Lufthans
Well Known Member
Hi guys,
Long time lurker and multiple homebuilding offender, I'm gearing up to the next project and could use some info and opinions.
Background: I've got significant experience with several Subaru engine conversions and am successfully racking up the hours on one on my own aircraft now (not the RV, unfortunately). Perfect cooling, turbine-like smooth, runs on anything that remotely smells like gas, dirt cheap to operate and completely trouble-free.
That engine is normally aspirated. I would like to take the concept up a notch now. The gear drive that I am using was designed to take over 500 hp, and every knowledgeable Subaru tuner and engine builder that I talk to tells me that it is quite doable to build an utterly reliable 4-cylinder turbo engine that will pump out 450 hp. Even if I were to limit that to 350 hp, I'd still have an impressive engine to play with.
And the good part: all included, that engine and cooling setup will weigh about the same as an IO360. And so might actually fit a modified RV-4 airframe quite nicely.
Except the RV-4 has that vne of just 212 mph. We all know that Vans is very conservative here, but still. On our own RV4, we have to throttle back even our measly O-320 with fixed pitch Catto prop or we'll bust that Vne even in a shallow climb. So imagine having nearly three times the power available...
Enter John Harmon.
From what I understand, on the HR2, the mods are:
* clipped wing. For reducing bending moments on the spar at heavier MTOW mostly. Since I'll be looking at regular RV weights, I don't really need this.
* Wider front part of the fuselage. Mostly to accommodate the bigger engine. Plus it's a style thing, I guess. I kind of like the RV-4 look, and the Subaru is no bigger than a Lycoming 4, so that would not be needed.
* The front part of the fuselage is using thicker skins. Again, with the engine not heavier than a Lycoming 4, this would not be necessary. Stress analysis shows that the pulling force from more engine hp is negligible when compared to the G-forces on the frame when pulling that stick back hard. Same engine weight - same strength needed.
* The fuselage is lengthened by 4 inches. Again, great to counterbalance a heavier engine, but therefore not needed in my case.
According to some stuff that I've read (from an old Sport Aviation article I think it was), the tail section and tail surfaces on a HR2 are stock RV-4.
But if the tail sections are the same, and the only modification to the wing is some clipping to accommodate a higher MTOW, then where does the significantly higher Vne come from?
According to that same article, John Harmon had tested his planes in 5 mph steps for flutter to 300 mph, which is nearly 90 mph beyond the RV4 Vne. Is he just that ballsy, or am I missing the point (or some more modifications) here?
For this project, I am mostly interested in the motorization. So if I could simply start with a ready-made RV4, rather than having to build a complete new airframe, that would give me quite a jump start.
I'd have to be certain about that Vne thing though. Any thoughts very welcome!
Thanks,
Hans
Long time lurker and multiple homebuilding offender, I'm gearing up to the next project and could use some info and opinions.
Background: I've got significant experience with several Subaru engine conversions and am successfully racking up the hours on one on my own aircraft now (not the RV, unfortunately). Perfect cooling, turbine-like smooth, runs on anything that remotely smells like gas, dirt cheap to operate and completely trouble-free.
That engine is normally aspirated. I would like to take the concept up a notch now. The gear drive that I am using was designed to take over 500 hp, and every knowledgeable Subaru tuner and engine builder that I talk to tells me that it is quite doable to build an utterly reliable 4-cylinder turbo engine that will pump out 450 hp. Even if I were to limit that to 350 hp, I'd still have an impressive engine to play with.
And the good part: all included, that engine and cooling setup will weigh about the same as an IO360. And so might actually fit a modified RV-4 airframe quite nicely.
Except the RV-4 has that vne of just 212 mph. We all know that Vans is very conservative here, but still. On our own RV4, we have to throttle back even our measly O-320 with fixed pitch Catto prop or we'll bust that Vne even in a shallow climb. So imagine having nearly three times the power available...
Enter John Harmon.
From what I understand, on the HR2, the mods are:
* clipped wing. For reducing bending moments on the spar at heavier MTOW mostly. Since I'll be looking at regular RV weights, I don't really need this.
* Wider front part of the fuselage. Mostly to accommodate the bigger engine. Plus it's a style thing, I guess. I kind of like the RV-4 look, and the Subaru is no bigger than a Lycoming 4, so that would not be needed.
* The front part of the fuselage is using thicker skins. Again, with the engine not heavier than a Lycoming 4, this would not be necessary. Stress analysis shows that the pulling force from more engine hp is negligible when compared to the G-forces on the frame when pulling that stick back hard. Same engine weight - same strength needed.
* The fuselage is lengthened by 4 inches. Again, great to counterbalance a heavier engine, but therefore not needed in my case.
According to some stuff that I've read (from an old Sport Aviation article I think it was), the tail section and tail surfaces on a HR2 are stock RV-4.
But if the tail sections are the same, and the only modification to the wing is some clipping to accommodate a higher MTOW, then where does the significantly higher Vne come from?
According to that same article, John Harmon had tested his planes in 5 mph steps for flutter to 300 mph, which is nearly 90 mph beyond the RV4 Vne. Is he just that ballsy, or am I missing the point (or some more modifications) here?
For this project, I am mostly interested in the motorization. So if I could simply start with a ready-made RV4, rather than having to build a complete new airframe, that would give me quite a jump start.
I'd have to be certain about that Vne thing though. Any thoughts very welcome!
Thanks,
Hans