Bryan Wood

Bryan Wood

Well Known Member
Often the performance specs are referred to in these lists for the 7A vs. 9A and there are some quirks that have me puzzled. I'll start with the 7A because as far as I can tell the numbers listed make sense, but the ones provided for the 9a don't add up. My question is why? For these examples I'm using the posted numbers for solo weight and 75% power for both airplanes.

Van's numbers for the 7A are 190mph on 160hp, 198mph on 180hp, and 205mph on 200hp.

After doing some learning about speed increases the math isn't terribly difficult to calculate how much the speeds will increase with additional horsepower on the same airframe. Simply put, the speed increases with the cube root of the power increase. So increasing the horsepower from 160 to 180hp is a 1.125% power increase. (180/160=1.125) The cube root of 1.125 is 1.040 and when this is multiplied by the 190mph of the 160hp RV-7A the result is 197.6, or rounded to 198. This is exactly what Van publishes, and when using the same formulas the numbers match Van's numbers for the 200hp plane also. Try it for yourself, here is a link to a cube root calculator.
http://www.csgnetwork.com/cuberootcubecalc.html

Now if you use this process to check the numbers for the 9a they are not coming up even close to what the factory publishes. If you start with the 135hp and 173mph and then jump the power to 160hp the numbers are significantly slower than the factory numbers. This is puzzling me. The only thing that I can come up with is that the props available for the smaller engines are not up to snuff.

According to the math the 160hp 9A should achieve 183mph at 75%, but the factory claims 187mph. Do any of you speedsters have an explanation for this? The factory plane is faster than 183mph. I know this personally having done my transition training in it. The CAFE Foundation report has it going even faster than what they publish. So why are the numbers skewed?

Best,
 
Prop, Cowl, Cooling Drag, power loading, wing loading, etc.

I think you are right about the prop being a big difference. There are other differences as well that take the comparrison out of the direct performance difference due to HP category.

Bob Axsom
 
So increasing the horsepower from 160 to 180hp is a 1.125% power increase.
Click to expand...
Not exactly right. A power increase from 160 to 180 hp is 1.125x, not 1.125%. It's actually an 8% power increase. Just a minor detail though, because the speed calculations are done with the ratio, which you've correctly calculated at 1.125.

My guess on the speed difference on the RV9 is the wing. It is more efficient, no?

Dave
 
Van's perf data for the 135 hp RV-9A is estimated, not measured. It seems to be based on the measured performance for 118 hp. Van appears to have used the cube root rule of thumb, and rounded the result for 135 hp down.

The perf data for 160 hp is listed as being measured, so I see no reason to question it. The cruz of your question seems to be "Why does the measured data differ at different hp not match the predictions from the cube root rule of thumb?"

Well, you need to understand the conditions under which the cube root rule of thumb is valid:

  • The drag coefficient must not change.
  • The prop efficiency must not change.
  • The ratio of engine powers must be known.

The drag coefficient changes if we change the angle of attack. The angle of attack changes if we change the weight or speed. So, the rule of thumb is only valid for small speed changes, which implies it is only valid for small power changes.

The 160 hp RV-9A prototype had a very different prop than the 118 hp prototype. So the prop efficiency is quite possibly very different.

The hp values (118 hp and 160 hp) are valid at sea level, standard day, with no ram recovery in the air box, and nominal pressure losses in the air box. If we now take those two engines to altitude, the hp we get depends on the rpm we achieve (which will vary depending on which prop we have) and the manifold pressure (temperature is assumed to be standard). The MP will depend on the amount of ram pressure recovery we get in the air box, and the amount of pressure loss in the induction tract. The O-235 and the O-320 may have different induction pressure losses. And, the faster speed of the O-320 powered aircraft could allow for a higher MP due to more ram air pressure recovery.

Bottom line - the power change is too large to expect the cube root rule of thumb to be accurate, even if the prop efficiency was not changed.
 
Kevin Horton said:
The hp values (118 hp and 160 hp) are valid at sea level, standard day, with no ram recovery in the air box, and nominal pressure losses in the air box. If we now take those two engines to altitude, the hp we get depends on the rpm we achieve (which will vary depending on which prop we have) and the manifold pressure (temperature is assumed to be standard). The MP will depend on the amount of ram pressure recovery we get in the air box, and the amount of pressure loss in the induction tract. The O-235 and the O-320 may have different induction pressure losses. And, the faster speed of the O-320 powered aircraft could allow for a higher MP due to more ram air pressure recovery.

Bottom line - the power change is too large to expect the cube root rule of thumb to be accurate, even if the prop efficiency was not changed.
Click to expand...

Kevin, not trying to be argumentitive, but I'm still not getting it. The spread you speak of and the ram effects etc not being consistant because the spread in HP is to great from 118hp to 160hp. This is a difference of 42hp and the difference between the 200hp and 160hp RV-7A is 40hp. However the cube root thing still works if you divide 200 by 160 and ... It comes out to 204.7 which rounds up to 205 matching Van's numbers. Do you think that the planes are operating in far different areas on their drag curves with the 118hp 9A compared to a 160hp 7A as a starting place for all of this?

Regards,
 
Bryan,

I don't recall which engine Van's first RV-7(A) had, but I'm betting Van has only done perf testing with one engine, and then used the cube root rule of thumb to produce predictions for other engines. On the RV-9A, Van actually did testing with 118 hp and with 160 hp, and he reported the actual results, which happen to differ a bit from the rule of thumb. That simply highlights that the rule of thumb is only an approximation. It should not be viewed as a problem with Van's perf numbers.
 
Kevin Horton said:
Bryan,

I don't recall which engine Van's first RV-7(A) had, but I'm betting Van has only done perf testing with one engine, and then used the cube root rule of thumb to produce predictions for other engines. On the RV-9A, Van actually did testing with 118 hp and with 160 hp, and he reported the actual results, which happen to differ a bit from the rule of thumb. That simply highlights that the rule of thumb is only an approximation. It should not be viewed as a problem with Van's perf numbers.
Click to expand...


Kevin,

Thanks for the second answer. This one makes sense and doesn't second guess your first one either. :D

Best,
 
You must log in or register to reply here.