Weight vs. Power

N941WR · #1 12-01-2008, 04:19 PM

OK, I'm not an engineer and all the talk about engine size over weight has had me thinking.

There has to be some breakeven point for adding weight when adding HP. In other words, how heavy must an engine be before the power gain it provides is limited by the weight of the installation?

It is a bit like the Vette vs. Lotus argument.

Do any of you know how to calculate this?

Geico266 · #2 12-01-2008, 04:29 PM

The simple ratio calculation is aircraft weight -v- total HP.

N941WR · #3 12-01-2008, 04:46 PM

Quote:

Originally Posted by Geico266

The simple ratio calculation is aircraft weight -v- total HP.

Yes, but that doesn't really tell me how the airplane performs.

Say you take one RV-7 with an empty weight of 1050 lbs with a 160 HP engine, that comes out to 6.6 lbs per HP.

Now swap that engine with 180 HP O-360 that weights 20 lbs more. Everything else being equal, that comes out to 5.9 lbs per hp.

That tells me the 180 HP should outperform the 160 HP version but I guess what I'm looking for is how well does one handle over the other.

Wing loading gives you an indication of how many pounds each sq-ft of wing has to lift but somewhere there has to be a combination of the two.

It might be that the difference is so small it is not noticeable. Take for instance a guy who mounts an IO-540 on an RV-7 vs. 160 HP RV-7. I hear the lighter -7 will fly better but how do you quantify that?

Mel · #4 12-01-2008, 04:56 PM

Some things depend on where the extra weight is. Everything else being equal, an airplane balanced toward the aft end of the CG will fly better than a nose heavy one.

ergie63 · #5 12-01-2008, 05:00 PM

For handling use wing loading. In all your examples, the wings are identical and have the same capacity to generate lift. So a heavier model will require a larger turning radius or a higher angle of attack.

Force = Mass * Acceleration
To get the same acceleration (turn) out of a heavier aircraft requires more force.

The IndyCar guys are all up in arm over Danica Patrick because she has a 40lb weight advantage over them. EDIT: For the 2008 season the weight rule was changed to include the driver. Wikipedia

L.Adamson · #6 12-01-2008, 05:05 PM

Quote:

Originally Posted by N941WR

Yes, but that doesn't really tell me how the airplane performs.

Say you take one RV-7 with an empty weight of 1050 lbs with a 160 HP engine, that comes out to 6.6 lbs per HP.

Now swap that engine with 180 HP O-360 that weights 20 lbs more. Everything else being equal, that comes out to 5.9 lbs per hp.

That tells me the 180 HP should outperform the 160 HP version but I guess what I'm looking for is how well does one handle over the other.

Wing loading gives you an indication of how many pounds each sq-ft of wing has to lift but somewhere there has to be a combination of the two.

It might be that the difference is so small it is not noticeable. Take for instance a guy who mounts an IO-540 on an RV-7 vs. 160 HP RV-7. I hear the lighter -7 will fly better but how do you quantify that?

I'll try not to hijack the thread too much....

And this way, I don't need to hijack your "build light" thread at all!

So lets take performance. It appears that my "heavier" 1172 lb. 6A with it's 180 HP C/S prop, is kind of running like a scalded dog, in comparison to a few 9A's with 150/160 C/S prop setups. I'm more maneuverable in comparison to being more sedate. Faster on the level, better climb rate, and less to worry about when it comes to density altitude. I may use a bit more fuel, but I much prefer the performance, as well as a livelier feel..

I won't compare mine to a lightweight "3" that's great for short flights around the countryside with minimal everything. Mine does very well for some long cross-country rides.

Yet, myself and the 9's have been going on quite a few 200 mile brunch runs lately. I get a lot of comparison that way. I've also got about 40 hrs in the two 9's myself.

The point is, mine is heavier, but it appears the horsepower difference, really DOES make a difference. It feels like a sports car, and I didn't have to give up a lot on the comfort end.

L.Adamson ---- RV6A

ergie63 · #7 12-01-2008, 05:19 PM

Quote:

Originally Posted by L.Adamson

The point is, mine is heavier, but it appears the horsepower difference, really DOES make a difference. It feels like a sports car, and I didn't have to give up a lot on the comfort end.

L.Adamson ---- RV6A

I disagree with your conclusion. Instead its the wing that is making the difference. The -6 wing is capable generating much more force, evidenced by its greater g load rating, than the -9 wing. This much larger force is apparently sufficient to overcome the additional weight noted.

N941WR · #8 12-01-2008, 05:21 PM

LA,

You missed the point of this post completely. I wasn't comparing a -9 to a -6 or any other plane.

The point is to find out where the tradeoff of power over weight cross.

I'm sure your -6 handles like a sports car to you, just like the guy who has only driven one type of sports car, say a Vette thinks his car is the best thing in the world. But there are other cars out there that can perform just as well, maybe better on less power, say a Lotus Exige which can stomp the Vette with only 240 HP because it is lighter. It is this light weight that allows the Exige to accelerate faster, stop quicker, and turn in faster than the Vette.

Of course you give up a lot with the Lotus for these performance numbers. The same tradeoffs are there in airplane construction, I just thought it might be nice to know where.

BTW, this thread was not aimed at you and your well documented 180HP CS sports car of an RV.

Kyle Boatright · #9 12-01-2008, 05:51 PM

Quote:

Originally Posted by N941WR

LA,

You missed the point of this post completely. I wasn't comparing a -9 to a -6 or any other plane.

The point is to find out where the tradeoff of power over weight cross.

<snip>

BTW, this thread was not aimed at you and your well documented 180HP CS sports car of an RV.

One hp does 33,000 lb*ft/minute of work. That means it can either lift 1 lb 33,000 feet in a minute or lift 33,000 lbs one foot in a minute.

In our situation, we factor this by prop efficiency. Say 80%, so an engine HP will lift 1 lb 80% x 33,000 feet in a minute = 26,400 ft.

If you have a 160 hp engine and a 1600 lb plane, theory is that it should climb at 160 x 80% x 33,000/1600 = 2640 fpm. But that's wrong since we didn't consider the power it takes to simply hold the aircraft in the air. Let's call that 50 hp off the top.

So for climb performance, you have a 110 surplus hp x 80% efficiency x 33,000 ft*lb/min = 1815 ft/min.

Modify the equations with the new aircraft weight and the new hp and see what happens.

Boil it all down and if you increase the ratio of surplus hp vs aircraft weight and you'll improve climb rate. In the example above, the ratio was 110/1600.

Hope this helps..

L.Adamson · #10 12-01-2008, 05:52 PM

Quote:

Originally Posted by N941WR

BTW, this thread was not aimed at you and your well documented 180HP CS sports car of an RV.

I know that. Yet your other "build light" thread, naturally caught my eye. The problem is, we can get into this notion, that building light is going to be the best of all things. When actually, it may not be.

I have a slider, I have a C/S prop. I have the 180HP engine, the two axis auto-pilot, and the leather seats. I've got the electric elevator & aileron trim. As it is, I didn't need to give these things up; and would not give them up a second time around. Besides, my RV probably penetrates wind better than the light weights.

L.Adamson

P.S. ---- If I had the time and money, I'd like to build a Cub Crafters Cub with a 180 HP engine and tundra tires. A different mission, but fun. The guy in the hangar across from me is building one, to go with his Cessna 180 & Pitts. It's a nice plane!