Aeromodifications for large motorhome

[orbywan]

Originally Posted by crmears

I do not think that we should attempt to copy the shape of a wing. Wings are designed to use airflow to create lift. The action of creating lift with a wing induces drag. The best thing to copy is the fuseloge of an airplane. Euromodder might be correct. Flat panels might give you the same basic result. Of course you would need to round the corners where the top and sides meet.
Randy

OK, I'll buy that, so can anyone tell me the radius for such a bend? If it's bent too tight the air will separate from the bend surface and negate the results, right?

[winkosmosis]

Originally Posted by crmears

I do not think that we should attempt to copy the shape of a wing. Wings are designed to use airflow to create lift. The action of creating lift with a wing induces drag. The best thing to copy is the fuseloge of an airplane. Euromodder might be correct. Flat panels might give you the same basic result. Of course you would need to round the corners where the top and sides meet.
Randy

The wing creates lift by directing air downward, along the top and bottom surfaces. That's what creates the pressure differential between top and bottom (Bernoulli effect is only a minor factor). To do that, the top of the wing has to be curved gently enough that flow stays attached. That's why the top of a wing is rounder than the bottom, and the front edge is rounded.

The aerofoil shape is made to be as aerodynamic as possible within the operating range of the wing, so it's a good design to copy.

[crmears]

Originally Posted by winkosmosis

The wing creates lift by directing air downward, along the top and bottom surfaces. That's what creates the pressure differential between top and bottom (Bernoulli effect is only a minor factor). To do that, the top of the wing has to be curved gently enough that flow stays attached. That's why the top of a wing is rounder than the bottom, and the front edge is rounded.

The aerofoil shape is made to be as aerodynamic as possible within the operating range of the wing, so it's a good design to copy.

It is incorrect to say that "wings create lift by directing air downward." Wings create lift by using Bernoulli's principle. Check wikipedia for a detailed explanation on how it works. Some aircraft can use deflection (that which you describe) but it takes a huge amount of energy and is not practical for sustained flight in conventional aircraft.

You are correct in saying that flow should stay attached to the top of the wing. Otherwise you get a "stall" and the wing loses lift.

The bottom of a wing is not "rounded." It is flat. Bernoulli's principle explains why it is flat.

Randy

[crmears]

Originally Posted by orbywan

OK, I'll buy that, so can anyone tell me the radius for such a bend? If it's bent too tight the air will separate from the bend surface and negate the results, right?

Zenith Aircraft Co. STOL CH 750 - Prototype Photos

The above is a link to a proven and popular kitplane. Note the fuselage. It is flat. I am not saying that the Zenithair is optimum.

Here is how I will create the bend for my boattail for my motorhome. Put one end of a eightfoot piece of 1/2 inch pvc pipe in a vise. (this is for about a six foot boattail) Go the the other end and bend it such that it would close the boat tail to a single point. That shape would be close to ideal.

There is a way to calculate the ideal shape based on the length of the boattail and the other dimensions. But it requires the use of calculus. I had that class 15 years ago and I remember an example in class that dealt with a similar problem.

Randy

[winkosmosis]

Originally Posted by crmears

It is incorrect to say that "wings create lift by directing air downward." Wings create lift by using Bernoulli's principle. Check wikipedia for a detailed explanation on how it works. Some aircraft can use deflection (that which you describe) but it takes a huge amount of energy and is not practical for sustained flight in conventional aircraft.

You are correct in saying that flow should stay attached to the top of the wing. Otherwise you get a "stall" and the wing loses lift.

The bottom of a wing is not "rounded." It is flat. Bernoulli's principle explains why it is flat.

Randy

Nope, it's a myth perpetrated by grade school textbooks that lift is created by air moving faster over the top of the wing.

Planes do fly by directing air downward-- that's why lift is adjusted by varying the pitch of the wings. Two myths I learned in college: bathtub drains and airplane wings — The Endeavour

Same goes for helicopters, which is why rotors push air downward like big propellers. A helicopter blade is basically a wing.

The application of the "Bernoulli effect" here is the attachment of flow to the top of the wing. Air gets pulled downward applies a low pressure effect above the wing that acts on the top surface.
Likewise on the bottom of the wing, the slope pushes air down and creates an upward force.



According to this thread, planes usually cruise with the wings at 1.5-3.5 degrees pitch. At zero pitch they wouldn't be able to stay in the air. http://forum.avsim.net/topic/126211-...during-cruise/

[winkosmosis]

What is Lift?

There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments. To help you understand lift and its origins, a series of pages will describe the various theories and how some of the popular theories fail.

Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

Incorrect Lift Theory

{Lifting airfoils are designed to have the upper surface longer than the bottom.} This is not always correct. The symmetric airfoil in our experiment generates plenty of lift and its upper surface is the same length as the lower surface. Think of a paper airplane. Its airfoil is a flat plate --> top and bottom exactly the same length and shape and yet they fly just fine. This part of the theory probably got started because early airfoils were curved and shaped with a longer distance along the top. Such airfoils do produce a lot of lift and flow turning, but it is the turning that's important, not the distance. There are modern, low-drag airfoils which produce lift on which the bottom surface is actually longer than the top. This theory also does not explain how airplanes can fly upside-down which happens often at air shows and in air-to-air combat. The longer surface is then on the bottom!

[UFO]

Wow, I learned about the incorrect lift in grade school. Ooops. Thanks for the explanations.

[jime57]

Just found this thread and I must say that it is very interesting. I also have a very similar rig, a 38' Newmar with a 300hp cat engine. I have experimented with the existing rig, without modification, since my purchase. So far I have just employed driving technique to improve my fuel economy. I have a long term average of around 10 mpg, with occasional interstate runs up to 11 mpg. I am also at 28-30000 pounds, depending on tank levels and cargo.

I believe that the single most effective measure is to simply slow down. My fuel economy is significantly better at 55 mph, or just above, than at 65 mph. In the end, it doesn't affect the arrival time all that much. But, I have derived a few other techniques which seem to help:
1. Drive just above 55 mph. In my rig, this allows the 6 speed Allison transmission to stay in top gear almost all the time.
2. Use the economy mode on the transmission. This mode delays down shifts and sometimes avoids a downshift entirely.
3. If terrain is very hilly, drive the rig without cruise control. Allow the rig to gain speed on downhills and try to anticipate how much momentum to give up on the uphill to avoid a downshift.
4. Travel with empty sewage tanks and only half a tank of fresh water.
5. Air the rear tires to the full spec 120 psi. The fronts may adversely effect a harsh ride at this pressure. I use 90 psi on the fronts, but one must use enough pressure to reach the required load capacity of the tire.
6. Empty the rig of all unnecessary loads such as excess cargo. (You could throw the wife overboard, but that probably has a negative impact )

I have also discovered that carefully shopping fuel prices can significantly effect the overall operating costs. The web site:

GasBuddy.com - Find Low Gas Prices in the USA and Canada

allows one to shop the diesel prices for any city along the route. Recently the best prices have been at WalMart fuel stations using their "gift" card to save the extra 3 cents, but these are sometimes tight so scout out your entry/exit before approaching the pumps.

I look forward to reading more of this thread. From my aero research I too think that the best potential in in some sort of boattail, wheel covers, and potentially some underbelly smooting. Good luck and we anxiouly await the numbers.

Jim E.

[crmears]

That which you label as "incorrect theory" is taught in FAA approved flight schools as fact. No point for me in debating this so I will leave it at that.

[JohnS]

Originally Posted by skyking

Hard to believe, right?
Somebody posted info from a test using a brick shape. Fairing in the back was the single biggest improvement, and here is my take on why:
No matter how you smooth the flow going onto your rig, it still displaces X amount in cross section. Still 13 high and 8 wide. No way to fool that.
At the back you can "let" that air off very gently and efficiently, or let it slam shut.
If you let it slam shut the turbulence and vacuum is so great, a car can ride with almost no power input in the wake of your motorhome. IF you think about it, you could pull a smaller motorhome along behind you

Thanks Sky! Now I think I'm finally starting to understand this aero stuff based on your explanation.

I was also one of the people who couldn't understand how the shape of the back of the vehicle was more important than the front. It just seemed counter-intuitive. Who the heck cared what happened to the air after the car was already through it, right? The most important part had to be the front of the car since that was cutting into the air first.

Over the last couple of months here, I've been reading about laminar flow, attached air, generated vortexes, etc. It seemed that everybody else was able to follow the conversation and I was...LOST.

Your explanation about "letting the air off very gently and efficiently or letting it slam shut" painted a picture of it that finally made it click in my thick head.

I kinda feel stupid that it took me this long to understand it.

And to think that I had ignored this thread until tonight because I thought that there wouldn't be anything discussed in a conversation of motor homes that would pertain to my little Camry...

Thanks!
John