Whats in 1" (ground clearance)?
n38139 said:
"Interested in advice on what size (length) prop to order for my RV-8, 72 or 74. The Van's site says either will work." Steve
-AND-
"72" blade diameters are recommended for the tailwheel aircraft due specifically to ground clearance issues." Van
Folks with tail wheels have found 74" works fine, even -4, -6, -7. Van's quote is a CYA comment in my opinion. I disagree that tail wheel planes have LESS ground clearance. Van is considering pilot skill or miss-skill by this recommendation. Clearly there is more room for error to drive the prop into the dirt with a tail dragger than a nose wheel RV.
There is no debate that you do have 1" less ground clearance with a 74" prop than a 72" prop, so what? If you prop tip is getting to with in 1" or 2" of the dirt, you are doing something wrong or driving over a drainage ditch.
Ironically, despite Vans quote, the tail wheel RV's have more prop clearance during ground ops than nose wheel RV's, obviously, with their nose in the air. Ground ops are the most likely time to snag a tip. There is the exception and that is during landing.
The critical time for the prop tips v. ground clearance on a tail wheel RV's, is during
wheel landings, but its not hard to keep the tips out of the dirt during wheel landings (with practice). I make a true wheel landing almost never. I just don't find a need and always land slightly tail low, even during a wheel landing. A true wheel landing in heavy winds does require you to "stick" the nose down to plant the mains, but I would only do that with strong cross wind. Landing in a cross wind I am in side slip and half the plane is still in the air (the down wind half). Prop clearance is not an issue.
During tail wheel RV takeoff's, during or just after the tail comes up, you usually are off the ground and flying, like right now. The prop tips do not get close to the ground.
I find
when people prang their prop they do a good job of it, and 72" or 74" would have made no difference.
Usually prop prangs mean it went on it's nose or some other non-normal reason; that 1" greater prop dia would not have made a critical difference.
I'm not saying that 1" could not save the day at some point, especially on a short gear RV-4, but you can be more careful and conservative in keeping the prop tips out the dirt.
You just may have to stay out of fields with rough, uneven and soft surfaces. Also pilot skill and technique makes a big difference.
If its a hard tip strike you will do damage, usually hitting by more than 1" (e.g., botched landing / hard bounce with gear splayed out and/or way nose down attitude). As long as you keep the nose fairly level and don't slam it on or drive into a ditch, prop to ground contact is unlikely in my opinion. However drive the nose in, bam-bam-bam, but than you have more problems than just 1" of extra prop dia.
In the previous Prop dia. threads on VAF, folks have chimed in on their successful use of 74" dia. props on all models of RV's, including the RV-8. The RV-4 (old gear) and RV-6 probably have the least ground clearance, but there are 74" props on those planes as well.
The RV-7/8 looks much taller on the gear (I've not measured the difference). I would be more inclined to go with 72" on short gear RV-4's/6's. I'd go with 74" on the RV-7/8. If prop clearance was my main criteria, 72" is a fine choice, but most people like the extra tip meat for future rework if needed. 72" is the min recommended dia. My Hartzell is 72", only because I bought it 2nd hand.
Like all things regarding
tail wheel RV's, more care needs to be taken. You can put a tail wheel plane on it's nose in the run-up area standing still! If you push the stick forward during run-up on a tail-wheel RV,
bam-bam-bam. Of course you have to totally not be paying attention for that to happen. 68" or 74" prop will not make a difference, keep the stick BACK. A nose wheel does have an advantage in running up, because you can ignore elevator position. Tail-wheel guys need to "fly the plane till it's tied down". You have to pay attention more to flight control position with a tail wheel plane, even during taxi. Regardless of gear type, pay attention to the ground surface conditions.
There are times when the ground is really uneven; a prop killer is the
big drop/divot/hole with a ridge opposite the hole. The killer for nose gear RV's is when the nose wheel drops into the hole and the prop hits the high ground on the far side of the hole. This applies more to nose wheel planes than tail wheel planes. I have seen or heard of one or two light tip strike in dirt with no damage to the prop.
Frankly I only go into soft fields I know to be hard packed and smooth and/or have a RV flyer PIREP on. Of course soft fields being made of earth, you can have a gopher attack overnight.
There are prop eating surfaces out there, regardless of prop dia. or gear configuration. Most people spend most of their time flying off of hard smooth fields, so run the bigger dia. prop, especially a RV-8. However you get 1" more clearance with the smaller prop. I think its splitting hairs (or an inch), especially with the tall gear RV's (-7/-8).
Bottom line: I was going to race, 72". If I was planning on flying off of rough, soft, rocky fields all the time, I might also go with 72". However 74" is probably the choice for most (more rework tip length and better overall everyday flying performance). I'm sure its the same exact blades, just 1" gets cut off each half.
Performance: 72" v. 74"?
You can search the VAF data base regarding performance v. prop dia. To break it down to the most basic, "on paper", 72" best for top speed high rpm (may be a tad faster/more efficient), 74" better takeoff, climb. Most people go for 74" for one practical reason, 72" is min diameter and they feel they have more rework room with 74".
