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Crush plate for aft CG

martinsrv7

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On numerous trips now with maximum luggage, I’ve found my RV7 to be very close If not right on the aft C of G when landing with less than say 50% fuel on board. Replacing my Sensenich metal fixed pitch prop with a Hartzell constant speed prop would be the obvious solution but unfortunately I’m not in a position to do this at the moment.

As it’s a reasonably light build with plenty of gross weight margin, I’m considering installing a crush plate forward of the front spinner plate to alleviate the aft CG situation. Saber Manufacturing can supply a 7 inch diameter 1 3/8” plate that weighs 15 pounds which is fairly close to the difference between a FP and CS prop. What is also required is to swap the currently installed 2 1/4” spacer with a 2 1/4” prop extension as there is a maximum bolt length that Saber can supply.

Has anyone tried such a solution for aft CG on an RV7 with a metal prop ? Or does anyone see any issues in doing this ?

Thanks,
Martin Russell
RV7 VH-IMR 360 hours
Sydney, Australia.
 
The option I used was a heavy alternator and starter. 14 pounds or so over some options, no changes to prop. Serviceable units can be quite reasonably priced if they don't require a core turn in.
 
On numerous trips now with maximum luggage, I’ve found my RV7 to be very close If not right on the aft C of G when landing with less than say 50% fuel on board. Replacing my Sensenich metal fixed pitch prop with a Hartzell constant speed prop would be the obvious solution but unfortunately I’m not in a position to do this at the moment.

As it’s a reasonably light build with plenty of gross weight margin, I’m considering installing a crush plate forward of the front spinner plate to alleviate the aft CG situation. Saber Manufacturing can supply a 7 inch diameter 1 3/8” plate that weighs 15 pounds which is fairly close to the difference between a FP and CS prop. What is also required is to swap the currently installed 2 1/4” spacer with a 2 1/4” prop extension as there is a maximum bolt length that Saber can supply.

Has anyone tried such a solution for aft CG on an RV7 with a metal prop ? Or does anyone see any issues in doing this ?

Thanks,
Martin Russell
RV7 VH-IMR 360 hours
Sydney, Australia.

You have a excellent engine and prop combination, and which should put you in the sweet spot. I would discourage the crush plate thing. I have the same combination with no cg issues. Please give us more info about the systems placed forward and aft of the flap motor like elt. wiring, lights, avionics trays, magnetometer etc. and maybe some info on your weighing procedure. Also what are your empty weight and balance numbers?
 
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I added the 15 lb crush on an RV9 which was rear CG. It had a composite , Prince FP prop on a 160 hp ,also replaced the spacer with Saber 2 1/2 extension. Results was a 2 inch move fwd on the RV9. Made great empty fuel single pilot landing differences. Prior to that 3 point was always tail wheel first. But usually always did wheel landing anyway.
 
I have the Saber extension and 20lb crush plate with a Catto 3-blade. I haven't flown yet but have weighed and the calculations look pretty good.
 
I have a 7a with a light catto 3-blade prop and earthx battery and io360. I had to put on a 20 pound crush plate to get more useful baggage capacity within aft CFG limits. Call Sabre and they will get you everything you need. It’s a bummer though to add weight to a light plane.
 
Useful ballast

Find a way to secure your toolkit and maybe a gallon or two of water on the floor just aft of the firewall between pilot's and copilot's feet. A couple of slotted brackets and some nylon straps and you're in business. Made a huge difference in our old 6A. The day I experienced sloppy stick on landing with minimum fuel was the day I moved my toolkit forward.

Ed Holyoke
 
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I added the 15 lb crush on an RV9 which was rear CG. It had a composite , Prince FP prop on a 160 hp ,also replaced the spacer with Saber 2 1/2 extension. Results was a 2 inch move fwd on the RV9. Made great empty fuel single pilot landing differences. Prior to that 3 point was always tail wheel first. But usually always did wheel landing anyway.
But i actually flew it for 8 years before i added the crush plate wt. It was not out of rear CG limit but would be at limit with EMPTY tanks and no passenger and 60 + iin Baggage. But in that config my 9 just did not like 3 point landings. Wheel landing were no problem and actually preferred.
 
20 lb plate from Saber on my -7A allowed me to swap out the PC-680 (15 lbs) with an EarthX 680C (3.5 lbs) and gain back 11.5 lb of useful load. Never a CG problem, even with 100 lb of baggage.
 
You have a excellent engine and prop combination, and which should put you in the sweet spot. I would discourage the crush plate thing. I have the same combination with no cg issues. Please give us more info about the systems placed forward and aft of the flap motor like elt. wiring, lights, avionics trays, magnetometer etc. and maybe some info on your weighing procedure. Also what are your empty weight and balance numbers?

Weighing was carried out as per standard aircraft weighing procedures with aircraft scales. Numbers are as follows:
Empty weight: 1040 lbs
Empty weight CG: 80.2” aft of datum.

I have a dual screen Garmin G3X with the magnetometer in the wingtip, ADHRS and engine modules behind the panel with Garmin pitch and roll autopilot servos. Garmin SL40 radio, GTX33ES transponder. PC680 15 lbs battery on the firewall. 0360-A1A engine, Vetterman exhaust with a standard Vans firewall forward electrical setup including a plane power 60 amp alternator.
 
Very interesting. Just for comparison these are my rv7 specs:
TmxIO360, sensi metal prop, pc680, g3x non touch with steam backup, gtx327, sl40, No autopilot servos, only one landing light, ahrs, and engine box in same location, magnatometer in wingtip also, aeroled pulsar in wintips.
No wiring aft of my flap motor. Composite tail wheel. Painted, 8 quarts oil, gearleg fairings and wheel pants. Elt just behind co-pilot seat. [email protected].

Some simple things to consider before adding heavy crush plate:
Lightweight or composite tail wheel upgrade
If your elt is located in your aft fuselage move it just behind the copilot seat
Did you weigh your plane with 8 quarts of oil?
Did you weigh your plane with wheel pants and gear fairings?
Was your plane weighed with a wooden prop on it? I ask that because your weight and CG
would reflect that configuration.
I recommend reweighing your plane somethimg just doesnt seem right.
 
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Thank you all for your input. From the various responses it would seem that crush plates have been installed on aircraft with relatively light composite props and not so much for metal props. I’m hesitant to add weight permanently to an already excellent setup so I’ve decided to go down the route of adding ballast only for the purpose of going on a trip when I have a high baggage weight. I already try to get heavy items such as tools etc as far forward as possible but the idea of adding ballast temporarily is appealing. In this way, the majority of my everyday flying is not affected and I’m not adding weight permanently for no real reason.

Thanks,
Martin Russell
Sydney, Australia
RV7 VH-IMR
 
Very interesting. Just for comparison these are my rv7 specs:
TmxIO360, sensi metal prop, pc680, g3x non touch with steam backup, gtx327, sl40, No autopilot servos, only one landing light, ahrs, and engine box in same location, magnatometer in wingtip also, aeroled pulsar in wintips.
No wiring aft of my flap motor. Composite tail wheel. Painted, 8 quarts oil, gearleg fairings and wheel pants. Elt just behind co-pilot seat. [email protected].

Some simple things to consider before adding heavy crush plate:
Lightweight or composite tail wheel upgrade
If your elt is located in your aft fuselage move it just behind the copilot seat
Did you weigh your plane with 8 quarts of oil?
Did you weigh your plane with wheel pants and gear fairings?
Was your plane weighed with a wooden prop on it? I ask that because your weight and CG
would reflect that configuration.
I recommend reweighing your plane somethimg just doesnt seem right.

I’ve since added gear leg fairings and wheel pants and autopilot servos now that I think about it. I do have a lightweight tail wheel. The aircraft was weighed with the Sensenich metal prop and 6 quarts of oil. ELT is in the cockpit. I think that’s a very good idea to reweigh before I go too much further.

Thanks,
Martin.
 
Useful ballast

Thank you all for your input. From the various responses it would seem that crush plates have been installed on aircraft with relatively light composite props and not so much for metal props. I’m hesitant to add weight permanently to an already excellent setup so I’ve decided to go down the route of adding ballast only for the purpose of going on a trip when I have a high baggage weight. I already try to get heavy items such as tools etc as far forward as possible but the idea of adding ballast temporarily is appealing. In this way, the majority of my everyday flying is not affected and I’m not adding weight permanently for no real reason.

Thanks,
Martin Russell
Sydney, Australia
RV7 VH-IMR

I built a carbon fiber box to cover my fuel filter/pump assembly and a battery. It extends from the fuel selector forward to the firewall and fills the entire center section tween pilot and co-pilot's legs. I mount my O2 tank on top of that with the valve facing aft. Since I usually only need oxygen for long trips, it coincides with when I have a lot of baggage and compensates for the aft CG condition. It also serves as tool kit storage and a glove box. Good place to keep the cannulas until I need them.

Ed Holyoke
 
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