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NorthernRV4 09-29-2018 10:23 AM

Engine/Prop options
I've really been trying NOT to think much about the engine I will use for my -4 because it's a ways off before I need it. I've had an idea of what I will likely use but still open to options. This morning I came across an ad on Barnstormers for an engine/CS prop very close to home and it seems like a potentially good buy. Looking for some opinions/direct experience from the community on this.

Lycoming IO-320 E1A
Hartzell prop HC-C2YL-4BF

both from a Decathlon. Condition aside, I have a few questions about the suitability for my -4

Fuel Injected: a plus since I plan to build it for inverted flight.
Dynafocal 1 mount
Induction: I'm told it's Horizontal induction, I know this affects the lower cowl/intake and FAB setup. Any potential problems here?

No idea on the suitability of the prop, wondering if there's any issues due to differences in speed range of a Decathlon compared to a -4... Being CS does the blade profile make a big difference for an RV application? Any advice here?

Lufthans 09-29-2018 11:07 AM

A common misconception is that the constant speed feature of a prop will make everything ok. It doesn't.

Let me elaborate.

A prop is a rotating wing. No news there. And like any wing, it can operate with its angle of attack in a certain range.

But what is the angle of attack of a prop?

How the air hits the airfoil of the prop depends on two factors: the forward speed of the aircraft and the rotation of the prop. The forward speed is the same for the entire prop, the "rotational speed" depends on the distance to the hub of the prop: at the hub itself it is zero, at the tips, the rotation of the prop is in the .7 to .8 Mach numbers.

If we want an efficient prop, we want each station along its span to be at optimum angle of attack (and so lowest drag). If we vector the forward and rotational speed together, we find that we will need quite some washout - with the prop flattening out towards the tip.

Now comes the trick: Imagine the forward speed difference between a fast and a slow aircraft and you'll see that on a fast aircraft, the tips need to accommodate for much more forward speed, and will thus be less flat than on a slow aircraft, whereas near the hub, the difference is much less pronounced. And thus washout for fast versus slow aircraft are quite different.

Note that turning the whole blade (as you do with a constant speed) will not alter the washout! A constant speed prop therefore really is only optimised for one particular speed.

Luckily, the relatively thick airfoils of a prop accept quite a range of angle of attacks, so ill-matched props WILL work. However, while simply adding more pitch will indeed slow down the engine, but not necessarily produce more thrust.

So far the theory.

In real life - On my Jodel, I went from an ill-matched Whirlwind to a well-matched MT, and needed 3 fewer inches of MAP to obtain the same speed, and gained nearly 500 fpm climb rate in the process.

So to get back to your question: yes, it will probably work, however you're likely to leave a fair bit of performance on the table when going with a prop that is meant for a much slower aircraft.

I'd consult with the prop manufacturer if I were you. Or take the engine and sell on the prop to save for a good one.

Happy building!


gasman 09-29-2018 12:52 PM

Now that's a very educational (to me) post!.....

"is only optimised for one particular speed." And what speed would that be for our 360 RV's?

smokyray 09-29-2018 01:29 PM

Prop me up...

Originally Posted by gasman (Post 1292049)
Now that's a very educational (to me) post!.....

"is only optimised for one particular speed." And what speed would that be for our 360 RV's?

Uitstekend Hans!
I assisted a friend installing the Decathlon engine/prop combo you mention onto an RV4 with no mods many years ago and it worked very well. The blade shape on this particular Hartzell model is optimized for the Decathlon's lower entry aerobatic speed range around 120 KIAS, but will still provide reasonable top end speed but at a slightly higher RPM/MP as I recall. We operated from my short turf strip and it provided decent STOL performance as well. Realize this prop is specifically designed for the I0-320 on the 8KCAB Decathlon. The 0-320 has a very different harmonic vibration footprint than the 0-360 as well which requires a placard for a 2000-2250 continuous RPM avoidance range in the 8KCAB. Food for thought. BTW, I had no problems keeping up with him in climb and cruise in my own 0-320/Sterba wood prop RV4.

As Hans so eloquently stated, propellers are compromises in performance and designers look at aircraft with specific blade designs in mind.
[Personally, I prefer a well designed Fixed Pitch Propeller for the 0-320 RV4. This produces a light nose, low flywheel effect and no RPM restrictions for Aerobatics and the occasional 1V1.:)


PS: The only Van's approved CS props specifically designed for the 0-320 powered RV are the Hartzell 2 blade, MT 2/3 blade listed on and the Whirlwind 200RV.
MT Props

NorthernRV4 09-30-2018 06:46 AM

First off, sorry for the double post, I wondered where the first one had gone :o

Thanks Smokey for the info, I figured somebody had tried this combination before. The info on the induction system is very helpful. I think the engine itself would be a good fit but might be worth going with high compression pistons for 160hp. The only downside is then I'd never be able to use Mogas.

For the prop, I hope to get a look at the logs soon and know a little more about the blade profile but as pointed out, I could sell it to fund more of the project and go with another prop, either fixed or CS, it's nice to have the option. I didn't realize the governor was specific to the prop so I'll have to make sure that's part of the deal if I go ahead with it.

"Make sure the seller includes the full Christen inverted system, boost pump and assorted hardware" I don't think this is part of the sale since the Decathlon is now flying with another engine and this one went for O/H. When you say "boost pump" is this part of the Christen system (sorry I'm not very familiar with the system) or do you mean the fuel boost pump? If it's the latter, is the fuel boost pump specific to the model of bendix injection? Sorry, so many FWF questions I haven't started looking into yet.

smokyray 09-30-2018 09:30 AM

Prop me up....
The boost pump is specific for Fuel Injection but attainable easily otherwise. It's not a deal breaker, just trying to save you a few $$$.
The IO-320 with the Bendix Servo will accept non-ethanol MoGas* and it's a Wide Deck engine which means higher Compression pistons are possible as you mentioned. After my original RV4's ND 0-320 reached TBO, I retrofitted a 0-320E2D (WD), rebuilt and balanced with 10.0:1 Comp forged air-boat racing rods and pistons in my own RV4 later and it was a worthy mod, significant improvement. No added weight and 170HP. With higher Comp I mixed my MoGas 25%/75% with 100LL based on a Canadian study I read.
Light+simple=Max bang for buck!

Good luck with your decision, my 2 cents notwithstanding :)

*Check this out:

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