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When is more too much?

T

TXFlyGuy

Well Known Member
Looking at two engines, one a V6, the other a V8. The V8 is 75 pounds heavier and produces 100 more hp, 400 vs. 300. If the weight and physical size of the bigger engine can be dealt with, and all aircraft limitations are religiously observed, is there a valid engineering reason not to install the V8?
This V8 is powering Supermarine Spitfire kit built aircraft and is a thing of beauty! Plus it has the best PSRU I have ever seen...a dual belt system. Several people have suggested this is too much power. If it is too much power, please explain why.
 
Can the larger engine's additional torque be absorbed and distributed into the existing airframe by whatever structural attach points are available for the engine mounting system?

Does the bigger engine produce any significantly different amounts or types of vibrations that will end up causing structural failures? (e.g. torsional, resonant, longitudinal, lateral, whatever...)?

Questions like this are just as important, sometimes even more so that simple horsepower output and weight & balance when it comes to fitting a particular powerplant to a particular airframe.

I remember reading a while back about some old flying boat or amphib seaplane with radial engines up high on the wings that was converted to turboprop engines of not really too much higher horsepower than the radial piston engines that were replaced, but despite the much lighter weight supported by the wing-engine mounts, some strange torsional vibration produced by the turboprop powerplant that wasn't there with the old heavy radial engine caused a catastrophic crack and the wing failed and the plane crashed. Stuff like that is what makes airframe and powerplant choice a little more complicated than simply power and weight. It needs to be engineered by someone who covers all the "what if's".
 
An extra 75 pounds on the nose will require an appropriate counter weight somewhere aft of CG. The result is a plane with greater moment of inertia around the pitch and yaw axis -- often not a good thing. Bigger motor probably also means greater gyroscopic forces at work. Not saying these are deal breakers, but somebody needs to take these into account.
 
You don't say which model RV you're looking at building. The Supermarine engine is indeed a beautiful-looking powerpant (and it is smooth - I've flow it...), but 430 HP is way beyond what most would think to be reasonable for an RV airframe - even the RV-10 in my opinion.

It is an interesting package, but yes, CG concerns ae going to be interesting.

Paul
 
Fuel burn vs available

What is the intended mission? If it's fast crosscountry, are we now talking shorter legs flown faster?
 
The V8 is not weight appropriate for a 6,7,8, 9 or 14 and is even pushing the limits for a -10. With most belt drives, you will also be stuck with a rather expensive electric prop. The often used IVO is not suitable in this HP range so most people have to use an MT.

300hp should be adequate for any RV and any time you add a heavier engine, the mounting structure around the firewall should be evaluated. I'd be tempted to go with the V6 to keep the weight down.

You also need to design and fit a radiator system which is not that easy given the structure in the 6-9s.

Unless you are a gearhead and like a challenge, I'd carefully evaluate your plan. This is not as easy as it seems and there will be many problems and design issues to solve along the way. You need to have the skills yourself or have the right team of experienced people to make this a successful and safe project.
 
same issue

This is the same issue I am facing with my rocket build, the V8 is 50 - 60 lbs heavier than the 540 but will put out 100 HP more. I am a gear head type but the Robinson gear setup is made for the LS4 and I am quite torn here.
 
HURMANAV8R said:
This is the same issue I am facing with my rocket build, the V8 is 50 - 60 lbs heavier than the 540 but will put out 100 HP more. I am a gear head type but the Robinson gear setup is made for the LS4 and I am quite torn here.
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You need to look at the V8 engines built by Supermarine Spitfire in Cisco, Texas. They have a beautiful LS-2 with a dual belt drive PSRU that is the best I have ever seen. They are building one for a Velocity right now.
 
HURMANAV8R said:
This is the same issue I am facing with my rocket build, the V8 is 50 - 60 lbs heavier than the 540 but will put out 100 HP more. I am a gear head type but the Robinson gear setup is made for the LS4 and I am quite torn here.
Click to expand...

The entire Robinson setup is quite heavy and heavily built but it is the only one out there with very extensive, reliable and successful flight time behind it. This means a whole lot if you are going the alternative route IMO. The rest are all recent with little flight time or a bunch of problems and issues/ crashes, forced landings. Robinson did a very good job from the get go and has well over a decade of flight experience with multiple examples and high time ones too.

Only time will tell on the others but something that "looks" really good is meaningless until flight proven extensively as we have seen many times in the past with other conversions. I think too many people get overly excited when a shiny new auto conversion shows up on the market- very few vendors have the time or money to do extensive engineering and flight testing before selling to the public.
 
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rv6ejguy said:
The entire Robinson setup is quite heavy and heavily built but it is the only one out there with very extensive, reliable and successful flight time behind it. This means a whole lot if you are going the alternative route IMO. The rest are all recent with little flight time or a bunch of problems and issues/ crashes, forced landings. Robinson did a very good job from the get go and has a decade of flight experience with multiple examples and high time ones too.

Only time will tell on the others but something that "looks" really good is meaningless until flight proven extensively as we have seem many times in the past with other conversions.
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Yes, Robinson is very good. In addition, the Supermarine Spitfire LS-2 has many hours and $$$ in R&D, plus they have been flying both here and in Australia for years with zero failures. Robinson employs a chain driven unit while the Spitfire is turned via dual belts. You take your choice. It is certainly sad that Geared Drives is no longer with us. Bud Warren was a leader in this technology. He is missed by all of us.
 
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TXFlyGuy said:
Yes, Robinson is very good. In addition, the Supermarine Spitfire LS-2 has many hours and $$$ in R&D, plus they have been flying both here and in Australia for years with zero failures. Robinson employs a chain driven unit while the Spitfire is turned via dual belts. You take your choice. It is certainly sad that Geared Drives is no longer with us. Bud Warren was a leader in this technology. He is missed by all of us.
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Not to be crossing swords or diminishing the accomplishments of others but Robinson has 4000 trouble free flight hours collectively and 1900 hours on the high time single example- nobody else in the V8 conversion world can make these claims. They have the ECU scienced out and can easily use a hydraulic C/S prop if desired. A few hundred hours on a few examples collectively, while encouraging, is comparatively meaningless as an indicator of long term cost savings and reliability.
 
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Toobuilder said:
I don't think he's talking about the RV line. This exact post is on another forum and there he's talking about an engine for his replica Mustang.
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You are very observant! In fact, the question pertains to a 3/4 scale replica P-51D Mustang.
After exhausting research, the decision has been made to stick with the V6.
 
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