Van's Air Force
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Which Prop
- Thread starter Rob Hanberg
- Start date
Lux Wrangler
Well Known Member
74 inch BA prop on the taildragger
John;
No, it is not generally agreed upon that the 74 inch is better. There is a discussion on the Matronics site that you may have read? called "Blended Airfoil 72" vs. 74"" started by "Jim Thorne" in March of '06. A search of this (Vansairforce) site, on the subject, gives more reading. Such as "Which Prop To Order" June '06. Many people feel that the larger diameter improves climb performance. Like many engineering compromises, this may come at the cost of cruise speed. We are probably talking small differences of cruise speed. No one wants a prop strike either and I wouldn't want a 1/8 inch interference between the prop and the ground. The early RV-4 had such problems and perhaps this is why Van's is being cautious? I would like to know what the typical clearance is on the RV-7?, both in tail-up and tail down, at gross weight?
Jim
John;
No, it is not generally agreed upon that the 74 inch is better. There is a discussion on the Matronics site that you may have read? called "Blended Airfoil 72" vs. 74"" started by "Jim Thorne" in March of '06. A search of this (Vansairforce) site, on the subject, gives more reading. Such as "Which Prop To Order" June '06. Many people feel that the larger diameter improves climb performance. Like many engineering compromises, this may come at the cost of cruise speed. We are probably talking small differences of cruise speed. No one wants a prop strike either and I wouldn't want a 1/8 inch interference between the prop and the ground. The early RV-4 had such problems and perhaps this is why Van's is being cautious? I would like to know what the typical clearance is on the RV-7?, both in tail-up and tail down, at gross weight?
Jim
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Which diameter?
This is another good question and in my opinion it probably comes down to personal preference after considering what Vans recommends. The larger diameter will generate slightly more thrust at lower speeds. However, since the RVs already have excellent field performance, is the larger diameter absolutely necessary? The shorter prop will have lower fly-over noise if that is important to any of you? I feel the difference in cruise thrust will be relatively small. I understand that Van's recommends the 72" for all the TDs because of ground clearance. I don't know what the ground clearance is on all the various models? Perhaps some of you that have flying airplanes can read 14 CFR 23.925 and go measure it so everybody knows?
One thing that everyone should be aware of is we approve the prop from 74" down to 72". If you buy a 72" you essentially have no length repair tolerance left. If you prang the tip, you don't have any tolerance to grind-off the damage. However, most of the time when a tip is pranged it causes enough damage/bend to scrap the hub too (our criteria: if the blade is damaged/bent beyond repair allowance, the hub is considered scrap because of undetectable fatigue damage). Ironically, with a 72" prop you are less likely to prang a tip because you have 1" more clearance so the issue of repair tolerance is sort of moot. I know 1" doesn't sound like a lot but it could be the difference between a scrapped prop or not (remember, its the last inch of spaghetti that gets the sauce on your shirt
).
Speaking for myself, I would buy the maximum diameter available that meets established ground clearance criteria. How's that for a non-commital, politically-correct answer? I would be happy with either prop.
I should have the approval/placard summary done early next week. I will provide it to Vans and post it in a new thread.
Les
This is another good question and in my opinion it probably comes down to personal preference after considering what Vans recommends. The larger diameter will generate slightly more thrust at lower speeds. However, since the RVs already have excellent field performance, is the larger diameter absolutely necessary? The shorter prop will have lower fly-over noise if that is important to any of you? I feel the difference in cruise thrust will be relatively small. I understand that Van's recommends the 72" for all the TDs because of ground clearance. I don't know what the ground clearance is on all the various models? Perhaps some of you that have flying airplanes can read 14 CFR 23.925 and go measure it so everybody knows?
One thing that everyone should be aware of is we approve the prop from 74" down to 72". If you buy a 72" you essentially have no length repair tolerance left. If you prang the tip, you don't have any tolerance to grind-off the damage. However, most of the time when a tip is pranged it causes enough damage/bend to scrap the hub too (our criteria: if the blade is damaged/bent beyond repair allowance, the hub is considered scrap because of undetectable fatigue damage). Ironically, with a 72" prop you are less likely to prang a tip because you have 1" more clearance so the issue of repair tolerance is sort of moot. I know 1" doesn't sound like a lot but it could be the difference between a scrapped prop or not (remember, its the last inch of spaghetti that gets the sauce on your shirt
).Speaking for myself, I would buy the maximum diameter available that meets established ground clearance criteria. How's that for a non-commital, politically-correct answer? I would be happy with either prop.
I should have the approval/placard summary done early next week. I will provide it to Vans and post it in a new thread.
Les
lesdoud said:
Les,
Great to know that the "tuned" angle valve induction system improves the torsional vibration situation. I am planning on mating an IO-360-A1A induction system to a 180 hp parallel valve engine. The one other modification will be a single Bendix impulse coupled magneto with one electronic ignition (ElectroAir). Have you done any testing on this combination?
Has any testing been done on a 360 using one of the aftermarket tuned 4 into 1 exhaust systems? I am referring to the units sold by Aircraft Exhaust Technologies and Sky Dynamics.
Charlie Kuss
mark manda
Well Known Member
I heard the shorter 72" prop was faster than the 74" Hartzell. Never heard by how much.
Bill Dicus
Well Known Member
A friend and RV-8 builder told me that the 74" blades can be shortened in the event of damage whereas the 72" cannot. I have the 72" BA prop. BillRV7AV8R said:Les,
According to Vans order chart I can use either a 72 inch dia or 74 inch dia BA on my RV7 with an O-360, what is the difference (besides 2 inches
John Adams
