max speed bc12-d

[tiketyboo]

hi
i have a 1946 bc 12 d with a 75 continental motor and a beechman roby adjustable propeller. the motor gets up to 2550 rpm well and we can adjust the prop to get 95 mph but once there any further adjustment just slows down the rpm's. seems like 95 mph is max, does this sound right, anyone got any thoughts. we have not tested the compression on the motor but it runs well.
tony

[Mark Julicher]

Re: max speed bc12-d

At what gross weight are you flight testing?
I experience a 10 mph difference between fully loaded/fueled and nearly out of gas solo. Weight is a big factor. I suspect you will get 100 mph tops. the extra horses will get you off the ground quicker than a 65 horse, but the top speed will only be about 3 mph faster.

[tiketyboo]

Re: max speed bc12-d

once we had two people on board for about 420 pounds plus full main tank and second time one person on board, 170 pounds and full main tank, did not seem to make much difference, i am wondering if i need to check the engine compression etc to make sure i am getting 75 hp or if this is just the max it will fly regardless of hp.
tony

[Mark Julicher]

Re: max speed bc12-d

Well, you may be a few ponies shy of 75, but you are still able to get the RPM up, so unless your cylinders feel "very tired" when you prop it, I don't think doing a compression check will show you much.
If memory serves me, change in top speed due to an increase in horsepower goes as the cube root of the change in horsepower. So the difference in top speed between a 75 and a 65 is the cube root of 10....which is the square root of %^$#&... or not very much.

Maybe you need to weigh the plane? Clean up draggy spots like crummy baffles if you want more speed. Perhaps shrink the engine cooling outlet if the temps will stay in limits.

I hope a few others will chime in here. This thread has the promise of teaching a lot of Taylorcraft lore...

[mulwyk]

Re: max speed bc12-d

Mark:
cube root doesn't sound right.
O-320: 85 additional HP = 4.4 additional mph?
O-200: 35 additional HP = 3.3 additional mph?
A-75: 10 additional HP = 2.2 additional mph?
square root maybe?
O-320: 85 additional HP = 9.2 additional mph
O-200: 35 additional HP = 5.9 additional mph
A-75: 10 additional HP = 3.2 additional mph

Your point that additional HP doesn't mean a big boost in speed is a valid point, I'm just not sure about the exact relationship. I know some of the 100HP guys claim a 20 mph speed increase...where's that come from? Dunno.

[3Dreaming]

Re: max speed bc12-d

Old rule of thumb. Four times the power to double the speed. 260hp for 190 MPH.

[jim cooper]

Re: max speed bc12-d

90-95 is about right I think. A BC12D is not a speed merchant. I agree with others that going to say an 0200, will get you off the ground a little sooner but not much faster in the air. It wasn't built to go fast. Its a low level , have fun cruise around type. I think its the very best of its class. Easy to fly, very forgiving, simple design, easy to annual (providing someone didn't screw it up before you got it.) cheap to fly. JC

[tiketyboo]

Re: max speed bc12-d

thanks for the replies, i am thinking you are right, 95 mph is about max for the bird and probably the best speed. i will set my engine to minimum revs and just enjoy the economy of the adjustable prop.
tony

[Tom Eggert]

Re: max speed bc12-d

For a while, I had both an F-19 and a BC-12. Both were clean, light and original. (Still have the BC-12). The big difference was in climb. In cruise, there was not as big a difference. (the F-19 would do about a solid 105) I was flying a 74-45 on the BC-12, and a 69-51 (I think - original prop) on the F-19.

[VictorBravo]

Re: max speed bc12-d

Everyone is forgetting the wooden Beech Roby propeller is not as efficient as a metal propeller. Perhaps it is not even as efficient as some fixed wood props.

If I'm remembering right, the Beech prop has a thick, rough airfoil. With brass leading edges that are NOT flush with the wood.

I don't care what the available pitch adjustment is, if you don't have a good high speed airfoil on the prop blade then the prop will not pull as hard.

Try installing a borrowed metal prop or two on your airplane and THEN see where your relative performance is comparatively. I'm willing to bet that the thin metal Sensenich fixed pitch will give you more of (either speed or climb) than the adjustable Beech prop.

[jim cooper]

Re: max speed bc12-d

In Western NY State where I fly some in summer, there are ALOT of tail draggers, T-CRAFTS, Super Cubs, j3 Cubs, and on and on. There are almost NO wooden props, none! Metal props make a BIG difference if they are spec'd correctly. Prop technology has come a long ways since the 1930's. I think Victor Bravo is 100% correct. JC

[Dano"T"]

Re: max speed bc12-d

I cruise at 105 with my 85HP with climb prop and 105 with my 90hP with a 40 pitch seaplane prop....heck, it cruises at 97 with the floats on!...verfied as true airspeed numerous times during FAA flight testing.....so I think Victor Bravo is n to something.

[Ragwing nut]

Re: max speed bc12-d

thr roby is your limiting factor. wood prop max about 95. metal prop worth 7-10 mph. Ours would run 105-107 trimmed out on step. My o-200 deluxe would do about 115 with a 74-43 wide open. Our F-21 would do about 125 with a 70-52

[jim cooper]

Re: max speed bc12-d

As someone else mentioned, these old speed indicators are all over the board. They are OLD! Best to check your forward progress with a GPS. I've had 3 pretty nice T crafts over the years and none did over 90-95. (65 hp in each case.) JC