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Turbo Subaru RV-8 Flies in Australia

rv6ejguy

Well Known Member
geoffrv8_zpspoay9ha4.jpg


Geoff Braddock flew his EJ257 powered RV8 on Dec. 27th and reports all went smoothly.

Empty weight was 1187lbs. He's using an Autoflight H6 300 gearbox with 1.95 ratio and an IVO 68 inch in-flight adjustable prop.

A few initial numbers: 2500fpm climb at 90 knots with full fuel running 5500 rpm and about 44 inches. Cruise at 4500 MSL, 4500rpm and unspecified MAP was 170 KTAS.

Seems it may be under propped a bit so he probably will be going up in diameter in the future.

Cooling was reported excellent with OATs of around 30 to 35C- never exceeded 80C.

Geoff did a nice job fitting the Subaru Sti engine and under wing rads complete with proper scoops and exit doors. Looks a bit Spitfire like. The intercooler is fed from the enlarged duct under the spinner.

Will be interesting to see more flight test numbers as he gets more hours on the aircraft.

This is pretty much the recipe I'd use today if I was doing another RV using an alternative engine.

Congrats Geoff, on completion of a challenging build!
 
Congratulations Geoff!
Wish you great success with your installation!

Ralph Inkster
Builder- RV7 Turbo STI C-FKRZ
- & others with varied Lycoming combinations
Calgary Alberta Canada
 
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Underwing Rads

Ross: Are there any pictures available that you could post showing how he plumbed his under-wing radiators? It seems that would be a great way to go; but I'm curious how he's moving the fluids all the way from the engine, through the cockpit and out to the wings...

It also will be interesting to hear how well he likes the IVO, since some of our Mazda rotary guys have reported they can't achieve VAN's top speed numbers with the IVO.

Doug Lomheim
RV-9A Mazda 13B/ FWF
RV-3A sold
 
Ross: Are there any pictures available that you could post showing how he plumbed his under-wing radiators? It seems that would be a great way to go; but I'm curious how he's moving the fluids all the way from the engine, through the cockpit and out to the wings...

It also will be interesting to hear how well he likes the IVO, since some of our Mazda rotary guys have reported they can't achieve VAN's top speed numbers with the IVO.

Doug Lomheim
RV-9A Mazda 13B/ FWF
RV-3A sold

I can see if I can find his Photobucket link at work today- that showed some rad details. He's just running heater hose to each rad

I have the IVO too. You must use the coarse pitched version on an RV (not the standard pitch version) and you don't want to shorten it too much. I run the full 76 inches and find it cannot absorb full power above 10-12,000 feet with my 2.2 redrive ratio at full coarse pitch, still trued 181 knots at 15,000 feet though the RPM was not staying down where I would have liked.

Geoff remarked that he was taking off at neutral pitch which is what I do too. From his description, his prop seems to work similarly to my setup. I suspect he's making a lot more power than me with the STi vs. my EJ22 and is running a lot more MAP. Probably making a lot more power at altitude than most atmo 13Bs as well.
 
Empty weight was 1187lbs.

2500fpm climb at 90 knots with full fuel running 5500 rpm and about 44 inches. Cruise at 4500 MSL, 4500rpm and unspecified MAP was 170 KTAS.

Cooling was reported excellent with OATs of around 30 to 35C- never exceeded 80C.

Going to be a little heavy with wheelpants and paint. The rest of the numbers are excellent, in particular for a first pass. 4500/1.97 is 2300 prop RPM, so 170 KTAS without pants is very reasonable.

It will be interesting to see what it will do at 9500~11500, where we NA guys run out of manifold pressure.

This is pretty much the recipe I'd use today if I was doing another RV using an alternative engine.

Me too.
 
Wow - I really like those rad scoops in the wing. I just can't shake the idea of doing something like this on my next build.

Oh well, the wife tells me my "next build" is going to be her new house - so I guess I have a little time to think about it before committing. :eek:
 
Wow - I really like those rad scoops in the wing. I just can't shake the idea of doing something like this on my next build.

Oh well, the wife tells me my "next build" is going to be her new house - so I guess I have a little time to think about it before committing. :eek:

Yeah -8s look cool already but I really like what Geoff has done with his rads here. Looks cooler.
 
turbo subaru

Looks fantastic, taxi testing my EJ25 powered RV 4, but as of this point my cowling is UGLY. My rad underneath similar to Ross RV6,but ducting aluminum instead of fiberglass. Hope to have test pilot fly soon.Then will try to post photos on VAF..Tom
 
A big congrats Geoff on getting her flying..greating looking with the underwing rad's
make it look like a Spit!
 
Alternative flight style....

geoffrv8_zpspoay9ha4.jpg


Geoff Braddock flew his EJ257 powered RV8 on Dec. 27th and reports all went smoothly.

Empty weight was 1187lbs. He's using an Autoflight H6 300 gearbox with 1.95 ratio and an IVO 68 inch in-flight adjustable prop.

A few initial numbers: 2500fpm climb at 90 knots with full fuel running 5500 rpm and about 44 inches. Cruise at 4500 MSL, 4500rpm and unspecified MAP was 170 KTAS.

Seems it may be under propped a bit so he probably will be going up in diameter in the future.

Cooling was reported excellent with OATs of around 30 to 35C- never exceeded 80C.

Geoff did a nice job fitting the Subaru Sti engine and under wing rads complete with proper scoops and exit doors. Looks a bit Spitfire like. The intercooler is fed from the enlarged duct under the spinner.

Will be interesting to see more flight test numbers as he gets more hours on the aircraft.

This is pretty much the recipe I'd use today if I was doing another RV using an alternative engine.

Congrats Geoff, on completion of a challenging build!


G'Day Geoff, fair dinkum!
From a fellow alternative engine guy...nicely done!
V/R
Smokey

PS: Spitfire paint scheme hopefully forthcoming...
 
Hat off to you for hanging it out there and doing a great job of it.

After 4 Sube installs I gave it up for a Lyc clone but seeing your effort makes me wish for some more of that turbo power...

Good luck with your testing and I hope you can report results here.

Congratulations!

Randall
 
RV-8 w/Subie

Here's the Photobucket link: http://s1277.photobucket.com/user/RV8Turbo/library/?sort=3&page=1

Yes, these things move. With MT props on the two RV7 STi turbos, 190 to 200 KTAS around 12000 feet at 40ish inches MAP. With pants and leg fairings on, looks like Geoff will be in the same zone.

Thanks for the link and initial numbers Ross! As Charlie said, we'll look forward to hearing more in the future and maybe he'll post some fuel flow numbers as well. In the pictures it shows he built a plug to create his "spitfire" style scoop; so when things slow down for him in the future, maybe he'll consider offering some scoops for sale? :)

Doug
RV-9A Mazda 13B/ FWF
 
Bad Weather

I got an email from Geoff today. He's not been able to fly at all this week due to terrible weather. He promises some more info when he can fly again.
 
Hat off to you for hanging it out there and doing a great job of it.

After 4 Sube installs I gave it up for a Lyc clone but seeing your effort makes me wish for some more of that turbo power...

Good luck with your testing and I hope you can report results here.

Congratulations!

Randall

You hung in there longer than me, Randall. I did only 2.

I do miss the smooth H6 with MT prop.

Lycoming (Superior) with Catto is so reliable it scares me. I keep wondering when the thing will do something unusual, so far it has not.

The H6 could be as bullet proof if it had an internal PSRU and remote radiators like WWII fighters. That technology is reliable and ancient, why no one has done it is matter of money and engineering, but it would be the cat's meow. If PSRU would allow 6000 rpm it would really snort.
 
The RWS drive from Tracy Crook (no longer available; he retired) is comfortable in the 7K rpm range. There's a guy down under working on an updated version with a couple of minor tweaks, but he hasn't given any projected production date.

Tracy has offered the drawings to anyone interested in resuming production.

Charlie
 
The beauty of the turbo is you don't need to rev the engine high like the atmo engine to make the power. No real need to rev over 5000 for takeoff and 4000 to 4500 in cruise. The engine is loafing down here and you rarely need to run over 40 inches in cruise (60 inches is stock MAP). The feeling is just of effortless power without all the noise of the atmo six.

Even running down here, the STi puts out at least 40 more hp than the Eggenfellner EZ30 engines which were heavy and strangled by poor intake and exhaust systems.
 
The RWS drive from Tracy Crook (no longer available; he retired) is comfortable in the 7K rpm range. There's a guy down under working on an updated version with a couple of minor tweaks, but he hasn't given any projected production date.

Tracy has offered the drawings to anyone interested in resuming production.

Charlie

The problem with planetary drives and the Subaru is the engine sits too high in the cowling for proper intake manifolds to be used. The Autoflight H 300 is well proven in front of Subaru, Suzuki and Honda engines up to 300hp.
 
Ross,

No argument about the effectiveness of a turbo for a/c use.

But.

While someone with your experience doesn't have many problems with properly sizing and installing a turbo, you aren't a typical homebuilder, and unfortunately, you're far from a typical alt engine installer.

This is hard to say, and no doubt hard to hear, but I'm afraid that there's often an inverse relationship between the builder's qualifications and his willingness to try an alt engine, and that's what drives the poor success rate.

Wouldn't you agree that for a more typical homebuilder, adding a turbo more than doubles the complexity of a custom alt engine install, and makes it much less likely to achieve success?

Charlie
 
Ross,

No argument about the effectiveness of a turbo for a/c use.

But.

While someone with your experience doesn't have many problems with properly sizing and installing a turbo, you aren't a typical homebuilder, and unfortunately, you're far from a typical alt engine installer.

This is hard to say, and no doubt hard to hear, but I'm afraid that there's often an inverse relationship between the builder's qualifications and his willingness to try an alt engine, and that's what drives the poor success rate.

Wouldn't you agree that for a more typical homebuilder, adding a turbo more than doubles the complexity of a custom alt engine install, and makes it much less likely to achieve success?

Charlie

Agreed. This sort of project is for the few rather than the many. I've told many people who ask about an auto conversion to just install the Lycoming as I don't feel they have the skill set required to do this.

I had another fellow contact me 2 days ago considering doing an installation like this. He's got the right design, welding, fabrication background and he's good friends with Shane Getson who has one of the other Sti Turbo RVs so that may work out for him. Certainly if people have no experience and don't heed the advice of those who successfully have done it before, they will double their headaches.

I offer to help anyone match a turbo for their application at no charge and to offer some guidance on what to do and what not to do to give the best chance of success. It's a matter of following a proven recipe for the most part. We have a number of very satisfied Subaru users with several hundred hours of reliable service to date.
 
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Update from Geoff:

He's hoping to be flying again in two weeks, waiting for a new drive coupler.

"Looking forward to flying again soon so will keep you posted.
Sadly, the first flight footage was only of the initial take off as the
camera person was not able to keep up with the plane and it disappeared out
of shot...
Have footage from inside as I had a GO Pro mounted so I could keep an eye on
the gauges etc. Need to get that uploaded so people can look at it. Will
hopefully do that in the next week."
 
Geoff said the problem was caused by the box being too close to the engine when set up, which flexed the coupler back towards the flywheel and distorted it. He has re-machined the flywheel central mount plate so the coupler is not in any tension when gearbox is re-mounted.
 
New Update

This was posted by Geoff today:

Flying again!! Sorry for lack of updates.

Happy to report no major issues of any sort. Cooling problems are non existant. The underwing rads are working exactly as planned and coolant temps rarely go past 95 degrees C ( Constant climb @ 5200rpm @ 8lbs boost @ 80 knots @ 2500ft/min with full fuel will see 95deg C coolant temps) and I have been flying in 35 to 40 deg C temps recently. In cruise coolant mainly runs 75 deg C, oil 100 deg C, redrive 75 deg C . Main issue was one of vibration which I traced to the drive coupler being compressed when PSRU bolted up to motor. I machined a new flywheel and coupler mount and replaced coupler and smooth sailing.

Now dust has settled I am seeing 150 knots in cruise @ 4200 rpm @ 3 lbs boost @ 28 liters an hour. EGTs run 1370f-1380f @ 9500ft AMSL. I still have to put gear leg fairings and wheel spats on.

Biggest issue I have is my prop and reduction ratio. I am using an IVO prop Magnum @ 68 inches ( I cut it down when I had the engine package in my other aircraft) and need to go back to 76 inches. My reduction ratio is 1.95: 1 which is turning the prop too fast, another reason I cut it down and need to go to approx. 2.2:1 which I think is about what you are using.

The whole package is so smooth its uncanny?just love it!!

The aircraft is absolutely beautiful to fly, a real credit to Vans. No vices.
 


Geoff sent me another update today:

"Have been flying for a while now and everything is working beautifully. 186 hours.

My data logging shows coolant temps constantly 79-80 deg C in cruise and sustained hard full power climbs, TOW 743kgs, 2500 ft per min @ 80 knots usually see 94-95 deg C max. My oil cooler is also incorporated into the cooling system so the system is performing both cooling tasks. Actual performance is not startling,( 155-160 Tas @ 7000ft, 4150 eng rpm, 2128 prop rpm, 110 kpa boost) as my prop is too small and my reduction ratio @ 1.95 : 1 is a bit fast to turn a larger prop efficiently. Will address that issue a bit further down the track. Just happy that all the effort has paid off."

Geoff is considering changes to the turbo in the future and we're discussing some possibilities there.
 
Ross, has he verified his TAS against a GPS box test?

To compare to say another outstanding RV8 that recently finished test flying here in SEQ, it does 180-182KTAS flat out, and in a cruise config, WOT/2500/LOP it does 164-165KTAS at max weight.

I am just curious, but his cooling drag does not seem to be as bad as it could have been.
 
Geoff didn't mention mention if it was 4 way GPS tested but after 186 hours I think most folks have a fairly realistic idea of what the real speeds are.

It's not the fastest RV8 around but the turbo and prop are far from optimal.

Another fellow emailed me today and ditched his tiny, mismatched IHI turbo on his Sube for a properly matched Garrett and dropped fuel flows about 8% at the same TAS, EGTs were down as well.

The IVO is not known as the best cruise prop but it usually climbs well.

Getting into the same ballpark as an Lycoming as far as performance on the first try is not bad. I think Geoff will improve on things a bit if he can change the prop and turbo.

Hard to know about the cooling drag aspect but it looks like it cools well, unlike many other conversions with cowling mounted rads.

The RV7 we did up here with turbo Sube power and MT prop had comparable cruise performance/ FF to Lycomings ones and when called on, could run away and hide from them at pretty mild power settings. 200 KTAS was easy above 8000 feet.
 
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Update

Geoff sent me some further info and updates on his plane:

"Have been doing a lot of flying over the past few months as the weather has been perfect?cool, calm and clear days?beautiful.

I have fitted out the rear seat with a full set of instruments and controls so the RV8 can be flown from the back seat?very interesting!!!! Anyway in the process when we calibrated the rear airspeed indicator, we discovered the front one was under reading by 10-12 knots. We did 4 way speed runs and the GPS data confirmed the rear airspeed indicator was spot on and the front one out!!! The upside is that we discovered our cruise speed is actually 160 knots @ 4200RPM..approx 2200 prop rpm for a throttle setting of 49% as shown from my MoTeC data logger. We are suffering in top end speed seeing only 185 knots flat out this due to a lack of prop efficiency. Climb performance is fabulous, seeing 3000 fpm @ 89 knots, with one POB and 120 litres of fuel and 20 kgs ballast on the rear seat.

Today (21/07/18) we were two up( approx 160 kgs), 90 litres of fuel and comfortably still climbed @ 2500 fpm @ 90 knots.

The fuel burn sits at 28 litres per hour (7.4 US Gallons)in cruise.

My MoTeC data logging confirms that cooling issues are non existant. The log line is almost dead flat from takeoff to landing!! Inlet air temp usually sits @ approx 10 Deg C above ambient, rising to 51-55 Deg C at 160 Kpa boost on takeoff and climb then very quickly dropping back in cruise. Again today, doing touch and goes the coolant temp never exceeded 83 deg C and oil and gearbox temps were all well within spec. EGTs were 1380 ? 1450 f and afr all below .95 lamda. Cannot fault the engine or airframe. Very happy so far.

Very hard to say if the underwing rads have affected performance much but even if they have, it is minimal and the trade off is effective and reliable cooling. The figures all mirror what I have seen in the summer months so I am confident I have solved the cooling issues that have plagued the Suby installations for years!!

I think someone asked how I had routed the cooling lines from the motor to the rads and back again. I routed the coolant pipes under the fuse to avoid the main wing spar cross member and brought them up behind the pilots seat and routed them out to the rads on the wings. I used 22mm OD 6061 alloy tube and formed silicon hose bends and straight sections to link everything together.

After a lot of research I found that 1 Sq ft of Rad surface was required to cool 100 hp in free air and that placing the rads in a duct improved the cooling efficiency considerably and also the specific shape and length of the duct had a dramatic effect on cooling performance. I designed the ducts to take advantage of the above criteria. The rads themselves I constructed as two rads in one, having a double pass from front to back. This allowed me to minimise the rad frontal area but effectively doubling the surface area exposed to the airflow. The actual surface area is approx. 2.5 sq ft.

The oil is also cooled by the engine coolant passing through an in line heat exchanger so the rads are doing double duty.

If anyone wants more info, feel free to drop me an e-mail. I have kept a comprehensive log of what I have done, made templates etc of the parts, brackets, for the cooling system and have an extensive list of all the parts necessary to construct the system. I also made very good moulds for the inner and outer ducts (8 incarnations until I was happy?just over a years work!!!!)"

 
I suspected he had an error ;)

Ross do you know what CR this engine is?

He certainly seems to have it sorted now. A labor of love for sure.
 
I suspected he had an error ;)

Ross do you know what CR this engine is?

He certainly seems to have it sorted now. A labor of love for sure.

I believe the long block is all OEM so around 8.3 to 1 CR.

He'd love to have the MT prop on there but that's not in the budget. We think he'd pick up about 10-20 knots with one, depending on power setting.

Yep, Geoff has done a nice job on this conversion. Kudos.
 
For reference, our Subaru conversion runs about 1.35 square feet of rad face area (130in2 rad and 64in2 more for the heater core) to cool around 160-170 hp so we're in the rough ballpark for area vs. hp.

Rad volume is an important variable too of course.
 
Props aint props the say.

We have a fellow down under with an extended RV8 built in Canada or Alaska (can't recall which) and it has an MT 3 blade prop with a TN IO390 I think it is.

Long story short, it is SLOW......during test flying one of my colleagues had access to a Hartzell two blade prop that would fit appropriately. No need to guess, yes it went about 15 or so knots faster.

The MT prop went back to Germany, the factory claim it is spot on for the job, no errors made. Apart from the fact it is as slow as a wet cold week.

So ya never know...... :cool:
 
Re Prop Performance...

I replaced anM2YR 74" Hartzell with a 70" Whirlwind CS Prop....everyone threw their hands up in horror!!....The aircraft is better balanced ..ie not so nose heavy..(25 lbs off nose!!)...trims well on final..doesn.t drop nose when power off in flare etc....TAS at 1500 ft...186 kts...(balls to wall!).....Crz at at 7500'...22MP..2350 Rpm...166kts 30ltr/hr...(170kts at34 l/hr)...throttle stiil retarded.....Tas have been well verified!...
 
Props aint props the say.

We have a fellow down under with an extended RV8 built in Canada or Alaska (can't recall which) and it has an MT 3 blade prop with a TN IO390 I think it is.

Long story short, it is SLOW......during test flying one of my colleagues had access to a Hartzell two blade prop that would fit appropriately. No need to guess, yes it went about 15 or so knots faster.

The MT prop went back to Germany, the factory claim it is spot on for the job, no errors made. Apart from the fact it is as slow as a wet cold week.

So ya never know...... :cool:

You can't compare MTs used in direct drive applications to geared ones where they turn much slower and are in the more efficient range according to MT engineers I talked to.

2 other Subaru STi powered RV7s with MT 3 blade props flew 200-210 KTAS, using very low boost, at 8-12,000 feet, making them the fastest RVs I know of outside of Dave Anders RV-4. The MT is a much better cruise prop than the IVO which is what Geoff and I use currently. Unfortunately they are 20 pounds heavier and 5X more expensive.

My IVO is the full 76 inch length with the coarse blades and still can't absorb the engine power above 9500 feet at full coarse pitch. Geoff has shorter blades and more power so is at even more of a disadvantage.

If you have doubts on how well the MT can work on a Subaru conversion, listen to the doppler shift near the end of this video, running only about 30 inches at 5500 feet. https://www.youtube.com/watch?v=No7BnIDAp-Q These 2.5L engines will easily slay a 6L Lycoming while loafing at low rpms and boost levels- just need the right prop on them.
 
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Like I said...........Props aint props they say :D

And sometimes you have to try a few. And that can be expensive :eek:
 
Another Update

"Just got back from an airshow here in QLD. Seem to be getting a handle on this aircraft at last. Cruising at 8500ft amsl, temp 42deg F, 4200rpm for a prop speed of 2320, 2 POB, luggage, full fuel etc, etc, we averaged 166knots. I am beginning to think I will stay with the IVO prop and just upgrade the blades and the turbo. Cooling is perfect?the logging for coolant temp varied 4-5 deg C for all conditions.

In constant climb to 8500amsl at 5 lbs boost,5000rpm, 90 knots, 2000fpm, 2 POB, 80 litres fuel and luggage, sees coolant temp at 84 deg C constant."
 
"Just got back from an airshow here in QLD. Seem to be getting a handle on this aircraft at last. Cruising at 8500ft amsl, temp 42deg F, 4200rpm for a prop speed of 2320, 2 POB, luggage, full fuel etc, etc, we averaged 166knots. I am beginning to think I will stay with the IVO prop and just upgrade the blades and the turbo. Cooling is perfect?the logging for coolant temp varied 4-5 deg C for all conditions.

In constant climb to 8500amsl at 5 lbs boost,5000rpm, 90 knots, 2000fpm, 2 POB, 80 litres fuel and luggage, sees coolant temp at 84 deg C constant."

Ross,

Thanks for posting the numbers, very valuable information.

Charlie
 
Let's face it... who wouldn't want two cool under-wing rads as neatly packaged as these ones? Can you say "shades of Spitfire"?

Pretty impressive performance numbers.
 
Awesome!

Next to our RV4, I am flying (500+ hours by now) a normally aspirated EJ25 with MT constant speed prop and under-wing radiators on my Jodel. I too have gone through loads of trials and tribulations to get this to run properly. I too now have perfect cooling, excellent performance (for a Jodel) and complete reliability.

My next project (once the current car project is done) will be an F1 Rocket (hopefully with the Evo 2 wings, which seem to be in the making), which I would like to outfit with an EJ257 with Turbo and under-wing radiators. So pretty much like this -8.

I'd be very curious to know the dimensions of the wing radiators! Ross, do you have these?

And I concur on the comments on Ivo, MT and redrive ratios.

I have run mine with 2.12:1, 2.2:1 and now 2.29:1. The higher the reduction ratio, the better the performance. In fact, I feel that the 2.29:1 is still too low for my Jodel. I get better top speeds from 4300 engine rpm than I get from 5000+ engine rpm, despite the fact that the engine then is producing significantly less power there. So all the extra power that I am producing at higher rpms is actually lost in the faster spinning of the prop, and then some. (That's 1875 vs 2185 prop rpm, with the 1875 being faster).

The fact that we are typically using around 2700 rpm for our props comes from necessity of our direct drive engines. It is a compromise between engine power and prop efficiency. Now if one has a redrive, one can choose prop rpm. Those who select a reduction ratio based on 2700 rpm just haven't gotten the point, if you ask me.

As for props - I have seen EJ engines with Warp drive props underperform significantly, losing most of their power in drag, rather than thrust. I have had 74" two-blade Whirlwind constant speed props (with NSI hub) on my first two engine/redrive combos, and now have a 74" three-blade scimitar MT on it. With the MT (and higher reduction drive ratio), for the same speed, I need about 3 fewer inches of MAP. Go figure the effects of a prop.

And thus my ideal setup would be the Rocket, with the same 2.29 redrive that I have now, EJ257 Turbo, under-wing radiators and a properly matched MT prop.

This is going to be fun!

Ross, if you have some contact details in Oz for me, that'd be awesome.

Thanks!

Hans
 
In the quote from post #32, he said to email him if we had questions, but I didn't see an address.

One reason to shoot for 2700 at the prop is that if you're using a fixed pitch wood prop (all some of us can afford), it's much simpler to get a properly carved prop. You'd probably be surprised at the name of one *very* well known and respected carver that made about a 200 hp mistake, carving one for a rotary & around 2200 rpm. The blades looked like canoe paddles, and he couldn't get above about 4000 rpm static. Builder had never seen a proper prop for his engine, so couldn't do a 'sanity check' looking at his new prop. He spent months building new intake manifolds, etc, thinking he'd made a build error. We had a rotary gathering at his airport; several experienced guys took one look at his prop and said, 'there's your problem. Swapped a prop from a flying rotary a/c, and instantly got proper static rpm.

Another, in many cases, is simple prop clearance (dia limit) vs HP. If all you can afford is a f/p 2 blade prop, it's tough fitting more than ~74" dia on a 2 seat RV and really tough efficiently absorbing a lot of HP with 74" and rpm down in the low 2000s.

Charlie
 
Yes, very valid points.

It seems that many people don't understand that besides diameter and pitch, the washout in a prop is of utmost importance. Also with a constant speed prop. ANY prop maker should take the engine and aircraft characteristics into account when specifying a prop for your application. If they are unable to produce a proper prop based on that, then it seems their engineering is more eyeball and less science than would be ideal.

Some background:

A prop is not there to absorb horsepower, it is there to create thrust. Rotating it will create drag for which you need power to overcome it, and you would like the maximum amount of thrust for the minimum amount of drag.

So we aim to achieve that:

A prop is a rotating wing. This is common knowledge. An aircraft has forward motion, and that forward motion is the same for each point on the prop. The prop rotates, giving it a sideward motion. This is different for different point on the prop. The further form the hub, the more sideward motion.

Now vector these. Forward motion versus sideward motion. The resulting vector is where the apparent wind is coming from. Now angle your airfoil (prop blade) for its optimum angle of attack (around 4 degrees for most airfoils).

You'll see that the further from the hub, the flatter your blade must be (since the rotating motion is getting more and more dominant with respect to the forward motion). Closer to the hub, you'll need the blades more into the airstream.

Now this washout is specific to a combination of aircraft speed and prop rotational speed. Faster aircraft need more washout than slower ones.

And thus simply bolting on a constant speed prop and thinking that it will self-adapt to your aircraft simply because its blades rotate will - unless you are lucky - only get you optimum airflow over your blades at one particular point along the span of the blades. And so most of your prop airfoil is operating at other than its optimum angle of attack, resulting in reduced thrust and increased drag.

How do you notice? You need more hp to achieve the same speed, whereas the aircraft with the optimized prop will be faster on the same hp.

I've discussed with MT when ordering my prop. They wanted my engine speed, reduction ratio, engine power and torque curves, my cruise speed, my climb speed, etc. Based on that, they advised a blade. In my case the MTV-12 188/53 (top of my head), where the 53 specifies the washout. A fair amount of data required, for what turn out to be VERY good results.

And indeed, slow rpms tend to need more prop area. Three blades are the way to go there, and as long as you can make them. Look at how a Robinson helicopter has enough thrust to go up vertically on an O-320 and we do not. The only difference - their "prop" is big and turns slowly. Hmmmm...

Hans
 
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Hans, I'll PM Geoff's email to you.

Turbo auto engines really need a VP or CS prop if you want to reap the benefits of the turbocharger. FP props are a massive compromise here.

When MT built my RV10 prop for my twin turbo EG33, they asked all the same questions and said most efficient range would be 2300-2550 rpm.

We had MT CS props on both the other turbo EJ257 RV7s and these worked really well- over 200 KTAS was easy stuff at low MAP and medium altitudes.

I agree with the comments on the prop rpm. No need to think in the same terms as direct drive engines when you are geared. I turn mine at around 2050 max and keep the engine rpm down to about 4500 max. This gets me close to torque peak and highest VE while reducing frictional losses.

Looking forward to seeing your project progress Hans. Cool stuff! They have some great parts available for the 257 these days.
 
Thanks Ross.

I'll contact Geoff shortly.

Unfortunately, it will be some time (could easily be 2 years or so) before I will be ready to start this project. First, I will have to finish this one: https://www.instagram.com/minottocars/ and hopefully sell a few of them.

There's no rush. I'm collecting information so that I can build my ultimate dream aircraft when the time comes.

How's your RV10 project doing by the way? It seems to have disappeared from your web site?

Cheers,

Hans



Hans, I'll PM Geoff's email to you.

Turbo auto engines really need a VP or CS prop if you want to reap the benefits of the turbocharger. FP props are a massive compromise here.

When MT built my RV10 prop for my twin turbo EG33, they asked all the same questions and said most efficient range would be 2300-2550 rpm.

We had MT CS props on both the other turbo EJ257 RV7s and these worked really well- over 200 KTAS was easy stuff at low MAP and medium altitudes.

I agree with the comments on the prop rpm. No need to think in the same terms as direct drive engines when you are geared. I turn mine at around 2050 max and keep the engine rpm down to about 4500 max. This gets me close to torque peak and highest VE while reducing frictional losses.

Looking forward to seeing your project progress Hans. Cool stuff! They have some great parts available for the 257 these days.
 
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