Mach .78 at SL _____Mach .80 at 6K' MSL (std temps)

Would that be okay for a carbon fiber covered, wood core prop? Based on your 48.5" pitch and estimated zero-lift estimates above at 3050 RPM, going to 3200 RPM would give us about 6 to 7 more MPH/knots before we run out of prop.

If we can do 170 mph with a turb airfoil and 150 HP, we will need 83,3% 125 hp at 160 mph. (3050/3350= 91%) so throttle back.
at 140 mph 55,85% = 84 HP
84 HP = approx 7 GPH for 140 MPH. Not bad, if it can be done.

Thanks for your patience.
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
It seems likely that, even with fixed pitch props, the pilot of a plane like the Beetlemaster would probably want manifold pressure readouts. The engines will be at high RPM/low load in cruise, so just using RPM won't be a good way to know what's going on with the engines. And/or fuel flow sensors are pretty cheap these days, they'd also provide handy information.
 

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,021
Location
USA.
It seems likely that, even with fixed pitch props, the pilot of a plane like the Beetlemaster would probably want manifold pressure readouts. The engines will be at high RPM/low load in cruise, so just using RPM won't be a good way to know what's going on with the engines. And/or fuel flow sensors are pretty cheap these days, they'd also provide handy information.
An experiential airplane should have a MP gauge to really know the load you are putting on the engine at different RPM's. I even had a 1956 Cessna 172 that came from the factory with a MP gauge. I think some of the Piper Tri-Pacers also came from the factory with MP gauges. Great Plains also recommended MP gauges with VW engines and cruise the engine at 22".
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
Jan, I finally had time to give your posts the attention that they deserve. Thanks again for putting the data through your software. Some questions:

Your Post 242 below (with the thrust, ROC and drag curves from your software): That was for 150 HP, and two 59" by 40" FP props, correct? The thrust curve for the output is not for two 57" x 48.5" props right? I think we made the prop change later.
About your observations below: Your software was showing that two 70 HP engines using the fixed-pitch 57" x 48.5" " JC Special Beetlemaster" props would push/pull a plane with a drag area of 3.6 sq ft to a speed of 170 MPH (148 Kts). If I've got that right, and if we disregard induced drag (which is negligible at these speeds), then the total thrust (both engines/props) needed to overcome that drag is 266 lbs if we're at SL. Do you recall if the prop software inputs were for sea level, or was it at higher altitude?
Yes and no, this is with the added drag from stopped propeller, others it will do some 135 mph.
interesting, in the prop soft, I have laminar and turb airfoils, (equations that change drag with CL) with turb the total Fo goes from 3.2 to 3.6, max theoretic speed goes from 175 to 170 mph
And the conditions were the same (prop/altitude etc) for the single-engine figures below, correct?
some thrust nr's at 70 HP and 57" prop
Mph - Lbs
65 = 217
70 = 213
75 = 209
80 = 205
90 = 197
100 = 189
110 = 182
120 = 175
Bad news: Snow is coming to my town on Monday. I'll have noting to do but shovel the driveway and work on crazy Beetlemaster ideas.:)
 

Jan Carlsson

Well-Known Member
Joined
Jan 11, 2009
Messages
1,860
Location
Sweden
Morning,

A. those propps for 170-175 mph need to be in 55" or so in diameter, but you would not get there with 57x48 props.
B. SL and 2 x 75 HP with the diameter it take. (the software don't like the props if they are Close to 60" for strengt reasons) (autocorrection think Close is a name?)
C. single Engine 70 HP 57"

All SL

Jan
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
Jan,
It's clear that we are asking the prop software to do something unusual. Rather than the normal "Here's my airplane, here's what I want to optimize for (climb/cruise/speed, etc), now figure out the best prop and show me the expected performance.) Instead, we have to go at the problem a bit sideways. Thanks for your the help!
A. those propps for 170-175 mph need to be in 55" or so in diameter, but you would not get there with 57x48 props.
B. SL and 2 x 75 HP with the diameter it take. (the software don't like the props if they are Close to 60" for strengt reasons)
C. single Engine 70 HP 57"

All SL

Jan
Dang. I had hoped our 57"D x 48.5"P prop was giving us the very nice thrust numbers for 60-70 Kt (70-81 MPH) climb (approx 210 lb each engine/prop) and 150 Kt (173 MPH) cruise (approx 133 lb each engine/prop).

(autocorrection think Close is a name?)
Maybe you have the Hollywood dictionary? Glenn Close
Still, with over 200K words in English, you'd think we wouldn't need to re-use them.
 
Last edited:

Jan Carlsson

Well-Known Member
Joined
Jan 11, 2009
Messages
1,860
Location
Sweden
To go 175 mph with 48.5" pitch we need some 3800 rpm.
Tip speed 979 ft-sec.
it will not be safe doing so with a Wood propeller.

173 mph and 3350 rpm 56" is max Before the software redflag for flutter (to narrow blade) pitch is 54,5"
Time for a modern aeromatic, the aeromaster prop?
 
Last edited:

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
Time for a modern aeromatic, the aeromaster prop?
Some thoughts:
A. At the end of the day, I would not be dissatisfied with cruise speeds of 150 MPH (130 Kts) if that's the limit with affordable props that can also produce safe climb on one engine. And if we can get that, then additional investments could be considered.
B. Adjustable props: According to their literature the Revmaster engines were designed for propellers weighing up to 25 lbs, and successfully flew many hours with the oil-pressure activated Maloof inflight adjustable props of 14-25 lbs without any crankshaft or bearing problems (the propellers themselves apparently did have some problems). I don't know what the inertial loadings were for these propellers, but the 25 lbs is a data point.
- Aeromatic/Aeromaster: It's a small company, I don't think they have the R&D resources to develop a new prop just for the VW. Also, their design depends on weights in the prop hub to automatically adjust the blades in flight (the Aeromatic for the O-200 weighs about 34 lbs). It might be a challenge to design one with a weight/ low enough for the Revmaster, but it might be possible. A big project, but if the price were right it would make for a lot of happy VW (and Rotax) pilots.
- Ivoprop makes an electric inflight-adjustable prop. As far as I know it is unique in that the root of each blade doesn't twist (it's firmly bolted to the hub. The hollow blade twists by means of an internal steel torsion rod attached to the tip which is twisted by an actuator in the hub. The model that they claim is good up to 140 HP has prop blades from 52"-74" diameter with pitches from 30"-90". Weight for the 2-blade is 15.7 lbs (and it looks like a lot of that is near the hub, which is good) and cost is $2460 USD. For $350 additional, they even sell a controller that makes them behave like a CS prop. I have no idea how reliable these props would be, the hollow blade idea is a bit . . . . scary. With a twin engine plane, the failure of the pitch adjustment mechanism wouldn't be a catastrophe, but a thrown blade would be a whole different matter. As I say, I know nothing about them, maybe they are as reliable as clockwork. Among those who know about props, is there a consensus about their reliability/safety?
C. "Almost-fixed pitch?": As mentioned previously, Prince props make a P-tip solid prop (carbon-fiber wrapped wood) that they claim flexes under load (as when the prop blades are "coning" during takeoff and climb) to reduce the effective pitch, but returns to the original higher pitch at cruise. They claim the pitch change is equal to about 4" in general but I'd believe it is less for our relatively short prop. Prince has a long involvement with the VW engine community and a good reputation. A 55"D carbon-fiber wrapped prop for the Beetlemaster would cost about $1200 USD and weigh about 5 lbs.

For now, unless there are dissenting views, I'll try to pin down what is possible with regular fixed-pitch props. Specifically, if we know we need about 205 lbs of single-engine thrust at 70 knots to generate safe climb on one engine (ref post 278), what's the prop that gives us that and also crosses our "required cruise thrust" line (future post) at the highest airspeed. That info or a way for me to figure it out is probably already in your posts. I'll put on my thinking cap (dunce cap? tin foil hat?) and see what comes of it.
 
Last edited:

Pops

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 1, 2013
Messages
9,021
Location
USA.
150 mph is good with me. Same as the cruise in my F-12.

I have the sizes of the verticals and horizontal worked out. Horizontal tail coefficient -- .52
Span of the horizontal is limited to the 8' from boom to boom. 11' from wing trailing edge to stab leading edge.
Vertical tails --------.045
Total length of airplane less spinner is 24.55 ft.
I wonder how close everyone gets.
Dan R
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
6,044
Location
US
I have the sizes of the verticals and horizontal worked out. Horizontal tail coefficient -- .52
. . . . Vertical tails --------.045
Dan,
Using your wing and your desired H-tail coefficient of .52, and a 14' lever arm (your 11' from TE of wing to front of H-stab + approx 2.5' from TE to CG + plus approx 1' from LE of H-stab to MAC of H-stab), I figured an H-stab+elevator size of 21 sq ft. Close to what you got?
Using the same method for the V-stab and rudders and your desired Vertical tail coefficient, I came up with 15.3 sq ft. In the ballpark?

I'll figure my numbers later, they'll be close to this. I was thinking theoretically the H-stab could probably be a bit smaller than the standard Raymer formula because of the endplates formed by the V-stabs and rudders. But it's always good to have a little bit extra pitch authority especially in a one-off with some unknowns.

25 feet long: Just a foot longer than a C-152 and your wings are just a foot longer. With wings folded back at the spars, they'll still be a bit shorter than than the tail. The whole thing would fit in a standard 40' long shipping container with room for parts and a workbench in the front.
 
Top