Van's has changed Hartzell recommendations

[gmcjetpilot]

Originally Posted by rv620mr

The Hartzel tech sheet, which is quoted above, lists tuned exhaust as having a potentially adverse affect on prop vibration and stress.

My sense is that most of us run tuned exhaust systems like Larry Vetterman's. I do. And, it's a great product. I see many threads addressing the potential impact of high compression pistons and electronic ignition on prop vibration and stress. But, I don't see much about tuned exhaust. Why?

Is the conventional wisdom that tuned exhaust has only a negligible impact on prop vibration and stress? I'm just lookin' to learn something.

Because no one really noticed, GOOD POINT! Gold star.... It has been mentioned but I don't think any one has really flagged it or tested it.

You are right the 4-into-1 "tuned exhaust" or headers do change the flow, power and pulses, no doubt.

I did ask about high compression pistons. Engineers don't like to speak off the record, but off the record the engineer I talked to thought HC pistons do make small changes to existing limits, such as a little border or wider range (not much). He did not think it would shift the whole range up or down. Basically higher compression does not make wholesale large changes in the vibration characteristics but small ones.

I suspect exhaust (wild guessing on my part) would have similar affect as HC pistons, in that it will increase the HP slightly. The cross-over Vetterman is no slouch; it's way better than any stock Cessna or Piper exhaust system. However 4-into-1 headers do have a little more savaging and affect on the power curve over cross-overs. I doubt there would be any huge shift, change or new limits that did not exist already (like HC pistons). However the HOTTER the engine (further from stock) the more you have to think about the affect on a metal prop, more so with the 7496 than the 7497 blade.

Power Flow makes those 4-into-1 exhaust systems for certified planes? Did they do prop vibration testing on every airframe for those STC's? They claim 15% or 20 HP power increase! Its not MORE than rated HP. It does not turn a 150/160 HP O-320 into 170/180HP, it just gets back close to rated hp of the engine, overcoming the poor stock exhaust, which cost about 20 HP. Still I wounder did the FAA consider the affect on the props. They make Power Flow systems for Mooney's, which of course uses Hartzell constant speed props.

Your exhaust point is super valid. It's why certified planes are so picky about having the original configuration not be altered, with out all kinds of hoop jumping. The cause and affect can not always be predicted, so each and every change needs to be tested.

[N787R]

Van offers both 72 & 74 for 7A.
What is the difference (except for the ground clearance)?

Ben

[Bill Dicus]

Len Kaufman (RV-8) told me you can grind down the 74" if you have tip damage that's severe, but the 72" can't be shortened to address a tip issue. I got the 72" for increased ground clearance before knowing the above. Bill

[Kevin Horton]

Originally Posted by Bill Dicus

Len Kaufman (RV-8) told me you can grind down the 74" if you have tip damage that's severe, but the 72" can't be shortened to address a tip issue. I got the 72" for increased ground clearance before knowing the above.

That was one of the reasons why I bought a 74" prop, but I am no longer convinced that it holds water. If you have a major prop strike, you will almost certainly damage more than the last inch on the blade. If you have a minor prop strike that only damages the last inch, you probably wouldn't have damaged a 72" prop, as the shorter blade wouldn't have hit.

I bought a 74" as the data I got from Hartzell suggest it would be very slightly more efficient at 65% power and below, which is where I expect to cruise. It will be a bit less efficient at high power and/or high rpm, so max speed, and 75% cruise would be better with the shorter blades. I did this analysis for the 7666 blades. It might not be relevant to the newer blended airfoil props.

[gmcjetpilot]

Originally Posted by Kevin Horton

That was one of the reasons why I bought a 74" prop, but I am no longer convinced that it holds water. If you have a major prop strike, you will almost certainly damage more than the last inch on the blade. If you have a minor prop strike that only damages the last inch, you probably wouldn't have damaged a 72" prop, as the shorter blade wouldn't have hit.

I bought a 74" as the data I got from Hartzell suggest it would be very slightly more efficient at 65% power and below, which is where I expect to cruise. It will be a bit less efficient at high power and/or high rpm, so max speed, and 75% cruise would be better with the shorter blades. I did this analysis for the 7666 blades. It might not be relevant to the newer blended airfoil props.

Excellent post as usual. Why not an ODD length? 73" Just asking.

The deal is any prop strike will be bad news so don't have them. Will the 1" gnd clearance between 74-72 make much difference? I guess it could but chance is if you really wack them, what ever the scenario, you just bought two new blades, prop overhaul and likely a tear down inspection of the engine as well. I don't even like to think about it.

This is one of the best arguments for a light relatively cheap fixed wood prop (not even fiberglass coated). If it hits dirt, it shatters and saves the engine, more likely than not. Replacement of the prop is relatively cheap. So if you have a constant speed prop or metal prop don't have a prop strike.

[Bill Dicus]

Kevin: Good thoughts. You make me feel better about the 72 incher. Was unaware of performance differences for the two props. Wonder if they apply identically to the BA model? Thanks for the info. Bill

[Pirkka]

If you look the Van's order form (http://www.vansaircraft.com/pdf/hartzell_order_form.pdf) you'll notice that they will recommend 72" propeller for 'any model' while 74" propellers for only A-models (tri-gear). Hope this helps to make you choice.

[Captain Avgas]

Originally Posted by Kevin Horton

If you have a major prop strike, you will almost certainly damage more than the last inch on the blade. If you have a minor prop strike that only damages the last inch, you probably wouldn't have damaged a 72" prop, as the shorter blade wouldn't have hit.

I'll give you another reason why a 72" hartzell makes more sense. If you have a prop strike and make an insurance claim (and a prop strike is virtually certain to be an insurance claim) you can be GUARANTEED of getting a new prop. But if the insurance company can repair that 74" prop then they certainly will to save a few dollars.

Builders who plan on insuring and who buy 74" props because they think they can be repaired in the event of a prop strike might just be shooting themselves in the foot.

[Bob Axsom]

Within the past couple of months I asked for and received efficiency graphs custom plotted for me by Hartzell and George's statements conform to those graphs and what I was told in the accompanying message. The pre-blended airfoil 7666 efficiency drops drastically above 165 KTAS while the 7496 declines very slowly after peaking at 185 kts. Because of the confidentiality of disclosure statements in the correspondence I cannot duplicate what I was told or shown but if I could find a 72" prop configuration with Hartzell 7496 blades there is no way I would settle for the same prop with 7497 blades. You will have to read Jack Norris' on propeller theory (a painful reading experience) to learn about tip loading problems, etc. The bottom line is the new 7497 is a performance compromise from all I have seen.

Bob Axsom

[gmcjetpilot]

Originally Posted by Bob Axsom

The bottom line is the new 7497 is a performance compromise from all I have seen. Bob Axsom

How much? It was expected the blade is thicker, which was done as a "compromise" to improve vibration characteristics and RPM limits. I talk in broad terms and in my discussion I heard the difference was small, 7496 v. 7497, may be a one mph? So Bob what is the deal? I think for the avg pilot the 7497 is better and what is recommended. Clearly the 7496 is a rocking blade for the 180HP, but if you go 200HP you have (suggested) to go 7497.