# Yet another VW power argument. Again.

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#### Klrskies

##### Member
I like the D Motor engines. I wonder if a flat head design that has more cooling capacity has been tried on a big VW engine? They need to worry about proper air flow thru the head with liquid cooling, but it has added weight and complexity, and the radiator(s) have to go somewhere. It's an impressive engine though. The big vw's need to be able to cool 20 hp / cylinder continuously.
Ken

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#### Topaz

##### Super Moderator
Staff member
Log Member
Not quite what I was referring to. I used to check/set the .006 valve lash at every oil change. The concerns that many are having with the Aerovee are valve warping/leaking and thus the need to pull the head and lap them back in if caught early enough. A few that didn't turn the prop over by hand now and again to check the health of the engine (mags and fuel OFF!) sometimes were surprised at annual at some really poor numbers.

The other concern is that using the big valve heads (I'm not certain that at prop RPMs these are needed) leaves little room between the valves and cracks appear pretty early in the life of the heads. Here's my thoughts on another site:

SonexBuilders.net View topic - End Play when hand turning prop

Dale
N319WF
Eeeesh. That sounds ugly. Not what I was expecting. But, then again, AeroVee is pushing the engine pretty hard, and that reflects in their own TBO numbers.

#### Aesquire

##### Well-Known Member
That doesn't look right. Shouldn't the torque & H.P. curves cross at 5252 rpm? ( in other words they shouldn't cross at all on that chart )

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
That doesn't look right. Shouldn't the torque & H.P. curves cross at 5252 rpm? ( in other words they shouldn't cross at all on that chart )

Only if the HP and torque scales on the left and right are the same. At the crossing point shown, 116 lb-ft * 3,675 rpm / 5252 = 81 hp.

#### Vigilant1

##### Well-Known Member
Part of the problem is that Sonex (and KR-2, etc.) has relatively stubby wings, meaning the airplane needs greater installed power to develop a decent climb rate. For speed at the top end, the wings are a plus, but in climb the airplane needs longer wings if a smaller engine is going to be used. Something more like a Aeronca C-3 or a motorglider would climb like the proverbial homesick angel on 65+hp, but would be slower on the top end.
While a CX5 may be "flyable" at gross wt. with the Revmaster engine, flyable doesn't always equal "enjoyable." I have flown some airplanes that were downright scary because of deficient performance.
I'm not as pessimistic regarding the prospects of adequate CX5 performance with the R2300 powerplant. Per Topaz's observation, good climb performance at relatively high wt/hp ratios requires low span loading. The CX5 has that.
For reference:

C-150........... 16.0 lb.hp....................48.19 lb/ft
C-152........... 15.18 lb/hp................. 50.14 lb/ft
CX5 ............. 15.29/16.5 (85/80hp)....47.14 lb/ft

Now, nobody ever said the C-150 and C-152 were rockets, but the FAA says they climb well enough at MTOW to be safe, and they've flown at/near (over) MTOW by flight schools all over the country for decades. So, I think the CX5 just might do as well at MTOW--it seems reasonable.

Now, what about the majority of flying--flying one-up (200 lb pilot) with 3 hours of fuel? Let's look at some numbers:

CX5..(80 HP)...........1011/80 = 12.64................1011/28= 36.11............. 8.02
C-152..................1398/110 = 12.7...............1398/33.3 = 41.98............ 8.73

This is a C-152 flown solo with less than 20 gallons of fuel (tanks less than 1/2 full)--that's a plane that climbs and cruises just fine. The CX5 looks to be better. I think it will be a fun cruiser.

I do think the CX5 should be scaled just enough to accept an 0-200. That along with conventional landing gear would make it a winner!
The conventional gear plans are available, maybe the next one flying will be a taildragger. While an O-200 version would be fun (might need to scoot the wing forward), a remanufactured O-200 costs more than what many builders of these airplanes probably hope to finish the entire plane for. Sure, they could try to find a used one or build one up from bits, but used O-200s are not nearly as available or cheap as they used to be. Writing a check to Revmaster for $8500 and having them deliver an entirely brand-new and factory assembled/test run engine a few weeks later is very appealing. And when it's time, owner can buy maintenance parts for a lot less than airplane-engine prices. #### mcrae0104 ##### Well-Known Member HBA Supporter Log Member Thanks for sharing that link, Dale. Interesting that of the three heads shown (CB044, Jabiru, and Corvair), the Jabiru appears to be the only one without a squish area. #### Topaz ##### Super Moderator Staff member Log Member I'm not as pessimistic regarding the prospects of adequate CX5 performance with the R2300 powerplant. Per Topaz's observation, good climb performance at relatively high wt/hp ratios requires low span loading. The CX5 has that.... I'm sure your math is fine. I just get a little twitchy when people start talking 80+ continuous horsepower out of a VW. Maybe this is me being conservative and old-fashioned, but the track record of VW's in that power range has never been particularly good. Maybe Revmaster has got a formula now that makes it possible; as I said, I'm not at all familiar with their newer products, so I'm hardly in a position to say anything with regard to their R2300. I'm not saying, "It's not gonna work." I'm just going to maintain a healthy skepticism until there's a longer track record with a larger sample of flying examples. #### Kyle Boatright ##### Well-Known Member I'm not as pessimistic regarding the prospects of adequate CX5 performance with the R2300 powerplant. Realize the C-15X is turning a prop at <2500 rpm on climbout and that prop is substantially longer than the prop on a CX5. Long props turning slowly generate low speed thrust. Little props turning faster deliver less thrust. #### Pops ##### Well-Known Member Log Member I'm not as pessimistic regarding the prospects of adequate CX5 performance with the R2300 powerplant. Per Topaz's observation, good climb performance at relatively high wt/hp ratios requires low span loading. The CX5 has that. For reference: At Max Gross: Power Loading...........Span loading C-150........... 16.0 lb.hp....................48.19 lb/ft C-152........... 15.18 lb/hp................. 50.14 lb/ft CX5 ............. 15.29/16.5 (85/80hp)....47.14 lb/ft Now, nobody ever said the C-150 and C-152 were rockets, but the FAA says they climb well enough at MTOW to be safe, and they've flown at/near (over) MTOW by flight schools all over the country for decades. So, I think the CX5 just might do as well at MTOW--it seems reasonable. Now, what about the majority of flying--flying one-up (200 lb pilot) with 3 hours of fuel? Let's look at some numbers: ........................Power loading (lbs/hp)..............Span loading...........Wing loading (lb/sf) CX5..(80 HP)...........1011/80 = 12.64................1011/28= 36.11............. 8.02 C-152..................1398/110 = 12.7...............1398/33.3 = 41.98............ 8.73 This is a C-152 flown solo with less than 20 gallons of fuel (tanks less than 1/2 full)--that's a plane that climbs and cruises just fine. The CX5 looks to be better. I think it will be a fun cruiser. The conventional gear plans are available, maybe the next one flying will be a taildragger. While an O-200 version would be fun (might need to scoot the wing forward), a remanufactured O-200 costs more than what many builders of these airplanes probably hope to finish the entire plane for. Sure, they could try to find a used one or build one up from bits, but used O-200s are not nearly as available or cheap as they used to be. Writing a check to Revmaster for$8500 and having them deliver an entirely brand-new and factory assembled/test run engine a few weeks later is very appealing. And when it's time, owner can buy maintenance parts for a lot less than airplane-engine prices.

I hate an under powered airplane. For me, CX5 would need 100 HP.

The JMR Special under the same conditions:
Flying with 200 lb pilot , with 3 hrs fuel..

Cont C-85---- 10.47 ----------- 31.50 -------------- 7.95

Dan

#### Daleandee

##### Well-Known Member
Thanks for sharing that link, Dale. Interesting that of the three heads shown (CB044, Jabiru, and Corvair), the Jabiru appears to be the only one without a squish area.
That photo is a bit deceiving in regards to the squish area. Try this one:

Dale
N319WF

#### Vigilant1

##### Well-Known Member
Realize the C-15X is turning a prop at <2500 rpm on climbout and that prop is substantially longer than the prop on a CX5. Long props turning slowly generate low speed thrust. Little props turning faster deliver less thrust.
True, the Cessnas can be expected to be getting more efficient thrust at low speeds. If angle of climb is important, then slower speeds/higher deck angles are called for and the Cessnas can be expected to more efficiently convert the HP available to climb. If rate of climb is the metric of choice, CX5 can use a lower deck angle and reduce the efficiency gap (and increase engine cooling in the bargain).

#### Turd Ferguson

##### Well-Known Member
I'm not as pessimistic regarding the prospects of adequate CX5 performance with the R2300 powerplant. Per Topaz's observation, good climb performance at relatively high wt/hp ratios requires low span loading. The CX5 has that.
For reference:

C-150........... 16.0 lb.hp....................48.19 lb/ft
C-152........... 15.18 lb/hp................. 50.14 lb/ft
CX5 ............. 15.29/16.5 (85/80hp)....47.14 lb/ft

Now, nobody ever said the C-150 and C-152 were rockets, but the FAA says they climb well enough at MTOW to be safe, and they've flown at/near (over) MTOW by flight schools all over the country for decades. So, I think the CX5 just might do as well at MTOW--it seems reasonable.
All that's assuming the R2300 can convert it's available brake HP into thrust HP as efficiently as the C-150. I'll standby for real world numbers.

#### Vigilant1

##### Well-Known Member
I hate an under powered airplane. For me, CX5 would need 100 HP.

The JMR Special under the same conditions:
Flying with 200 lb pilot , with 3 hrs fuel..

Cont C-85---- 10.47 ----------- 31.50 -------------- 7.95

Dan
That oughta get off the ground in a jiffy--and climb out of those WV "hollers" without a problem.

#### BJC

##### Well-Known Member
HBA Supporter
The standard propeller on the C152 is poorly designed, plus it is sized to limit WOT RPM to 2550. Changing to a propeller that allows 2800 RPM makes a notable difference in performance, although it still is less than spectacular.

BJC

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#### Aesquire

##### Well-Known Member
KITPLANES The Independent Voice for Homebuilt Aviation - Pondering Powerplants - KITPLANES Magazine Article

"Because of the limited displacement, all VW-based engines produce power in rpm ranges higher than typical direct-drive engines, but lower than gear-reduced engines like the Rotax. Typically this is around 3500 rpm—a rather awkward spot from the propeller point of view. Props running at this speed (on direct-drive engines) must be quite short to prevent tip speeds that lose efficiency and create a lot of noise. Reduction drives are possible—both gear and belt drives are available—but they add weight and complexity. In rough terms, short, high-rpm props work well enough on fast, slippery airframes, and long, low-rpm props are better for slow, draggy (or STOL) designs. That’s why you’ll see direct-drive engines on airplanes such as the KR-2 and reduction drive units on WW-I replicas...."

So, better cooling & prop match for faster aircraft. Hmm.

Thanks Mcrae, Makes sense. Mental note: be careful reading graphs on phone...... better make that all the time.

HBA Supporter
Log Member

#### Pops

##### Well-Known Member
Log Member
That oughta get off the ground in a jiffy--and climb out of those WV "hollers" without a problem.

That's the idea. Plus if I need more, install a 0-200 crank, etc. For a power to weight of 8.9

Dan

#### nheistand

##### Member
KITPLANES The Independent Voice for Homebuilt Aviation - Pondering Powerplants - KITPLANES Magazine Article

"Because of the limited displacement, all VW-based engines produce power in rpm ranges higher than typical direct-drive engines, but lower than gear-reduced engines like the Rotax. Typically this is around 3500 rpm—a rather awkward spot from the propeller point of view. Props running at this speed (on direct-drive engines) must be quite short to prevent tip speeds that lose efficiency and create a lot of noise. Reduction drives are possible—both gear and belt drives are available—but they add weight and complexity. In rough terms, short, high-rpm props work well enough on fast, slippery airframes, and long, low-rpm props are better for slow, draggy (or STOL) designs. That’s why you’ll see direct-drive engines on airplanes such as the KR-2 and reduction drive units on WW-I replicas...."

So, better cooling & prop match for faster aircraft. Hmm.

Thanks Mcrae, Makes sense. Mental note: be careful reading graphs on phone...... better make that all the time.
To understand the air-cooling challenge, look at the Jabiru engines that have engineered good airflow around the critical areas. A VW install could do the same thing for reasonable cost.

Norm Heistand

#### mcrae0104

##### Well-Known Member
HBA Supporter
Log Member
Yes, good airflow is essential, but so is adequate fin area to do the deed of heat transfer.

#### HapHazard

##### Well-Known Member

More interesting is the reactions from a lot of readers who are all hooked up on how much HP VW’s can produce. I have some experience with the humble VW, but it isn’t in the same league as some of you out there. What is clear from my experience that there are limitations to the engine in producing sustained power and it isn’t good at soaking up excess heat.

Bob Hoover was right and incredibly articulate in explaining why he wouldn’t, and couldn’t, commit to high end sluggers poking out 75hp continuous. He had learnt from experience and understood the VW for what it is and its limitations and the trade-offs in trying to push the thing. Kirskies (Ken) also makes some interesting points which should go towards our understanding of some of the VW limitations . In the engine world, you don’t get energy for free, there’s always a price to pay!

Others however, see only figures. First and foremost, get over HP. It isn’t the be all and end to the argument (and is only part of the equation of other very important considerations), because cars are not aircraft and vice-versa. Sure a VW can be modified to produce way above 100hp, but not for sustained periods. Also there is a trade-off, higher RPM’s, lots of heat and short engine lives, virtues that have little attraction to aircraft.

Now take a moment and consider your propeller maker. Ask him what he would like in a real world, and now you have a formula for an engine that the VW was never designed for, with torque curves peaking at lower RPM ranges…..now you begin to understand where Bob Hoover was coming from. If you want an aircraft with higher end power from a VW, then forget direct drive….but like all things evil, there is yet another trade-off with reduction drive, that old enemy of homebuilders, weight!

The VW is a great little engine, if you use it for the right aircraft. Revmaster, etc, have developed it to its limits, but if you are after a serious 2 seat aircraft engine, then make another choice. If however, you are after a more sedate 40-50hp, then it’s a lovely little thing (albeit a bit porky on weight). It has quite happily powered single seat lovelies like the Turbulent, D9, Fred, Corby Starlets and Taylor Mono’s (etc, etc) for decades and continues to do so. I would argue that no other auto engine has ticked so many boxes for the homebuilder, but understand it for what it is.

My upcoming project flew on just 18hp with a Douglas engine, with more powerful examples flying on Sarolea’s at around 30hp (no doubt around 26hp cont), so the VW is a good contender and allows me stacks of leeway for experimentation regarding set up and torque curves. The only thing that prevents it being perfect for my case, is its dreaded weight with around 65-70kg to contend with against the originals 40kg.

If however I needed an engine of around 80-100hp, the VW wouldn’t be my choice, I’d definitely be looking elsewhere.

Moral of the story, understand the beast for what it is, not what you want it to be…..