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Wood Prop question

Hey wood prop guys,

I have an RV7A with a wood prop. Did all my weight and balance numbers and I am within weight and balance with one pilot, 3 gallons of fuel and 100 pounds of luggage in the back. With that being said, when I have less than 10 gallons of fuel and two people flying, I can tell I am aft heavy. It may fall within W&B limits but I know I am way aft heavy. As a matter of a fact, when the passenger steps on the step, many times the front wheel will lift off the ground. Anyway, I just ordered a 20 pound crush plate to install. I anticipate this is going to improve many characteristics of the plane especially the aft heavy issue. Anybody else done this? Any comments? Also I have noticed that in straight and level flight, the elevator is ever so slightly down deflected and I anticipate this correcting that too.
 
Weight

Tommy
An option to losing 20lbs of useful load could be to add spacers between your engine mount and the firewall.Try re running your weight and balance with this shift in your engine position, it may not take much. There may be side effects of this repositioning like changes to the baffles and a custom length prop spacer. Alternatively, I believe the RV9 uses a longer engine mount to adjust the W&B for typically lighter engine/prop combinations.
 
I have a 320 with a catto and struggle with aft CG issues. I have a 19# crush plate and it helps a lot. They can be finicky to get installed with good balance. It just takes trial and error attempts. In addition to the WB help, it also smooths out the engine and allows lower idle RPM due to the extra rotating mass.

Larry
 
I have a 320 with a catto and struggle with aft CG issues. I have a 19# crush plate and it helps a lot. They can be finicky to get installed with good balance. It just takes trial and error attempts. In addition to the WB help, it also smooths out the engine and allows lower idle RPM due to the extra rotating mass.

Larry

Larry,

I got the 20# plate and installed it. Really changes the flying characteristics of the plane. I mean really changes it!!! Also, I did have to loosen and retighten the plate due to it being out of balance. Amazing that a mm can make a huge difference. Trial and error was the only way I got it balanced.
 
I will be installing a Catto 3 blade w NLE and planning to install either a 15 or 20 lb plate for the same reason. You'll find that many others have done the same if you search the forum for Saber.
 
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Follow up after 20# crush plate installation

Follow up after 20# crush plate installation. After the extra weight up front, I am climbing out faster and my cruising speed in straight and level flight has increased by 10 MPH. The plane also seems to be much smoother in flight. I guess the increased speed is due to the angle of attack of the wings, horizontal stabilizer and elevator position in straight and level flight. Plane also flies much much different. Very glad I added the weight even though balancing it is difficult.
 
Late to the party as usual!

I was going to recommend moving things forward to avoid adding weight. The easiest thing is the ELT with a remote 'shorty' antenna on the back of the seat. You can move it way up front, almost to the firewall.
 
Follow up after 20# crush plate installation. After the extra weight up front, I am climbing out faster and my cruising speed in straight and level flight has increased by 10 MPH. The plane also seems to be much smoother in flight. I guess the increased speed is due to the angle of attack of the wings, horizontal stabilizer and elevator position in straight and level flight. Plane also flies much much different. Very glad I added the weight even though balancing it is difficult.

That seems like a lot of speed increase. Maybe you had an issues with the incidence angle on the tail. Unless you found a hidden speed mod;)

Larry
 
That seems like a lot of speed increase. Maybe you had an issues with the incidence angle on the tail. Unless you found a hidden speed mod;)

Larry

I found the opposite effect on my RV6. With all my camping gear loaded in the back and the cg hitting the aft limits when the fuel tanks get close to empty, it gains almost 5 MPH. I'd always surmised that the tail was having to do less work to hold level flight and the lower induced drag from that was responsible for the extra speed.
 
I'd have to agree with Larry. If a plane is set up correctly, as cg moves aft the tail has to use less down force, and speed goes up due to less drag. Only way a speed increase with cg moving forward makes sense, is if the cg was so far aft the tail was *lifting* prior to the cg move. That would be *very* unstable.
 
Well, maybe Larry's plane is much more nose heavy than mine is and by adding aft weight it puts the horizontal stabilizer in the "sweet spot" with regard to drag. I can assure you that the plane was not unstable at all prior to the weight on the front. All W&B calculations worked. Recovery from stalls, etc very easy. With the weight added, it is indeed faster. I suspect it is from the elevator having to lift the tail without the weight and that increased drag. Anyway, it climbs better, flies faster and I am very happy with the results of adding the weight. keep in mind too, I was very light up front. O-320 with wood prop.
 
I'd have to agree with Larry. If a plane is set up correctly, as cg moves aft the tail has to use less down force, and speed goes up due to less drag. Only way a speed increase with cg moving forward makes sense, is if the cg was so far aft the tail was *lifting* prior to the cg move. That would be *very* unstable.

That is a wive's tale. Most airplanes will have the tail lifting at the aft CG limit. It has no impact on stability. Stability is the rate of change of pitching moment with angle of attack. Having the tail lift or push down affects trim, not stability. The two are very often mixed up. This subject is very poorly taught in all private and commercial pilot courses that I have seen. I am an Aero Eng specializing in stability and control (25 yrs +) and I have measured this stuff in wind tunnels and verified it with flight test data.

I find a 10kt increase very strange due to nose weight. Trim drag is almost always worse at forward CG. That is why all takeoff and climb performance is required to be calculated at full forward CG for certified airplanes. I think there must be something else at play here. Removing a tiny bit of up-lift on the tail would not be worth 10 kts - that's huge. That's like having 20 more HP. There are so many variables that really nailing down a speed difference due to a mod is difficult. You need the same weight, alt, temps etc.

I have no trouble believing that the airplane flies VERY differently. I have experienced that too. It can make a huge difference and is well worth doing.
 
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Details

Tommy
Would you share details of your 7a please.
Engine?
Interior?
Avionics?
Weight?
I know every plane is different.
I'm considering W&B for my 7a build.
 
I will be glad to show my W&B numbers. The W&B was done before the 20# weight was added to the front. I want to redo it with the weight added but haven't done it yet.

As far as the speed increase, it is around 10 MPH not knots. I have flown it enough to tell there is a definite increase. I consistently got around 171 mph true air speed before the weight. I had a definite downward deflection on the elevator in straight and level flight (lifting the tail). That is one of the reasons I decided to add weight. Now I am consistently getting 180-181 mph TAS. Now the elevator is still deflected down in straight and level flight, but only minimally.

The flying characteristics are significantly different. Before the weight was added, I never turned the trim more than 1 turn in any direction. As a matter of a fact, I really never turned it more than a half a turn. Now I use it much more like the Cherokee I use to fly. I need a trim to help take the pressure of the control surface.

The glide slope has also changed significantly. I use to come in very steep with full flaps. I could come in as steep as I wanted and speed was never a concern. Now, I have to come in much more shallow or I can easily get too fast even with full flaps. Just takes some getting use to. The difference is just really impressive. At least to me!
 
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